view src/p2_deco.c @ 543:a92cecf8cee4

BUGFIX: "lost gas" showed current set of gases (Diluents in CC, OC in Bailout) for both "lost gas" menus
author heinrichsweikamp
date Wed, 06 Sep 2017 11:08:59 +0200
parents 015b7fdd90a7
children a5d2e6083b1d
line wrap: on
line source

// **************************************************************
// p2_deco.c
//
//  Created on: 12.05.2009
//  Author: chsw
//
// **************************************************************

//////////////////////////////////////////////////////////////////////////////
// OSTC - diving computer code
// Copyright (C) 2011 HeinrichsWeikamp GbR
//
//    This program is free software: you can redistribute it and/or modify
//    it under the terms of the GNU General Public License as published by
//    the Free Software Foundation, either version 3 of the License, or
//    (at your option) any later version.
//
//    This program is distributed in the hope that it will be useful,
//    but WITHOUT ANY WARRANTY; without even the implied warranty of
//    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
//    GNU General Public License for more details.
//
//    You should have received a copy of the GNU General Public License
//    along with this program.  If not, see <http://www.gnu.org/licenses/>.
//
//////////////////////////////////////////////////////////////////////////////

// *****************************
// ** I N T R O D U C T I O N **
// *****************************
//
// OSTC
//
// code:
// p2_deco_main_c_v101.c
// part2 of the OSTC code
// code with constant O2 partial pressure routines
// under construction !!
//
// summary:
// decompression routines
// for the OSTC experimental project
// written by Christian Weikamp
// last revision __________
// comments added _________
//
// additional files:
// p2_tables_v100.romdata (other files)
// 18f4685_ostc_v100.lkr (linker script)
//
// history:
// 01/03/08 v100: first release candidate
// 03/13/08 v101: start of programming ppO2 code
// 03/13/25 v101a: backup of interrim version with ppO2 calculation
// 03/13/25 v101: open circuit gas change during deco
// 03/13/25 v101: CNS_fraction calculation
// 03/13/26 v101: optimization of tissue calc routines
// 07/xx/08 v102a: debug of bottom time routine
// 09/xx/08 v102d: Gradient Factor Model implemenation
// 10/10/08 v104: renamed to build v103 for v118 stable
// 10/14/08	v104: integration of char_I_depth_last_deco for Gradient Model
// 03/31/09 v107: integration of FONT Incon24
// 05/23/10 v109: 5 gas changes & 1 min timer
// 07/13/10 v110: cns vault added
// 12/25/10 v110: split in three files (deco.c, main.c, definitions.h)
// 2011/01/20: [jDG] Create a common file included in ASM and C code.
// 2011/01/24: [jDG] Make ascenttime an short. No more overflow!
// 2011/01/25: [jDG] Fusion deco array for both models.
// 2011/01/25: [jDG] Use CF(54) to reverse deco order.
// 2011/02/11: [jDG] Reworked gradient-factor implementation.
// 2011/02/15: [jDG] Fixed inconsistencies introduced by gas switch delays.
// 2011/03/21: [jDG] Added gas consumption (CF56 & CF57) evaluation for OCR mode.
// 2011/04/15: [jDG] Store low_depth in 32bits (w/o rounding), for a better stability.
// 2011/04/25: [jDG] Added 1mn mode for CNS calculation, to allow it for decoplanning.
// 2011/04/27: [jDG] Fixed char_O_gradient_factor calculation when model uses gradient-factor.
// 2011/05/02: [jDG] Added "Future TTS" function (CF58).
// 2011/05/17: [jDG] Various cleanups.
// 2011/08/08: [jDG] Computes CNS during deco planning ascent.
// 2011/11/24: [jDG] Slightly faster and better NDL computation.
// 2011/12/17: [mH]  Remove of the useless debug stuff
// 2012/02/24: [jDG] Remove missed stop bug.
// 2012/02/25: [jDG] Looking for a more stable LOW grad factor reference.
// 2012/09/10: [mH]  Fill char_O_deco_time_for_log for logbook write
// 2012/10/05: [jDG] Better deco_gas_volumes accuracy (average depth, switch between stop).
// 2013/03/05: [jDG] Should vault low_depth too.
// 2013/03/05: [jDG] Wrobell remark: ascent_to_first_stop works better with finer steps (2sec).
// 2013/05/08: [jDG] A. Salm remark: NOAA tables for CNS are in ATA, not bar.
// 2013/12/21: [jDG] Fix CNS calculation in decoplan w/o marked gas switch
// 2014/06/16: [jDG] Fix Helium diluant. Fix volumes with many travel mix.
// 2014/06/29: [mH]  Compute int_O_ceiling
// 2015/06/12: [jDG] Fix NDL prediction while desaturating with the Buhlmann model.
// 2017/08/04: [mH] Switch to absolute GF everywhere and apply safety margin parameters to both models (GF and non-GF), fixes from Ralph Lembcke
//
// TODO:
//
// Literature:
// Buhlmann, Albert: Tauchmedizin; 4. Auflage [2002];
// Schr"oder, Kai & Reith, Steffen; 2000; S"attigungsvorg"ange beim Tauchen, das Modell ZH-L16, Funktionsweise von Tauchcomputern; http://www.achim-und-kai.de/kai/tausim/saett_faq
// Morrison, Stuart; 2000; DIY DECOMPRESSION; http://www.lizardland.co.uk/DIYDeco.html
// Balthasar, Steffen; Dekompressionstheorie I: Neo Haldane Modelle; http://www.txfreak.de/dekompressionstheorie_1.pdf
// Baker, Erik C.; Clearing Up The Confusion About "Deep Stops"
// Baker, Erik C.; Understanding M-values; http://www.txfreak.de/understanding_m-values.pdf
//
//

// *********************
// ** I N C L U D E S **
// *********************
#include <math.h>

// ***********************************************
// ** V A R I A B L E S   D E F I N I T I O N S **
// ***********************************************

#include "p2_definitions.h"
#define TEST_MAIN
#include "shared_definitions.h"

// Water vapour partial pressure in the lungs
#define ppWater        0.0627
#define METER_TO_BAR   0.09985
#define BAR_TO_METER   10.0150      // (1.0/METER_TO_BAR)

// Surface security factor
#define SURFACE_DESAT_FACTOR    0.7042

// *************************
// ** P R O T O T Y P E S **
// *************************

static void calc_hauptroutine(void);
static void calc_nullzeit(void);

static void calc_tissue(PARAMETER unsigned char period);
static void calc_limit(void);

static void clear_tissue(void);
static void calc_ascenttime(void);
static void update_startvalues(void);
static void clear_deco_table(void);
static unsigned char update_deco_table(void);

static void sim_tissue(PARAMETER unsigned char period);
static void sim_limit(PARAMETER float GF_current);
static void sim_extra_time(void);
static void calc_dive_interval(void);

static void calc_gradient_factor(void);
static void calc_wo_deco_step_1_min(void);

static void calc_hauptroutine_data_input(void);
static void calc_hauptroutine_update_tissues(void);
static void calc_hauptroutine_calc_deco(void);
static void sim_ascent_to_first_stop(void);

static unsigned char gas_switch_deepest(void);
static void gas_switch_set(void);

static unsigned char calc_nextdecodepth(void);

//---- Bank 5 parameters -----------------------------------------------------
#ifndef UNIX
#   pragma udata bank5=0x500
#endif

static float			GF_low;
static float			GF_high;
static float			GF_delta;
static float			locked_GF_step;             // GF_delta / low_depth

static unsigned char    temp_depth_limit;
float                   low_depth;                  // Depth of deepest stop

// Simulation context: used to predict ascent.
static unsigned char	sim_lead_tissue_no;         // Leading compatiment number.
static float			sim_lead_tissue_limit;      // Buhlmann tolerated pressure.

// Real context: what we are doing now.
static float			calc_lead_tissue_limit;     //

static unsigned char	internal_deco_time[NUM_STOPS];
static unsigned char	internal_deco_depth[NUM_STOPS];

static float cns_vault;
static float low_depth_vault;
static float pres_tissue_N2_vault[NUM_COMP];
static float pres_tissue_He_vault[NUM_COMP];

//---- Bank 6 parameters -----------------------------------------------------
#ifndef UNIX
#   pragma udata bank6=0x600
#endif

static unsigned char	ci;
static float            pres_respiration;
static float            pres_surface;
static float            temp_deco;
static float            ppN2;
static float            ppHe;
static float            temp_tissue;
static float            N2_ratio;                       // Breathed gas nitrogen ratio.
static float            He_ratio;                       // Breathed gas helium ratio.
static float            var_N2_a;                       // Buhlmann a, for current N2 tissue.
static float            var_N2_b;                       // Buhlmann b, for current N2 tissue.
static float            var_He_a;                       // Buhlmann a, for current He tissue.
static float            var_He_b;                       // Buhlmann b, for current He tissue.
static float            var_N2_e;                       // Exposition, for current N2 tissue.
static float            var_He_e;                       // Exposition, for current He tissue.
static float            var_N2_ht;                      // Half-time for current N2 tissue.
static float            var_He_ht;                      // Half-time for current N2 tissue.

static float            pres_diluent;                   // new in v.101
static float            const_ppO2;                     // new in v.101

static unsigned char    sim_gas_last_depth;             // Depth of last used gas, to detected a gas switch.
static unsigned char    sim_gas_last_used;              // Number of last used gas, to detected a gas switch.
static unsigned short   sim_dive_mins;                  // Simulated dive time.
static float            calc_N2_ratio;                  // Simulated (switched) nitrogen ratio.
static float            calc_He_ratio;                  // Simulated (switched) helium ratio.
static float            CNS_fraction;                   // new in v.101
static float            float_saturation_multiplier;    // new in v.101
static float            float_desaturation_multiplier;  // new in v.101
static float            float_deco_distance;            // new in v.101

static unsigned char    deco_gas_change[NUM_GAS];       // new in v.109
static unsigned char	internal_deco_gas  [NUM_STOPS];

//---- Bank 7 parameters -----------------------------------------------------
#ifndef UNIX
#   pragma udata bank7=0x700
#endif
    // Keep order of 0x700 variables
float  pres_tissue_N2[NUM_COMP];
float  pres_tissue_He[NUM_COMP];
float  sim_pres_tissue_N2[NUM_COMP];             // 16 floats = 64 bytes.
float  sim_pres_tissue_He[NUM_COMP];             // 16 floats = 64 bytes.

