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Parameter Passing and Calling Conventions
=========================================

ELKS kernel system calls
    System call calling convention requires registers set in order of parameters,
    regardless of which compiler is used:
        BX, CX, DX, DI, SI
    AX is set to the system call number defined in elks/arch/i86/kernel/syscall.dat,
    then an INT 0x80 executed.  Return value is in AX.
    Error are indicated by negative AX (-errno) return values.


ia16-elf-gcc C compiler
    Cdecl (default) calling convention: stack has parms, passed in reverse order,
        caller pops all args after call.

    Regparmcall calling convention: args passed in following registers in order:
        AX, DX, CX, DI, SI
        callee pops args > 3 before return.


OpenWatcom C compiler
    Register calling convention by default, picked from the following registers,
    according to the compilation model and function argument signature:
        AX, DX, BX, CX, then use stack for arguments

    Cdecl calling convention (push onto stack) always used for varargs functions.

    Can be overridden using a variation of:
        #pragram aux func __parm [__ax] [__bx] [__cx] [__dx] [__di] [__si] __value [__ax]

    Examples of register allocation in small model:
        main(int ac, char __far **av)  uses AX, CX:BX
        main(int ac, char **av)        uses AX, DX
        strlen(char __far *)           uses DX:AX
        write(int, char __far *, int)  uses AX, CX:BX, DX

    Compiler also supports __stdcall like __cdecl except callee pops args on return.


Function Prologues Produced by C Compilers
==========================================

Possible ia16-elf-gcc function prologues
----------------------------------------

SI only
 193:   56                      push   %si

SI & DI only
   0:   56                      push   %si
   1:   57                      push   %di

NORMAL
   0:   55                      push   %bp
   1:   89 e5                   mov    %sp,%bp

NORMAL w/SI
  3d:   56                      push   %si
  3e:   55                      push   %bp
  3f:   89 e5                   mov    %sp,%bp

NORMAL w/SI & DI
 13e:   56                      push   %si
 13f:   57                      push   %di
 140:   55                      push   %bp
 141:   89 e5                   mov    %sp,%bp

NORMAL w/SI & DI and small locals
 1c4:   56                      push   %si
 1c5:   57                      push   %di
 1c6:   55                      push   %bp
 1c7:   89 e5                   mov    %sp,%bp
 1c9:   83 ec 24                sub    $0x24,%sp

NORMAL w/SI & DI and large locals
  6d:   56                      push   %si
  6e:   57                      push   %di
  6f:   55                      push   %bp
  70:   89 e5                   mov    %sp,%bp
  72:   81 ec 84 00             sub    $0x84,%sp


Function start address determination for backtrace
--------------------------------------------------

Suggested algorithm for calculating function start:
Search for 55 89 e5 in reverse order at IP-3 (NORMAL), IP-6 (NORMAL w/small locals)
and IP-6 (NORMAL w/large locals).
If found, BP_PUSHED, IP=matched IP;
Then search for 57 DI_PUSHED, --IP;
then search for 56 SI_PUSHED, --IP;
Last location (IP) is then function start.

Use -finstrument-functions-simple.          # function profiling
Possibly use -fno-omit-frame-pointer.       # always push BP in prologue
Possibly use -fno-optimize-sibling-calls.   # disable tail call optimization
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