//---- Bank 8 parameters -----------------------------------------------------
#ifndef UNIX
#   pragma udata overlay bank8=0x800
    static char	  md_pi_subst[256];
#   define C_STACK md_pi_subst      // Overlay C-code data stack here, too.
#endif

// Back to bank6 for further tmp data
#ifndef UNIX
#   pragma udata bank6
#endif

//////////////////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////////////////
///////////////////////////// THE LOOKUP TABLES //////////////////////////////
//////////////////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////////////////
//
// End of PROM code is 17F00, So push tables on PROM top...
//
#ifndef UNIX
#   pragma romdata buhlmann_tables = 0x1DD00  // Needs to be in UPPER bank.
#endif

rom const float buhlmann_ab[4*16] = {
// Data ZH-L16C, from Bühlmann Tauchmedizin 2002, option 1a (4mn)
// a for N2    b for N2     a of He     b for He
	1.2599,     0.5050,     1.7424,     0.4245,
	1.0000,     0.6514,     1.3830,     0.5747,
	0.8618,     0.7222,     1.1919,     0.6527,
	0.7562,     0.7825,     1.0458,     0.7223,
	0.6200,     0.8126,     0.9220,     0.7582,
	0.5043,     0.8434,     0.8205,     0.7957,
	0.4410,     0.8693,     0.7305,     0.8279,
	0.4000,     0.8910,     0.6502,     0.8553,
	0.3750,     0.9092,     0.5950,     0.8757,
	0.3500,     0.9222,     0.5545,     0.8903,
	0.3295,     0.9319,     0.5333,     0.8997,
	0.3065,     0.9403,     0.5189,     0.9073,
	0.2835,     0.9477,     0.5181,     0.9122,
	0.2610,     0.9544,     0.5176,     0.9171,
	0.2480,     0.9602,     0.5172,     0.9217,
	0.2327,     0.9653,     0.5119,     0.9267
};

rom const float buhlmann_ht[2*16] = {
// Compartiment half-life, in minute
//-- N2 ---- He ---------------------------------------------------------------------
	  4.0,    1.51,
	  8.0,    3.02,
	 12.5,    4.72,
	 18.5,    6.99,
	 27.0,   10.21,
	 38.3,   14.48,
	 54.3,   20.53,
	 77.0,   29.11,
	109.0,   41.20,
	146.0,   55.19,
	187.0,   70.69,
	239.0,   90.34,
	305.0,  115.29,
	390.0,  147.42,
	498.0,  188.24,
	635.0,  240.03
};

rom const float e2secs[2*16] = {
// result of  1 - 2^(-1/(30sec*HT))
//---- N2 ------------- He ------------
	5.75958E-03,    1.51848E-02,  
	2.88395E-03,    7.62144E-03,
	1.84669E-03,    4.88315E-03,
    1.24813E-03,    3.29997E-03,
    8.55371E-04,    2.26041E-03,
    6.03079E-04,    1.59437E-03,
    4.25414E-04,    1.12479E-03,
    3.00019E-04,    7.93395E-04,
    2.11949E-04,    5.60641E-04,
    1.58240E-04,    4.18555E-04,
    1.23548E-04,    3.26795E-04,
    9.66686E-05,    2.55722E-04,
    7.57509E-05,    2.00387E-04,
    5.92416E-05,    1.56716E-04,
    4.63943E-05,    1.22734E-04,
    3.63850E-05,    9.62538E-05
//-------------------------------------
};

rom const float e1min[2*16] = {
// Integration constant for 1 minute,
// Ie. 1- 2^(-1/HT)
//----- N2 --------- e 1min He --------
	1.59104E-01,    3.68109E-01,  	
    8.29960E-02,   	2.05084E-01,     
    5.39424E-02,    1.36579E-01,
    3.67742E-02,    9.44046E-02,
    2.53454E-02,    6.56359E-02,
    1.79351E-02,    4.67416E-02,
    1.26840E-02,    3.31991E-02,
    8.96152E-03,    2.35301E-02,
    6.33897E-03,    1.66832E-02,
    4.73633E-03,    1.24808E-02,
    3.69981E-03,    9.75753E-03,
    2.89600E-03,    7.64329E-03,
    2.27003E-03,    5.99417E-03,
    1.77572E-03,    4.69082E-03,
    1.39089E-03,    3.67548E-03,
    1.09097E-03,    2.88359E-03
//-------------------------------------
};

rom const float e10min[2*16] = {
// The 10 min Value in float notation:
//  result of 1 - 2^(-10/ht)
//---- N2 -------------- He -----------
	8.23223E-01,    9.89851E-01,  
	5.79552E-01,  	8.99258E-01,
    4.25651E-01,    7.69737E-01,
    3.12487E-01,    6.29027E-01,
    2.26416E-01,    4.92821E-01,
    1.65547E-01,    3.80407E-01,
    1.19840E-01,    2.86538E-01,
    8.60863E-02,    2.11886E-01,
    6.16117E-02,    1.54849E-01,
    4.63665E-02,    1.18026E-01,
    3.63881E-02,    9.34005E-02,
    2.85855E-02,    7.38569E-02,
    2.24698E-02,    5.83504E-02,
    1.76160E-02,    4.59303E-02,
    1.38222E-02,    3.61528E-02,
    1.08563E-02,    2.84646E-02
//-------------------------------------
};

//////////////////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////////////////
////////////////////////////// THE SUBROUTINES ///////////////////////////////
//////////////////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////////////////
//
// all new in v.102
// moved from 0x0D000 to 0x0C000 in v.108
#ifndef UNIX
#   pragma code p2_deco = 0x0C000
#endif

//////////////////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////////////////
///////////////////////  U T I L I T I E S   /////////////////////////////////
//////////////////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////////////////

//////////////////////////////////////////////////////////////////////////////
// Bump to blue-screen when an assert is wrong
#ifdef __DEBUG
void assert_failed(PARAMETER short int line)
{
}
#endif

//////////////////////////////////////////////////////////////////////////////
// When calling C code from ASM context, the data stack pointer and
// frames should be reset. Bank8 is used by stack

#ifdef CROSS_COMPILE
#       define RESET_C_STACK
#else
#   ifdef __DEBUG
#       define RESET_C_STACK fillDataStack();
        void fillDataStack(void)
        {
            _asm
                LFSR    1,C_STACK
                MOVLW   0xCC
        loop:   MOVWF   POSTINC1,0
                TSTFSZ  FSR1L,0
                BRA     loop

                LFSR    1,C_STACK
                LFSR    2,C_STACK
            _endasm
        }
#   else
#       define RESET_C_STACK    \
        _asm                    \
            LFSR    1, C_STACK  \
            LFSR    2, C_STACK  \
        _endasm
#   endif
#endif

//////////////////////////////////////////////////////////////////////////////
// Fast subroutine to read timer 5.
// Note: result is in 1/32 of msecs (30,51757813 us/bit to be precise)
static unsigned short tmr5(void)
{
#ifndef CROSS_COMPILE
    _asm
        movff   0xf7c,PRODL     // TMR5L
        movff   0xf7d,PRODH     // TMR5H
    _endasm                     // result in PRODH:PRODL.
#else
    return 0;
#endif
}


//////////////////////////////////////////////////////////////////////////////
// read buhlmann tables A and B for compatriment ci
//
static void read_buhlmann_coefficients(void)
{
#ifndef CROSS_COMPILE
    // Note: we don't use far rom pointer, because the
    //       24 bits is too complex, hence we have to set
    //       the UPPER page ourself...
    //       --> Set zero if tables are moved to lower pages !
    _asm
        movlw 1
        movwf TBLPTRU,0
    _endasm
#endif

    assert( ci < NUM_COMP );

    // Use an interleaved array (AoS) to access coefficients with a
    // single addressing.
    {
        overlay rom const float* ptr = &buhlmann_ab[4*ci];
        var_N2_a = *ptr++;
        var_N2_b = *ptr++;
        var_He_a = *ptr++;
        var_He_b = *ptr++;
    }
}

//////////////////////////////////////////////////////////////////////////////
// read buhlmann tables for compatriment ci
// If period == 0 : 2sec interval
//              1 : 1 min interval
//              2 : 10 min interval.
static void read_buhlmann_times(PARAMETER char period)
{
#ifndef CROSS_COMPILE
    // Note: we don't use far rom pointer, because the
    //       24 bits is to complex, hence we have to set
    //       the UPPER page ourself...
    //       --> Set zero if tables are moved to lower pages !
    _asm
        movlw 1
        movwf TBLPTRU,0
    _endasm
#endif

    assert( ci < NUM_COMP );

    // Integration intervals.
    switch(period)
    {
    case 0: //---- 2 sec -----------------------------------------------------
        {
            overlay rom const float* ptr = &e2secs[2*ci];
            var_N2_e = *ptr++;
            var_He_e = *ptr++;
        }
        break;

    case 1: //---- 1 min -----------------------------------------------------
       {
            overlay rom const float* ptr = &e1min[2*ci];
            var_N2_e = *ptr++;
            var_He_e = *ptr++;
        }
        break;

    case 2: //---- 10 min ----------------------------------------------------
        {
            overlay rom const float* ptr = &e10min[2*ci];
            var_N2_e = *ptr++;
            var_He_e = *ptr++;
        }
        break;

    default:
        assert(0);  // Never go there...
    }
}

//////////////////////////////////////////////////////////////////////////////
// read buhlmann tables for compatriment ci
//
static void read_buhlmann_ht(void)
{

#ifndef CROSS_COMPILE
    // Note: we don't use far rom pointer, because the
    //       24 bits is to complex, hence we have to set
    //       the UPPER page ourself...
    //       --> Set zero if tables are moved to lower pages !
    _asm
        movlw 1
        movwf TBLPTRU,0
    _endasm
#endif

    assert( ci < NUM_COMP );
    {
        overlay rom const float* ptr = &buhlmann_ht[2*ci];
        var_N2_ht = *ptr++;
        var_He_ht = *ptr++;
    }

    assert( 4.0    <= var_N2_ht && var_N2_ht <= 635.0 );
    assert( 1.5099 <= var_He_ht && var_He_ht <= 240.03 );
}

//////////////////////////////////////////////////////////////////////////////
// calc_nextdecodepth
//
// new in v.102
//
// INPUT, changing during dive:
//      temp_deco
//      low_depth
//
// INPUT, fixed during dive:
//      pres_surface
//      GF_delta
//      GF_high
//      GF_low
//      char_I_depth_last_deco
//      float_deco_distance
//
// RETURN TRUE iff a stop is needed.
//
// OUTPUT
//      locked_GF_step
//      temp_depth_limt
//      low_depth
//
static unsigned char calc_nextdecodepth(void)
{
    //--- Max ascent speed ---------------------------------------------------
    // Recompute leading gas limit, at current depth:
    overlay float depth = (temp_deco - pres_surface) * BAR_TO_METER;

    // At most, ascent 1 minute, at 10m/min == 10.0 m.
    overlay float min_depth = (depth > 10.0) ? (depth - 10.0) : 0.0;

    // Do we need to stop at current depth ?
    overlay unsigned char need_stop = 0;

    assert( depth >= -0.2 );        // Allow for 200mbar of weather change.

    //---- ZH-L16 + GRADIENT FACTOR model ------------------------------------
    if( char_I_deco_model != 0 )
    {
        overlay unsigned char first_stop = 0;
        overlay float p;

        sim_limit( GF_low );
        p = sim_lead_tissue_limit - pres_surface;
        if( p <= 0.0f )
            goto no_deco_stop;          // We can surface directly...

        p *= BAR_TO_METER;

        // Store the deepest point needing a deco stop as the LOW reference for GF.
        // NOTE: following stops will be validated using this LOW-HIGH gf scale,
        //       so if we want to keep coherency, we should not validate this stop
        //       yet, but apply the search to it, as for all the following stops afterward.
        if( p > low_depth )
        {
            low_depth = p;
            locked_GF_step = GF_delta / low_depth;
        }

        if( p < min_depth )
            goto no_deco_stop;          // First stop is higher than 1' ascent.

        // Round to multiple of 3m.
        first_stop = 3 * (short)(0.9995f + p*0.333333f);
        assert( first_stop < 128 );

        // Apply correction for the shallowest stop.
        if( first_stop == 3 )                           // new in v104
            first_stop = char_I_depth_last_deco;        // Use last 3m..6m instead.

        // We have a stop candidate.
        // But maybe ascending to the next stop will diminish the constraint,
        // because the GF might decrease more than the preassure gradient...
        while(first_stop > 0)
        {
            overlay unsigned char next_stop;            // Next depth (0..90m)

            // Check max speed, or reaching surface.
            if( first_stop <= min_depth )
                goto no_deco_stop;

            if( first_stop <= char_I_depth_last_deco )  // new in v104
                next_stop = 0;
            else if( first_stop == 6 )
                next_stop = char_I_depth_last_deco;
            else
                next_stop = first_stop - 3;             // Index of next (upper) stop.

            // Total preassure at the new stop candidate:
            p = next_stop * METER_TO_BAR
              + pres_surface;

            // Recompute limit for this new stop:
            if( !low_depth || next_stop > low_depth )
                sim_limit( GF_low );
            else
                sim_limit( GF_high - next_stop * locked_GF_step );

            // Check upper limit (lowest ambiant pressure tolerated):
            if( sim_lead_tissue_limit >= p )
                goto deco_stop_found;       // Ascent to next_stop forbiden.

            // Else, validate that stop and loop...
            first_stop = next_stop;
        }

no_deco_stop:
        temp_depth_limit = min_depth;
        goto done;

deco_stop_found:
        // next stop is the last validated depth found, aka first_stop
        need_stop = 1;                  // Hit.
        temp_depth_limit = first_stop;  // Stop depth, in meter.

done:
        ;
    }
    else //---- ZH-L16 model -------------------------------------------------
    {
        overlay float pres_gradient;

        // Original model
        // optimized in v.101
        // char_I_depth_last_deco included in v.101

        // Compute sim_lead_tissue_limit too, but just once.
        sim_limit(1.0);

        pres_gradient = sim_lead_tissue_limit - pres_surface;
        if (pres_gradient >= 0)
        {
            pres_gradient *= BAR_TO_METER/3;                        // bar --> stop number;
            temp_depth_limit = 3 * (short) (pres_gradient + 0.99);  // --> metre : depth for deco
            need_stop = 1;                                          // Hit.

            // Implement last stop at 4m/5m/6m...
            if( temp_depth_limit == 3 )
                temp_depth_limit = char_I_depth_last_deco;
        }
        else
            temp_depth_limit = 0;
    }

    //---- Check gas change --------------------------------------------------
    need_stop |= gas_switch_deepest();  // Update temp_depth_limit if there is a change,

    return need_stop;
}

//////////////////////////////////////////////////////////////////////////////
// copy_deco_table
//
// Buffer the stops, once computed, so we can continue to display them
// while computing the next set.
//
static void copy_deco_table(void)
{
    // Copy depth of the first (deepest) stop, because when reversing
    // order, it will be hard to find...
    char_O_first_deco_depth = internal_deco_depth[0];
    char_O_first_deco_time  = internal_deco_time [0];

    {
        overlay unsigned char x, y;

        for(x=0; x<NUM_STOPS; x++)
        {
            char_O_deco_depth[x] = internal_deco_depth[x];
            char_O_deco_time [x] = internal_deco_time [x];
            char_O_deco_gas  [x] = internal_deco_gas  [x];
        }

        //Now fill the char_O_deco_time_for_log array
        //---- First: search the first non-null depth
        for(x=(NUM_STOPS-1); x != 0; --x)
            if( internal_deco_depth[x] != 0 ) break;

        //---- Second: copy to output table (in reverse order)
        for(y=0; y<NUM_STOPS; y++, --x)
        {
            char_O_deco_time_for_log[y] = internal_deco_time [x];

            // Stop only once the last transfer is done.
            if( x == 0 ) break;
        }

        //---- Third: fill table end with null
        for(y++; y<NUM_STOPS; y++)
        {
            char_O_deco_time_for_log [y] = 0;
        }
    }
}

//////////////////////////////////////////////////////////////////////////////
// temp_tissue_safety //
//
// outsourced in v.102
//
// Apply safety factors for both ZH-L16 models.
//
static void temp_tissue_safety(void)
{
    assert( 0.0 <  float_desaturation_multiplier && float_desaturation_multiplier <= 1.0 );
    assert( 1.0 <= float_saturation_multiplier   && float_saturation_multiplier   <= 2.0 );

        if( temp_tissue < 0.0 )
            temp_tissue *= float_desaturation_multiplier;
        else
            temp_tissue *= float_saturation_multiplier;
}

//////////////////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////////////////
// ** THE JUMP-IN CODE **
// ** for the asm code **
//////////////////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////////////////

//////////////////////////////////////////////////////////////////////////////
// Called every 2 seconds during diving.
// update tissues every time.
//
// Every 6 seconds (or slower when TTS > 16):
//    - update deco table (char_O_deco_time/depth) with new values.
//    - update ascent time,
//    - set status to zero (so we can check there is new results).
//
void deco_calc_hauptroutine(void)
{
    RESET_C_STACK
    calc_hauptroutine();
    int_O_desaturation_time = 65535;
}

//////////////////////////////////////////////////////////////////////////////
// Reset decompression model:
// + Set all tissues to equilibrium with Air at ambient pressure.
// + Reset last stop to 0m
// + Reset all model output.
void deco_clear_tissue(void)
{
    RESET_C_STACK
    clear_tissue();
}

//////////////////////////////////////////////////////////////////////////////
// Called every 1 min during decoplanning.
// Update tissues for 1 min.
//
void deco_calc_tissue(void)
{
    RESET_C_STACK
    calc_hauptroutine_update_tissues();
}

//////////////////////////////////////////////////////////////////////////////

void deco_calc_wo_deco_step_1_min(void)
{
    RESET_C_STACK
    calc_wo_deco_step_1_min();
    deco_calc_desaturation_time();
}

//////////////////////////////////////////////////////////////////////////////

void deco_calc_dive_interval(void)
{
    RESET_C_STACK
    calc_dive_interval();
}

//////////////////////////////////////////////////////////////////////////////
// Find current gas in the list (if any).
//
// Input:  char_I_current_gas = 1..6
//
// Output: sim_gas_last_depth = 0..5, temp_depth_limit.
//
static void gas_switch_find_current(void)
{
    assert( 0 < char_I_current_gas && char_I_current_gas <= (2*NUM_GAS) );

    if( char_I_current_gas <= NUM_GAS )                 // Gas1..Gas5
    {
        sim_gas_last_used  = char_I_current_gas;

        // Note: if current is first gas, we must find it, but not set
        //       last depth change to surface.
        if( char_I_deco_gas_change[sim_gas_last_used-1] )
            sim_gas_last_depth = char_I_deco_gas_change[sim_gas_last_used-1];
    }
    else
        sim_gas_last_used = 0;                          // Gas 6 = manual set
}

//////////////////////////////////////////////////////////////////////////////
// Find deepest available gas.
//
// Input:  temp_depth_limit,
//         deco_gas_change[]
//         sim_gas_depth_used, sim_dive_mins.
//
// RETURNS TRUE if a stop is needed for gas switch.
//
// Output: temp_depth_limit, sim_gas_depth_used IFF the is a switch.
//
// NOTE: might be called from bottom (when sim_gas_delay and sim_gas_depth_used
//       are null), or during the ascent to make sure we are not passing a
//       stop (in which case both can be already set).
//
static unsigned char gas_switch_deepest(void)
{
    overlay unsigned char switch_deco = 0, switch_last = 0;

    if (char_I_const_ppO2 == 0)
    {
        overlay unsigned char j;

        // Loop over all enabled gas, to find the deepest one,
        // above last used gas, but below temp_depth_limit.
        for(j=0; j<NUM_GAS; ++j)
        {
            // Gas not (yet) allowed ? Skip !
            if( temp_depth_limit > deco_gas_change[j] )
                continue;

            // Gas deeper (or equal) than the current one ? Skip !
            if( sim_gas_last_depth && deco_gas_change[j] >= sim_gas_last_depth )
                continue;

            // First, or deeper ?
            if( switch_deco < deco_gas_change[j] )
            {
                switch_deco = deco_gas_change[j];
                switch_last = j+1;  // 1..5
            }
        }
    }

    // If there is a better gas available
    if( switch_deco )
    {
        assert( !sim_gas_last_depth || sim_gas_last_depth > switch_deco );

        sim_gas_last_depth = switch_deco;
        sim_gas_last_used  = switch_last;
    }
    return 0;
}

//////////////////////////////////////////////////////////////////////////////
// Calculate gas switches
//
//
// Input:  N2_ratio, He_ratio.
//         sim_gas_last_used
//
// Output: calc_N2_ratio, calc_He_ratio
//
static void gas_switch_set(void)
{
    assert( sim_gas_last_used <= NUM_GAS );

    if( sim_gas_last_used == 0 )    // Gas6 = manualy set gas.
    {
        calc_N2_ratio = N2_ratio;
        calc_He_ratio = He_ratio;
    }
    else
    {
        calc_N2_ratio = char_I_deco_N2_ratio[sim_gas_last_used-1] * 0.01;
        calc_He_ratio = char_I_deco_He_ratio[sim_gas_last_used-1] * 0.01;
    }

    assert( 0.0 <= calc_N2_ratio && calc_N2_ratio <= 0.95 );
    assert( 0.0 <= calc_He_ratio && calc_He_ratio <= 1.00 );
    assert( (calc_N2_ratio + calc_He_ratio) <= 1.00 );
}

//////////////////////////////////////////////////////////////////////////////
//
// Input: calc_N2_ratio, calc_He_ratio : simulated gas mix.
//        temp_deco : simulated respiration pressure
//        float_deco_distance : security factor.
//        Water-vapor pressure inside limbs (ppWater).
//
// Output: ppN2, ppHe.
//
static void sim_alveolar_presures(void)
{
    overlay float deco_diluent = temp_deco;                 // new in v.101

    // Take deco offset into account, but not at surface.
    // Note: this should be done on ambiant pressure, hence before
    //       computing the diluant partial pressure...
    if( deco_diluent > pres_surface )
        deco_diluent += float_deco_distance;

    //---- CCR mode : deco gas switch ? --------------------------------------
    if( char_I_const_ppO2 != 0 )
    {
        // In CCR mode, use calc_XX_ratio instead of XX_ratio.
        // Note: PPO2 and ratios are known outside the lumbs, so there is no
        //       ppWater in the equations below:
        deco_diluent -= const_ppO2;
        deco_diluent /= calc_N2_ratio + calc_He_ratio;// potential DIV/0 issue when O2 is used as diluent!
        
        if(calc_N2_ratio==0&calc_He_ratio==0) deco_diluent = 0.0; // workaround for potential DIV/0 issue
    }

    if( deco_diluent > ppWater )
    {
        ppN2 = calc_N2_ratio * (deco_diluent - ppWater);
        ppHe = calc_He_ratio * (deco_diluent - ppWater);
    }
    else
    {
        ppN2 = 0.0;
        ppHe = 0.0;
    }
    assert( 0.0 <= ppN2 && ppN2 < 14.0 );
    assert( 0.0 <= ppHe && ppHe < 14.0 );
}

//////////////////////////////////////////////////////////////////////////////
// clear_tissue
//
// optimized in v.101 (var_N2_a)
//
// preload tissues with standard pressure for the given ambient pressure.
// Note: fixed N2_ratio for standard air.
//
static void clear_tissue(void)
{
    overlay float p;

    // Kludge: the 0.0002 of 0.7902 are missing with standard air.
    N2_ratio = 0.7902;
    pres_respiration = int_I_pres_respiration * 0.001;

    p = N2_ratio * (pres_respiration -  ppWater);
    for(ci=0; ci<NUM_COMP; ci++)
    {
        // cycle through the 16 Buhlmann N2 tissues
        pres_tissue_N2[ci] = p;

        // cycle through the 16 Buhlmann tissues for Helium
        pres_tissue_He[ci] = 0.0;
    }

    clear_deco_table();
    char_O_deco_status = 0;
    char_O_nullzeit = 0;
    int_O_ascenttime = 0;
    char_O_gradient_factor = 0;

    calc_lead_tissue_limit = 0.0;
    char_O_gtissue_no = 0;
}

//////////////////////////////////////////////////////////////////////////////
// calc_hauptroutine
//
// this is the major code in dive mode calculates:
// 		the tissues,
//		the bottom time,
//		and simulates the ascend with all deco stops.
//
// The deco_state sequence is :
//       3 (at surface)
// +---> 0 : calc nullzeit
// |     2 : simulate ascent to first stop (at 10m/min, less that 16x 1min simu)
// | +-> 1 : simulate up to 16min of stops.
// | +------< not finished
// +--------< finish
//
// Added steps 6,5 for @+5 calculation:
//      6 = ascent to first stop (same as 2), except continue to 7
//      7 = same as 1, except loop to 7.
//
static void calc_hauptroutine(void)
{
    static unsigned char backup_gas_used  = 0;
    static unsigned char backup_gas_depth = 0;

    calc_hauptroutine_data_input();

    calc_hauptroutine_update_tissues();
    calc_gradient_factor();

    // toggle between calculation for nullzeit (bottom time),
    //                deco stops
    //                and more deco stops (continue)
    switch( char_O_deco_status )
    {
    case 3: //---- At surface: start a new dive ------------------------------
        clear_deco_table();
        copy_deco_table();
        int_O_ascenttime = 0;       // Reset DTR.
        int_O_extra_ascenttime = 0;
        char_O_nullzeit = 0;        // Reset bottom time.
        char_O_deco_status = 0;     // Calc bottom-time/nullzeit next iteration.

        // Values that should be reset just once for the full real dive.
        // This is used to record the lowest stop for the whole dive,
        // Including ACCROSS all simulated ascent.
        low_depth = 0.0;
        locked_GF_step = 0.0;

        // Reset gas switch history.
        backup_gas_used  = sim_gas_last_used  = 0;
        backup_gas_depth = sim_gas_last_depth = 0;
        sim_dive_mins = 0;
        break;

    case 0: //---- bottom time -----------------------------------------------
    default:
        gas_switch_find_current();              // Lookup for current gas & time.
        gas_switch_set();                       // setup calc_ratio's

        calc_nullzeit();
        if( char_O_nullzeit > 0 )               // Some NDL time left ?
        {
            char_O_deco_status = 0;             // YES: recalc ndl next time.
            clear_deco_table();                 // Also clear stops !
            copy_deco_table();
            char_O_deco_last_stop = 0;          // And last stop (OSTC menu anim)
        }
        else
            char_O_deco_status = 2;             // NO: calc ascent next time.
        break;

    case 2: //---- Simulate ascent to first stop -----------------------------
    case 6: // @+5min variation
        // Check proposed gas at begin of ascent simulation
        sim_dive_mins = int_I_divemins;         // Init current time.

        gas_switch_find_current();              // Lookup for current gas & time.
        gas_switch_set();                       // setup calc_ratio's

        backup_gas_used  = sim_gas_last_used;   // And save for later simu steps.
        backup_gas_depth = sim_gas_last_depth;  // And save for later simu steps.

        sim_ascent_to_first_stop();

        // Calc stops next time (deco or gas switch).
        char_O_deco_status = 1 | ( char_O_deco_status & 4 );
        break;

    case 1: //---- Simulate stops --------------------------------------------
    case 5: // @+5 variation.
        calc_hauptroutine_calc_deco();

        // If simulation is finished, restore the GF low reference, so that
        // next ascent simulation is done from the current depth:
        if( (char_O_deco_status & 3) == 0 )
        {
            sim_gas_last_used  = backup_gas_used;
            sim_gas_last_depth = backup_gas_depth;
        }
        break;
    }
}

//////////////////////////////////////////////////////////////////////////////
// calc_hauptroutine_data_input
//
// Reset all C-code dive parameters from their ASM-code values.
// Detect gas change condition.
//
void calc_hauptroutine_data_input(void)
{
    overlay short int_temp;
    overlay unsigned char g;

    pres_respiration    = int_I_pres_respiration * 0.001;
    pres_surface        = int_I_pres_surface     * 0.001;
    N2_ratio            = char_I_N2_ratio        * 0.01;
    He_ratio            = char_I_He_ratio        * 0.01;
    float_deco_distance = char_I_deco_distance   * 0.01;     // Get offset in mbar

    // ____________________________________________________
    //
    // _____________ G A S _ C H A N G E S ________________
    // ____________________________________________________

    // Keep a margin of 150mbar = 1.50m
    int_temp = (int_I_pres_respiration - int_I_pres_surface)
             + MBAR_REACH_GASCHANGE_AUTO_CHANGE_OFF;

    // Gas are selectable if we did not pass the change depth by more than 1.50m:
    for(g=0; g < NUM_GAS; ++g)
    {
        deco_gas_change[g] = 0;
        if(char_I_deco_gas_change[g])
            if( int_temp > 100 *(short)char_I_deco_gas_change[g] )
                deco_gas_change[g] = char_I_deco_gas_change[g];
    }

    const_ppO2 = char_I_const_ppO2 * 0.01;
    float_desaturation_multiplier = char_I_desaturation_multiplier * 0.01;
    float_saturation_multiplier   = char_I_saturation_multiplier   * 0.01;
    GF_low   = char_I_GF_Low_percentage  * 0.01;
    GF_high  = char_I_GF_High_percentage * 0.01;
    GF_delta = GF_high - GF_low;
}

//////////////////////////////////////////////////////////////////////////////
//
//
void calc_hauptroutine_update_tissues(void)
{
    assert( 0.00 <= N2_ratio && N2_ratio <= 1.00 );
    assert( 0.00 <= He_ratio && He_ratio <= 1.00 );
    assert( (N2_ratio + He_ratio) <= 1.00 );
    assert( 0.800 < pres_respiration && pres_respiration < 14.0 );

    pres_diluent = pres_respiration;
    if( char_I_const_ppO2 != 0 )
    {
        overlay float flush_ppO2 = pres_respiration * (1.0 - N2_ratio - He_ratio);

        pres_diluent -= const_ppO2;
        pres_diluent /= N2_ratio + He_ratio;            // potential DIV/0 issue when O2 is used as diluent!
        if( pres_diluent < 0.0 )
            pres_diluent = 0.0;
        if(N2_ratio==0&He_ratio==0) pres_diluent = 0.0; // workaround for potential DIV/0 issue

        char_O_diluent = (unsigned char)(pres_diluent/pres_respiration*100.0 + 0.5);

        if( flush_ppO2 > 2.545) flush_ppO2 = 2.55;
        if( flush_ppO2 < 0.0  ) flush_ppO2 = 0.0;
        char_O_flush_ppO2 = (unsigned char)(flush_ppO2*100.0 + 0.5);
    }

    if( pres_diluent > ppWater )
    {
        overlay float EAD, END;

        ppN2 = N2_ratio * (pres_diluent - ppWater);
        ppHe = He_ratio * (pres_diluent - ppWater);

        // EAD : Equivalent Air Dive. Equivalent depth for the same N2 level
        //       with plain air.
        //       ppN2 = 79% * (P_EAD - ppWater)
        //       EAD = (P_EAD - Psurface) * 10
        //   ie: EAD = (ppN2 / 0.7902 + ppWater -Psurface) * 10
        EAD = (ppN2 / 0.7902 + ppWater - pres_surface) * BAR_TO_METER;
        if( EAD < 0.0 || EAD > 245.5 ) EAD = 0.0;
        char_O_EAD = (unsigned char)(EAD + 0.5);

        // END : Equivalent Narcotic Dive.
        //       Here we count O2 as narcotic too. Hence everything but helium (has a narcosis factor of
        //       0.23 btw). Hence the formula becomes:
        //       END * BarPerMeter * (1.0 - 0.0) - ppWater + Psurface == Pambient - ppHe - ppWater
        //  ie:  END = (Pambient - ppHe - Psurface) * BAR_TO_METER
        //
        // Source cited:
        //       The Physiology and Medicine of Diving by Peter Bennett and David Elliott,
        //       4th edition, 1993, W.B.Saunders Company Ltd, London.
        END = (pres_respiration - ppHe - pres_surface) * BAR_TO_METER;
        if( END < 0.0 || END > 245.5 ) END = 0.0;
        char_O_END = (unsigned char)(END  + 0.5);
    }
    else																		// new in v.101
    {
        ppN2 = 0.0;
        ppHe = 0.0;
        char_O_EAD = char_O_END = 0;
    }

    if(!char_I_step_is_1min)
        calc_tissue(0);
    else
        calc_tissue(1);

    // Calc limit for surface, ie. GF_high.
    calc_limit();

    // Fill int_O_ceiling if ceiling is below the surface
    if ((calc_lead_tissue_limit-pres_surface)>0)
        int_O_ceiling = (short)((calc_lead_tissue_limit-pres_surface)*1000);
    else
        int_O_ceiling = 0;

    int_O_gtissue_press = (short)((pres_tissue_N2[char_O_gtissue_no] + pres_tissue_He[char_O_gtissue_no]) * 1000);
}


//////////////////////////////////////////////////////////////////////////////
// Compute stops.
//
// Note: because this can be very long, break on 16 iterations, and set state
//       to 0 when finished, or to 1 when needing to continue.
// Note: because each iteration might be very long too (~ 66 ms in 1.84beta),
//       break the loop when total time > 512msec.
//
void calc_hauptroutine_calc_deco(void)
{
    overlay unsigned char loop;

    for(loop = 0; loop < 16; ++loop)
    {
        // Limit loops to 512ms, using timer 5:
        if( tmr5() & (512*32) )
            break;

            if( calc_nextdecodepth() )
            {
                if( temp_depth_limit == 0 )
                    goto Surface;

                //---- We hit a stop at temp_depth_limit ---------------------
                temp_deco = temp_depth_limit * METER_TO_BAR // Convert to relative bar,
                          + pres_surface;                   // To absolute.
                if( !update_deco_table() )                  // Adds a one minute stops.
                    goto Surface;                           // Deco table full: abort...
            }
            else
            {
                //---- No stop -----------------------------------------------
                temp_deco -= (10*METER_TO_BAR);             // Ascend 10m, no wait.

                //---- Finish computations once surface is reached -----------
                if( temp_deco <= pres_surface )
                {
Surface:
                    if( char_O_deco_status == 1 )   // Don't in @+5min variant.
                        copy_deco_table();

                    calc_ascenttime();
                    char_O_deco_status = 0;         // calc nullzeit next time.
                    char_O_deco_last_stop = 0;      // Surface reached (to animate menu)
                    return;
                }
            }
        //---- Then update tissue --------------------------------------------
        sim_dive_mins++;            // Advance simulated time by 1 minute.
        gas_switch_set();           // Apply any simulated gas change, once validated.
        sim_alveolar_presures();    // Updates ppN2 and ppHe.
        sim_tissue(1);              // Simulate compartiments for 1 minute.
    }

    // Surface not reached, need more stops... for menu animation.
    char_O_deco_last_stop = temp_depth_limit;   // Reached depth.
}


//////////////////////////////////////////////////////////////////////////////
// Simulation ascention to first deco stop.
//
// Note: because we ascent with a constant speed (10m/mn, ie. 1bar/mn),
//       there is no need to break on more that 16 iterations
//       (or we are already in deep shit).
//
// Input:  pres_respiration
// Output: temp_deco
//
// if char_O_deco_status indicate @+5 variant, add extra time at current depth,
// before ascent.
void sim_ascent_to_first_stop(void)
{
    overlay unsigned char fast = 1; // 1min or 2sec steps.

    update_startvalues();
    clear_deco_table();

    temp_deco = pres_respiration;       // Starts from current real depth.

    // Are we doing the special @+5min variation ?
    if(char_O_deco_status & 4)
        sim_extra_time();

    //---- Loop until first stop, gas switch, or surface is reached ----------
    for(;;)
    {
        overlay float old_deco = temp_deco;     // Pamb backup (bars)

        // Try ascending 1 full minute (fast) or 2sec (!fast):
        if( fast )
            temp_deco -= 10*METER_TO_BAR;   // 1 min, at 10m/min. ~ 1bar.
        else
            temp_deco -= (10.0/30.0)*METER_TO_BAR;  // 2sec at 10m/min.

        if( temp_deco < pres_surface )  // But don't go over surface.
            temp_deco = pres_surface;

        // Recompute sim_lead_tissue_limit at GF_low (deepest stop), because
        // one minute passed.
        sim_limit(GF_low);

        // Did we reach deepest remaining stop ?
        if( temp_deco < sim_lead_tissue_limit )
        {
            temp_deco = old_deco;           // Restore last correct depth,

            if( fast )
            {
                fast = 0;                   // Retry with 2sec steps.
                continue;
            }
            else
                break;                      // Done...
        }

        // Did we reach surface ?
        // NOTE: we should round BEFORE checking surface is reached.
        temp_depth_limit = (unsigned char)(0.5 + (temp_deco - pres_surface) * BAR_TO_METER);
        if( temp_depth_limit == 0 )
        {
            temp_deco = pres_surface;   // Yes: finished !
            break;
        }

        // Check for gas change below new depth ?
        if( gas_switch_deepest() )
        {
            assert( temp_depth_limit > 0);

            temp_deco = temp_depth_limit * METER_TO_BAR + pres_surface;
            break;
        }

        if( fast )
            sim_dive_mins++;            // Advance simulated time by 1 minute.
        sim_alveolar_presures();        // temp_deco --> ppN2/ppHe
        sim_tissue(fast);               // and update tissues for 1 min.
    }
}

//////////////////////////////////////////////////////////////////////////////
// Simulation extra time at the current depth.
//
// This routine is used for @+5min feature.
void sim_extra_time(void)
{
    overlay unsigned char extra = char_I_extra_time;
    do {
        sim_dive_mins++;                // Advance simulated time by 1 minute.
        sim_tissue(1);                  // and update tissues for 1 min.
    } while( --extra != 0 );
}

//////////////////////////////////////////////////////////////////////////////
// calc_tissue
//
// optimized in v.101
//
static void calc_tissue(PARAMETER unsigned char period)
{
    assert( 0.00 <= ppN2 && ppN2 < 11.2 );  // 80% N2 at 130m
    assert( 0.00 <= ppHe && ppHe < 12.6 );  // 90% He at 130m

    for (ci=0;ci<NUM_COMP;ci++)
    {
        read_buhlmann_times(period);        // 2 sec or 1 min period.

        // N2
        temp_tissue = (ppN2 - pres_tissue_N2[ci]) * var_N2_e;
        temp_tissue_safety();
        pres_tissue_N2[ci] += temp_tissue;

        // He
        temp_tissue = (ppHe - pres_tissue_He[ci]) * var_He_e;
        temp_tissue_safety();
        pres_tissue_He[ci] += temp_tissue;
    }
}

//////////////////////////////////////////////////////////////////////////////
// calc_limit
//
// New in v.111 : separated from calc_tissue(), and depends on GF value.
//
static void calc_limit(void)
{
    char_O_gtissue_no = 255;
    calc_lead_tissue_limit = 0.0;

    for(ci=0; ci<NUM_COMP;ci++)
    {
        overlay float N2 = pres_tissue_N2[ci];
        overlay float He = pres_tissue_He[ci];
        overlay float p = N2 + He;

        read_buhlmann_coefficients();
        var_N2_a = (var_N2_a * N2 + var_He_a * He) / p;
        var_N2_b = (var_N2_b * N2 + var_He_b * He) / p;

        // Apply the Eric Baker's varying gradient factor correction.
        // Note: the correction factor depends both on GF and b,
        //       Actual values are in the 1.5 .. 1.0 range (for a GF=30%),
        //       so that can change who is the leading gas...
        // Note: Also depends of the GF. So the calcul is different for
        //       GF_low, current GF, or GF_high...
        //       *BUT* calc_tissue() is used to compute bottom time,
        //       hence what would happend at surface,
        //       hence at GF_high.
        if( char_I_deco_model != 0 )
            p = ( p - var_N2_a * GF_high) * var_N2_b
              / (GF_high + var_N2_b * (1.0 - GF_high));
        else
            p = (p - var_N2_a) * var_N2_b;
        if( p < 0.0 ) p = 0.0;

        if( p > calc_lead_tissue_limit )
        {
            char_O_gtissue_no = ci;
            calc_lead_tissue_limit = p;
        }
    }

    assert( char_O_gtissue_no < NUM_COMP );
    assert( 0.0 <= calc_lead_tissue_limit && calc_lead_tissue_limit <= 14.0);
}

//////////////////////////////////////////////////////////////////////////////
// calc_nullzeit
//
// calculates the remaining bottom time
//
// NOTE: Erik Baker's closed formula works for Nitroxes. Trimix adds a second
//       exponential term to the M-value equation, making it impossible to
//       invert... So we have to make a fast-simu until we find a better way.
//
// Input:  pres_respiration
// Output: char_O_nullzeit
//
static void calc_nullzeit(void)
{
    //---- Compute ppN2 and ppHe ---------------------------------------------
    temp_deco = pres_respiration;
    sim_alveolar_presures();

    char_O_nullzeit = 240;
    for(ci=0; ci<NUM_COMP; ci++)
    {
        //---- Read A/B values and loading factor for N2 and He --------------
        overlay float tN2 = pres_tissue_N2[ci];
        overlay float tHe = pres_tissue_He[ci];
        overlay float t = tN2 + tHe;
        overlay unsigned char ndl;
        overlay unsigned char period = 10;

        read_buhlmann_coefficients();
        read_buhlmann_times(2);             // Starts with a 10min period.

        //---- Simulate for that tissue --------------------------------------
        // NOTE: No need to simulate for longuer than the already found NDL.
        for(ndl=0; ndl<char_O_nullzeit;)
        {
            //---- Compute updated mix M-value at surface
            overlay float a = (var_N2_a * tN2 + var_He_a * tHe) / t;
            overlay float b = (var_N2_b * tN2 + var_He_b * tHe) / t;
            overlay float M0 = (a + pres_surface/b);

            //---- Add 10min/1min to N2/He tissues
            overlay float dTN2 = (ppN2 - tN2) * var_N2_e;
            overlay float dTHe = (ppHe - tHe) * var_He_e;

            //---- Apply security margin for both models
                // NDL can be computed while ascending... SO we have
                // to check wether we are saturating or desaturating.
                if( dTN2 > 0.0 ) dTN2 *= float_saturation_multiplier;
                else             dTN2 *= float_desaturation_multiplier;

                if( dTHe > 0.0 ) dTHe *= float_saturation_multiplier;
                else             dTHe *= float_saturation_multiplier;
            
            if (char_I_deco_model != 0 )
                M0 = GF_high * (M0 - pres_surface) + pres_surface;

            //---- Simulate off-gasing while going to surface
            // TODO !
            // dTN2 -= exp( ... ascent time ... ppN2...)
            // dTHe -= exp( ... ascent time ... ppHe...)

            //---- Ok now, and still ok to surface after 1 or 10 minutes ?
            if( (t <= M0) && (t + dTN2 + dTHe <= M0) )
            {
                tN2 += dTN2;    // YES: apply gas loadings,
                tHe += dTHe;
                t = tN2 + tHe;
                ndl += period;  // increment NDL,
                continue;       // and loop.
            }

            //---- Should we retry with smaller steps ?
            if( period == 10 )
            {
                read_buhlmann_times(1); // 1min coefs.
                period = 1;
                continue;
            }

            //---- ELSE make a linear approx for the last minute
            // Usefull to have a meaningfull rounding of NDL.
            // But ONLY it positive (negativ casted to unsigned is bad).
            if( M0 > t )
                ndl += (unsigned char)(0.5f + (M0-t)/(dTN2+dTHe));
            break;
        }

        // Keep the shortest NDL found
        if( ndl < char_O_nullzeit )
            char_O_nullzeit = ndl;
    }
}

//////////////////////////////////////////////////////////////////////////////
// calc_ascenttime
//
// Summup ascent from bottom to surface, at 1 bar/min, 1min for last 3 meters,
// and all stops.
//
// Result in int_O_ascenttime, or int_O_extra_ascenttime if in @+5min variant.
static void calc_ascenttime(void)
{
    overlay unsigned char x;
    overlay unsigned short sum;

    // + 0.7 to count 1 minute ascent time from 3 metre to surface
    overlay float ascent = pres_respiration - pres_surface + 0.7;
    if (ascent < 0.0)
        ascent = 0.0;
    sum = (unsigned short)(ascent + 0.99);

    for(x=0; x<NUM_STOPS && internal_deco_depth[x]; x++)
        sum += (unsigned short)internal_deco_time[x];

    if( char_O_deco_status == 1 )
        int_O_ascenttime = sum;
    else
        int_O_extra_ascenttime = sum;

}

//////////////////////////////////////////////////////////////////////////////
// update_startvalues
//
// updated in v.102
//
void update_startvalues(void)
{
    overlay unsigned char x;

    // Start ascent simulation with current tissue partial pressures.
    for(x=0; x<NUM_COMP; x++)
    {
        sim_pres_tissue_N2[x] = pres_tissue_N2[x];
        sim_pres_tissue_He[x] = pres_tissue_He[x];
    }

    // No leading tissue (yet) for this ascent simulation.
    sim_lead_tissue_limit = 0.0;
    sim_lead_tissue_no = 255;
}

//////////////////////////////////////////////////////////////////////////////
// sim_tissue
//
// optimized in v.101
//
// Function very simular to calc_tissue, but:
//   + Use a 1min or 10min period.
//   + Do it on sim_pres_tissue, instead of pres_tissue.
static void sim_tissue(PARAMETER unsigned char period)
{
    assert( 0.00 <= ppN2 && ppN2 < 11.2 );  // 80% N2 at 130m
    assert( 0.00 <= ppHe && ppHe < 12.6 );  // 90% He at 130m

    for(ci=0; ci<NUM_COMP; ci++)
    {
        read_buhlmann_times(period);        // 1 or 10 minute(s) interval

        // N2
        temp_tissue = (ppN2 - sim_pres_tissue_N2[ci]) * var_N2_e;
        temp_tissue_safety();
        sim_pres_tissue_N2[ci] += temp_tissue;

        // He
        temp_tissue = (ppHe - sim_pres_tissue_He[ci]) * var_He_e;
        temp_tissue_safety();
        sim_pres_tissue_He[ci] += temp_tissue;
    }
}

//////////////////////////////////////////////////////////////////////////////
// sim_limit()
//
// New in v.111
//
// Function separated from sim_tissue() to allow recomputing limit on
// different depth, because it depends on current gradient factor.
//
static void sim_limit(PARAMETER float GF_current)
{
    assert( 0.0 < GF_current && GF_current <= 1.0f);

    sim_lead_tissue_limit = 0.0;
    sim_lead_tissue_no = 0;             // If no one is critic, keep first tissue.

    for(ci=0; ci<NUM_COMP; ci++)
    {
        overlay float N2 = sim_pres_tissue_N2[ci];
        overlay float He = sim_pres_tissue_He[ci];
        overlay float p = N2 + He;

        read_buhlmann_coefficients();
        var_N2_a = (var_N2_a * N2 + var_He_a * He) / p;
        var_N2_b = (var_N2_b * N2 + var_He_b * He) / p;

        // Apply the Eric Baker's varying gradient factor correction.
        // Note: the correction factor depends both on GF and b,
        //       Actual values are in the 1.5 .. 1.0 range (for a GF=30%),
        //       so that can change who is the leading gas...
        // Note: Also depends of the GF_current...
        if( char_I_deco_model != 0 )
            p = ( p - var_N2_a * GF_current)
              / (GF_current / var_N2_b + 1.0 - GF_current);
        else
            p = (p - var_N2_a) * var_N2_b;

        if( p > sim_lead_tissue_limit )
        {
            sim_lead_tissue_no = ci;
            sim_lead_tissue_limit = p;
        }
    } // for ci

    assert( sim_lead_tissue_no < NUM_COMP );
    assert( 0.0 <= sim_lead_tissue_limit && sim_lead_tissue_limit <= 14.0 );
}

//////////////////////////////////////////////////////////////////////////////
// clear_deco_table
//
// unchanged in v.101
//
static void clear_deco_table(void)
{
    overlay unsigned char x;

    for(x=0; x<NUM_STOPS; ++x)
    {
        internal_deco_time [x] = 0;
        internal_deco_depth[x] = 0;
    }
}

//////////////////////////////////////////////////////////////////////////////
// update_deco_table
//
// Add 1 min to current stop.
//
// Inputs:
//      temp_depth_limit = stop's depth, in meters.
// In/Out:
//      internal_deco_depth[] : depth (in metres) of each stops.
//      internal_deco_time [] : time (in minutes) of each stops.
//
static unsigned char update_deco_table()
{
    overlay unsigned char x;
    assert( temp_depth_limit < 128 );   // Can't be negativ (overflown).
    assert( temp_depth_limit > 0 );     // No stop at surface...

    for(x=0; x<NUM_STOPS; ++x)
    {
        // Make sure deco-stops are recorded in order:
        assert( !internal_deco_depth[x] || temp_depth_limit <= internal_deco_depth[x] );

        if( internal_deco_depth[x]== temp_depth_limit )
        {
            // Do not overflow (max 255')
            if( internal_deco_time[x] < 255 )
            {
                internal_deco_time[x]++;
                return 1;
            }
            // But store extra in the next stop...
        }

        if( internal_deco_depth[x] == 0 )
        {
            internal_deco_depth[x] = temp_depth_limit;

            internal_deco_time[x]  = 1;
            internal_deco_gas[x] = sim_gas_last_used;
            return 1;
        }
    }

    // Can't store stops at more than 96m.
    // Or stops at less that 3m too.
    // Just do nothing with that...
    return 0;
}

//////////////////////////////////////////////////////////////////////////////
// calc_gradient_factor
//
// optimized in v.101 (var_N2_a)
// new code in v.102
//
static void calc_gradient_factor(void)
{
    overlay float gf;
    overlay float N2 = pres_tissue_N2[char_O_gtissue_no];
    overlay float He = pres_tissue_He[char_O_gtissue_no];

    assert( char_O_gtissue_no < NUM_COMP );
    assert( 0.800 <= pres_respiration && pres_respiration < 14.0 );

    // tissue > respiration (currently off-gasing)
    // GF =   0% when respiration == tissue, ie. bubbles are at equilibrium.
    // GF = 100% when respiration == limit.
    temp_tissue = N2 + He;
    if( temp_tissue <= pres_respiration )
        gf = 0.0;
    else
    {
        overlay float limit = calc_lead_tissue_limit;
        // NOTE: in GF model, calc_lead_tissue_limit include already the
        //       correction due to gradient factor. To compute the actual
        //       current GF, we need to (re-)compute the raw ambiant-pressure
        //       limit from the Buhlmann model.
        if( char_I_deco_model != 0 )
        {
            ci = char_O_gtissue_no;
            read_buhlmann_coefficients();
            var_N2_a = (var_N2_a * N2 + var_He_a * He) / temp_tissue;
            var_N2_b = (var_N2_b * N2 + var_He_b * He) / temp_tissue;
            limit = (temp_tissue - var_N2_a) * var_N2_b;
        }

        gf = (temp_tissue  - pres_respiration)
           / (temp_tissue  - limit)
           * 100.0;
        if( gf > 254.5 ) gf = 255.0;
        if( gf < 0.0   ) gf = 0.0;
    }
    char_O_gradient_factor = (unsigned char)(gf+0.5f);

}

//////////////////////////////////////////////////////////////////////////////
// deco_calc_desaturation_time
//
// FIXED N2_ratio
// unchanged in v.101
// Inputs:  int_I_pres_surface, ppWater, char_I_desaturation_multiplier
// Outputs: int_O_desaturation_time, char_O_tissue_saturation[0..31]
//
void deco_calc_desaturation_time(void)
{
    RESET_C_STACK

    assert( 800 < int_I_pres_surface && int_I_pres_surface < 1100 );
    assert( 0 < char_I_desaturation_multiplier && char_I_desaturation_multiplier <= 100 );

    N2_ratio = 0.7902; // FIXED sum as stated in buhlmann
    pres_surface = int_I_pres_surface * 0.001;
    ppN2 = N2_ratio * (pres_surface - ppWater);
    int_O_desaturation_time = 0;
    float_desaturation_multiplier = char_I_desaturation_multiplier * (0.01 * SURFACE_DESAT_FACTOR);

    for(ci=0; ci<NUM_COMP; ci++)
    {
        overlay unsigned short desat_time;    // For a particular compartiment, in min.
        overlay float temp1;
        overlay float temp2;
        overlay float temp3;
        overlay float temp4;

        read_buhlmann_ht();

        // saturation_time (for flight) and N2_saturation in multiples of halftime
        // version v.100: 1.1 = 10 percent distance to totally clean (totally clean is not possible, would take infinite time )
        // new in version v.101: 1.07 = 7 percent distance to totally clean (totally clean is not possible, would take infinite time )
        // changes in v.101: 1.05 = 5 percent dist to totally clean is new desaturation point for display and NoFly calculations
        // N2
        temp1 = 1.05 * ppN2 - pres_tissue_N2[ci];
        temp2 = ppN2 - pres_tissue_N2[ci];
        if (temp2 >= 0.0)
            temp1 = 0.0;
        else
            temp1 = temp1 / temp2;

        if( 0.0 < temp1 && temp1 < 1.0 )
        {
            // 0.6931 is ln(2), because the math function log() calculates with a base of e not 2 as requested.
            // minus because log is negative.
            temp1 = log(1.0 - temp1) / -0.6931; // temp1 is the multiples of half times necessary.
            temp2 = var_N2_ht * temp1 / float_desaturation_multiplier; // time necessary (in minutes ) for complete desaturation (see comment about 5 percent)
        }
        else
        {
            temp1 = 0.0;
            temp2 = 0.0;
        }

        // He
        temp3 = 0.1 - pres_tissue_He[ci];
        if (temp3 >= 0.0)
            temp3 = 0.0;
        else
            temp3 = - temp3 / pres_tissue_He[ci];

        if( 0.0 < temp3 && temp3 < 1.0 )
        {
            temp3 = log(1.0 - temp3) / -0.6931; // temp1 is the multiples of half times necessary.
                                                // 0.6931 is ln(2), because the math function log() calculates with a base of e  not 2 as requested.
                                                // minus because log is negative
            temp4 = var_He_ht * temp3 / float_desaturation_multiplier; // time necessary (in minutes ) for "complete" desaturation, new in v.101 float_desaturation_multiplier
        }
        else
        {
            temp3 = 0.0;
            temp4 = 0.0;
        }

        // saturation_time (for flight)
        if (temp4 > temp2)
            desat_time = (unsigned short)temp4;
        else
            desat_time = (unsigned short)temp2;

        if(desat_time > int_O_desaturation_time)
            int_O_desaturation_time = desat_time;

        // N2 saturation in multiples of halftime for display purposes
        temp2 = temp1 * 20.0;   // 0 = 1/8, 120 = 0, 249 = 8
        temp2 = temp2 + 80.0;   // set center
        if (temp2 < 0.0)
            temp2 = 0.0;
        if (temp2 > 255.0)
            temp2 = 255.0;
        char_O_tissue_N2_saturation[ci] = (char)temp2;

        // He saturation in multiples of halftime for display purposes
        temp4 = temp3 * 20.0;   // 0 = 1/8, 120 = 0, 249 = 8
        temp4 = temp4 + 80.0;   // set center
        if (temp4 < 0.0)
            temp4 = 0.0;
        if (temp4 > 255.0)
            temp4 = 255.0;
        char_O_tissue_He_saturation[ci] = (char)temp4;
    } // for
}

//////////////////////////////////////////////////////////////////////////////
// calc_wo_deco_step_1_min
//
// FIXED N2 Ratio
// optimized in v.101 (...saturation_multiplier)
// desaturation slowed down to 70,42%
//
static void calc_wo_deco_step_1_min(void)
{
    assert( 800 < int_I_pres_surface && int_I_pres_surface < 1100 );
    assert( 800 < int_I_pres_respiration && int_I_pres_respiration < 1100 );
    assert( 100 <= char_I_saturation_multiplier && char_I_saturation_multiplier < 200 );
    assert( 0 < char_I_desaturation_multiplier && char_I_desaturation_multiplier <= 100 );

    N2_ratio = 0.7902; // FIXED, sum lt. buehlmann
    pres_respiration = pres_surface = int_I_pres_surface * 0.001;
    ppN2 = N2_ratio * (pres_respiration - ppWater);
    ppHe = 0.0;
    float_desaturation_multiplier = char_I_desaturation_multiplier * (0.01 * SURFACE_DESAT_FACTOR);
    float_saturation_multiplier   = char_I_saturation_multiplier   * 0.01;

    calc_tissue(1);  // update the pressure in the tissues N2/He in accordance with the new ambient pressure

    clear_deco_table();
    char_O_deco_status = 3;     // surface new in v.102 : stays in surface state.
    char_O_nullzeit = 0;
    int_O_ascenttime = 0;
    int_O_extra_ascenttime = 0;
    calc_gradient_factor();
}

//////////////////////////////////////////////////////////////////////////////
// calc_dive_interval
//
// Prepare tissue for delay before the next dive simulation.
//
// Inputs:  char_I_dive_interval == delay before dive (in 10' steps).
// Outputs: pres_tissue_N2/He[], CNS_fraction
//
// Should be protected by deco_push_tissues_to_vault(),
//                        deco_pull_tissues_from_vault()
//
// desaturation slowed down to 70,42%.
//
static void calc_dive_interval(void)
{
    overlay unsigned char t;
    
    //---- Initialize simulation parameters ----------------------------------
    N2_ratio = 0.7902; // FIXED, sum lt. buehlmann
    pres_respiration = pres_surface = int_I_pres_surface * 0.001;
    ppN2 = N2_ratio * (pres_respiration - ppWater);
    ppHe = 0.0;
    float_desaturation_multiplier = char_I_desaturation_multiplier * (0.01 * SURFACE_DESAT_FACTOR);
    float_saturation_multiplier   = char_I_saturation_multiplier   * 0.01;

    //---- Perform simulation ------------------------------------------------
    for(t=0; t<char_I_dive_interval; ++t)
    {
        calc_tissue(2);  // period = 10min.
        CNS_fraction =  0.92587471 * CNS_fraction;  // Half-time = 90min: (1/2)^(1/9)
    }
    assert( 0.0 <= CNS_fraction && CNS_fraction <= 9.99 ); // 999 %
    int_O_CNS_fraction = (unsigned short)(CNS_fraction * 100.0 + 0.5);

}

//////////////////////////////////////////////////////////////////////////////
// deco_clear_CNS_fraction
//
// new in v.101
//
void deco_clear_CNS_fraction(void)
{
    RESET_C_STACK

    CNS_fraction = 0.0;
    int_O_CNS_fraction = 0;
}

//////////////////////////////////////////////////////////////////////////////
// deco_calc_CNS_fraction
//
// Input:  char_I_actual_ppO2   : Current condition (in decibars).
//         char_I_step_is_1min  : use 1min or 10min steps instead of 2sec.
//         CNS_fraction         : velue before period.
// Output: CNS_fraction, int_O_CNS_fraction
//
void deco_calc_CNS_fraction(void)
{
    overlay float time_factor = 1.0f;
    RESET_C_STACK

    assert( 0.0 <= CNS_fraction && CNS_fraction <= 9.99 );
    assert( char_I_actual_ppO2 > 15 );

    if( char_I_step_is_1min == 1 )
        time_factor = 30.0f;
    else if( char_I_step_is_1min == 2  )
        time_factor = 300.0f;
    //------------------------------------------------------------------------
    // Don't increase CNS below 0.5 bar, but keep it steady.
    if (char_I_actual_ppO2 < 50)
        ;   // no changes
    //------------------------------------------------------------------------
    // Below (and including) 1.60 bar
    else if (char_I_actual_ppO2 < 61)
        CNS_fraction += time_factor/(-533.07 * char_I_actual_ppO2 + 54000.0);
    else if (char_I_actual_ppO2 < 71)
        CNS_fraction += time_factor/(-444.22 * char_I_actual_ppO2 + 48600.0);
    else if (char_I_actual_ppO2 < 81)
        CNS_fraction += time_factor/(-355.38 * char_I_actual_ppO2 + 42300.0);
    else if (char_I_actual_ppO2 < 91)
        CNS_fraction += time_factor/(-266.53 * char_I_actual_ppO2 + 35100.0);
    else if (char_I_actual_ppO2 < 111)
        CNS_fraction += time_factor/(-177.69 * char_I_actual_ppO2 + 27000.0);
    else if (char_I_actual_ppO2 < 152)
        CNS_fraction += time_factor/( -88.84 * char_I_actual_ppO2 + 17100.0);
    else if (char_I_actual_ppO2 < 167)
        CNS_fraction += time_factor/(-222.11 * char_I_actual_ppO2 + 37350.0);
    //------------------------------------------------------------------------
    // Arieli et all.(2002): Modeling pulmonary and CNS O2 toxicity:
    // J Appl Physiol 92: 248--256, 2002, doi:10.1152/japplphysiol.00434.2001
    // Formula (A1) based on value for 1.55 and c=20
    // example calculation: Sqrt((1.7/1.55)^20)*0.000404
    else if (char_I_actual_ppO2 < 172)
        CNS_fraction += time_factor*0.00102;
    else if (char_I_actual_ppO2 < 177)
        CNS_fraction += time_factor*0.00136;
    else if (char_I_actual_ppO2 < 182)
        CNS_fraction += time_factor*0.00180;
    else if (char_I_actual_ppO2 < 187)
        CNS_fraction += time_factor*0.00237;
    else if (char_I_actual_ppO2 < 192)
        CNS_fraction += time_factor*0.00310;
    else if (char_I_actual_ppO2 < 198)
        CNS_fraction += time_factor*0.00401;
    else if (char_I_actual_ppO2 < 203)
        CNS_fraction += time_factor*0.00517;
    else if (char_I_actual_ppO2 < 233)
        CNS_fraction += time_factor*0.0209;
    else
        CNS_fraction += time_factor*0.0482; // value for 2.5

    if( CNS_fraction > 9.99)    // Limit display to 999%
        CNS_fraction = 9.99;
    if( CNS_fraction < 0.0 )
        CNS_fraction = 0.0;

    int_O_CNS_fraction = (unsigned short)(100.0 * CNS_fraction + 0.5);
}

//////////////////////////////////////////////////////////////////////////////
// deco_calc_CNS_planning
//
// Compute CNS during predicted ascent.
//
// Note:    Needs a call to deco_push_tissues_to_vault(),
//          deco_pull_tissues_from_vault() to avoid trashing everything...
//
// Input:   CNS_fraction, char_O_deco_time[], char_O_deco_depth[]
// Output:  CNS_fraction, int_O_CNS_fraction
//
void deco_calc_CNS_planning(void)
{
    overlay unsigned char  backup_gas_last_depth;
    overlay unsigned char  backup_gas_last_used;
    overlay unsigned short backup_dive_mins;
    overlay unsigned char  backup_actual_ppO2;

    RESET_C_STACK

    // Backup state machine
    backup_gas_last_depth = sim_gas_last_depth;
    backup_gas_last_used  = sim_gas_last_used;
    backup_dive_mins      = sim_dive_mins;
    backup_actual_ppO2    = char_I_actual_ppO2;

    // Uses 1min CNS period:
    char_I_step_is_1min = 1;

    //---- Retrieve bottom Gas used, and set variables.
    sim_gas_last_used  = char_I_first_gas;
    sim_gas_last_depth = 0;             // Surface gas marker.
    gas_switch_set();                   // Sets initial calc_N2/He_ratio

    //---- CCR mode : do the full TTS at once --------------------------------
    if( char_I_const_ppO2 != 0 )
    {
        overlay unsigned short t;       // Needs 16bits here !
        char_I_actual_ppO2 = char_I_const_ppO2;
        for(t=0; t<int_O_ascenttime; ++t)
            deco_calc_CNS_fraction();
    }
    else //---- OC mode : have to follow all gas switches... -----------------
    {
        overlay unsigned char i = 0;    // Decostop loop counter
        overlay float actual_ppO2;
        overlay unsigned char time, t;

        //---- Ascent to surface delay
        // NOTE: count as if time is spent with bottom pressure,
        //       AND the bottom gas
        actual_ppO2 = (pres_surface + char_I_bottom_depth * METER_TO_BAR)
                    * (1.0 - calc_N2_ratio - calc_He_ratio);
        if( actual_ppO2 < 0.0  ) actual_ppO2 = 0.0;
        if( actual_ppO2 > 2.50 ) actual_ppO2 = 2.55;
        char_I_actual_ppO2 = (unsigned char)(100.0 * actual_ppO2 + 0.5);

        // Ascent time (rounded up):
        time = (unsigned char)(0.1 * char_I_bottom_depth + 0.5);

        for(t=0; t<time; ++t)
        {
            deco_calc_CNS_fraction();
            sim_dive_mins++;
        }

        //---- Do all further stops ------------------------------------------
        for(i=0; i<NUM_STOPS; ++i)
        {
            overlay unsigned char stop_gas;

            //---- Get next stop ---------------------------------------------
            {
                time             = char_O_deco_time[(NUM_STOPS-1)-i];
                temp_depth_limit = char_O_deco_depth[(NUM_STOPS-1)-i];
                stop_gas         = char_O_deco_gas[(NUM_STOPS-1)-i];
            }
            if( time == 0 ) continue;

            //---- Gas Switch ? ----------------------------------------------
            if( stop_gas != sim_gas_last_used )
            {
                sim_gas_last_depth = deco_gas_change[stop_gas-1];
                sim_gas_last_used  = stop_gas;
                gas_switch_set();
            }

            //---- Convert Depth and N2_ratio to ppO2 ------------------------
            actual_ppO2 = (pres_surface + temp_depth_limit * METER_TO_BAR)
                        * (1.0 - calc_N2_ratio - calc_He_ratio);
            if( actual_ppO2 < 0.0  ) actual_ppO2 = 0.0;
            if( actual_ppO2 > 2.50 ) actual_ppO2 = 2.55;
            char_I_actual_ppO2 = (unsigned char)(100.0 * actual_ppO2 + 0.5);

            //---- Apply the stop
            for(t=0; t<time; ++t)
            {
                deco_calc_CNS_fraction();
                sim_dive_mins++;
            }
        }
    }

    //---- Back to normal mode... --------------------------------------------
    char_I_step_is_1min = 0;
    sim_gas_last_depth  = backup_gas_last_depth;
    sim_gas_last_used   = backup_gas_last_used;
    sim_dive_mins       = backup_dive_mins;
    char_I_actual_ppO2  = backup_actual_ppO2;
}

//////////////////////////////////////////////////////////////////////////////
// deco_calc_CNS_decrease_15min
//
// new in v.101
//
// calculates the half time of 90 minutes in 6 steps of 15 min
// (Used in sleepmode, for low battery mode).
//
// Output: int_O_CNS_fraction
// Uses and Updates: CNS_fraction
//
void deco_calc_CNS_decrease_15min(void)
{
    RESET_C_STACK
    assert( 0.0 <= CNS_fraction && CNS_fraction <= 9.99 );

    CNS_fraction =  0.890899 * CNS_fraction;
    int_O_CNS_fraction = (unsigned short)(CNS_fraction * 100.0 + 0.5);
}

//////////////////////////////////////////////////////////////////////////////
// deco_calc_percentage
//
// new in v.101
//
// calculates int_I_temp * char_I_temp / 100
// output is int_I_temp
//
// Used to compute NoFly remaining time.
//
void deco_calc_percentage(void)
{
    RESET_C_STACK

    assert( 60 <= char_I_temp && char_I_temp <= 100 );
    assert( int_I_temp  < 5760 );      // Less than 4 days = 96h...

    int_I_temp = (unsigned short)(((float)int_I_temp * (float)char_I_temp) * 0.01 );

    assert( int_I_temp < 5760 );                            // Less than 96h too...
}

//////////////////////////////////////////////////////////////////////////////
// deco_gas_volumes
//
// new in v.111
//
// calculates volumes for each gas.
//
// Input:   char_I_bottom_depth, char_I_bottom_time for planned dive.
//          Gas list.
//          char_I_first_gas is the bottom gas.
//          decoplan (char_O_deco_depth, char_O_deco_time).
//          char_I_bottom_usage is bottom liters/minutes (5 .. 50) or bar/min.
//          char_I_deco_usage is deco liters/minutes (5 .. 50) or bar/min.
// Output:  int_O_gas_volumes[0..4] in litters * 0.1
//
void deco_gas_volumes(void)
{
    overlay float volumes[NUM_GAS];
    overlay float bottom_usage, deco_usage;
    overlay unsigned char i;
    overlay unsigned char gas, depth;
    overlay unsigned char lastGasStop;
    RESET_C_STACK

    //---- initialize --------------------------------------------------------
    for(i=0; i<NUM_GAS; ++i)                            // Nothing yet...
        volumes[i] = 0.0;

    bottom_usage = char_I_bottom_usage;      // In liter/minutes.
    deco_usage   = char_I_deco_usage;        // In liter/minutes.

    // Early return if not defined:
    if( deco_usage <= 0.0 || bottom_usage <= 0.0 )
        goto done;

    //---- Bottom usage -----------------------------------------------------
    assert(1 <= char_I_first_gas && char_I_first_gas <= NUM_GAS);
    gas = char_I_first_gas - 1;

    if( char_I_const_ppO2 == 0 )
        volumes[gas]
            = (char_I_bottom_depth*0.1 + 1.0)           // Use Psurface = 1.0 bar.
            * char_I_bottom_time                        // in minutes.
            * bottom_usage;                             // In liter/minutes.

    //---- Ascent usage ------------------------------------------------------
    depth = char_I_bottom_depth;
    lastGasStop = 255;          // Allow deco gas at or below bottom depth

    for(i=0; i<NUM_STOPS; ++i)
    {
        overlay unsigned char newDepth, time;

        time = char_O_deco_time [i];
        if( time == 0 ) break;          // End of table: done.

        newDepth = char_O_deco_depth[i];
        assert(0 < newDepth && newDepth <= depth);

        //---- Any gas switch before this stop -------------------------------
        for(;;)
        {
            overlay unsigned char newGas  = 0;
            overlay unsigned char newStop = 0;  // Mark as NO CHANGE yet
            overlay unsigned char j;

            for(j=0; j<NUM_GAS; ++j)
            {
                // Skip gas without changing depth:
                if( ! char_I_deco_gas_change[j] )
                    continue;
                // Select gas changed between [newDepth .. lastGasStop[
                // Note that <= means changing gas at BEGINNING of this stop.
                // Note that < means we cant use the same gas twice
                if( newDepth <= char_I_deco_gas_change[j]
                 && char_I_deco_gas_change[j] < lastGasStop )
                {
                    // Keep the DEEPEST gas in that range:
                    if( char_I_deco_gas_change[j] >= newStop )
                    {
                        newGas  = j;
                        newStop = char_I_deco_gas_change[j];
                    }
                }
            }

             // Did we find something ?
            if( !newStop )
                break;

            //---- usage BEFORE gas switch (if any), at 10m/min :
            if( depth > newStop )
                // Plus usage during ascent to the next stop, at 10m/min.
                volumes[gas] += ((depth+newStop)*0.05 + 1.0)    // average depth --> bar.
                              * (depth-newStop)*0.1             // metre --> min
                              * deco_usage;

            //---- Do gas switch:
            gas = newGas;

            lastGasStop = newStop;          // Mark last used gas
            if( newStop < depth )           // ascent to gas switch,
                depth = newStop;
        }

        // Are we back to gas from the deco list (just in case):
        assert(gas == char_O_deco_gas[i]-1);

        //---- usage AFTER gas switch (if any), at 10m/min :
        if( depth > newDepth )
            volumes[gas] += ((depth+newDepth)*0.05 + 1.0)    // average depth --> bar.
                          * (depth-newDepth)*0.1             // metre --> min
                          * deco_usage;

        //---- Do stop:
        depth = newDepth;

        //---- Usage at stop:
        volumes[gas] += (depth*0.1 + 1.0)   // depth --> bar.
                      * time                // in minutes.
                      * deco_usage;         // in xxx / min @ 1bar.
    }

    // From last stop to surface
    volumes[gas] += (depth*0.05 + 1.0)      // avg depth --> bar.
                  * depth * 0.1             // time to surface, in minutes.
                  * deco_usage;             // in xxx / min @ 1bar.

    //---- convert results for the ASM interface -----------------------------
done:
    for(i=0; i<NUM_GAS; ++i)
        if( volumes[i] > 65534.0 )
            int_O_gas_volumes[i] = 65535;
        else
            int_O_gas_volumes[i] = (unsigned short)(volumes[i] + 0.5);
}

//////////////////////////////////////////////////////////////////////////////

void deco_push_tissues_to_vault(void)
{
    overlay unsigned char x;
    RESET_C_STACK

    cns_vault = CNS_fraction;
    low_depth_vault = low_depth;

    for (x=0;x<NUM_COMP;x++)
    {
        pres_tissue_N2_vault[x] = pres_tissue_N2[x];
        pres_tissue_He_vault[x] = pres_tissue_He[x];
    }
}

void deco_pull_tissues_from_vault(void)
{
    overlay unsigned char x;
    RESET_C_STACK

    for (x=0; x<NUM_COMP; x++)
    {
        pres_tissue_N2[x] = pres_tissue_N2_vault[x];
        pres_tissue_He[x] = pres_tissue_He_vault[x];
    }

    // Restore both CNS variable, too.
    CNS_fraction = cns_vault;
    int_O_CNS_fraction = (unsigned short)(CNS_fraction * 100.0 + 0.5);

    // GF history too:
    low_depth = low_depth_vault;
    locked_GF_step = GF_delta / low_depth;
}

//////////////////////////////////////////////////////////////////////////////
//
#ifndef CROSS_COMPILE
void main() {}
#endif