/* X0.c

This is the startup module for external functions.
Recompiling requires Borland C++ or Turbo C and Assembler.

There is an alternate startup module in this file for external
functions which do not require DS=SS.  The math library, ld.lib,
which has been provided, does not require DS=SS.  Programs built
with this save some disk space.

Options.

You must set Opt=1 or Opt=2.

1. External function sets up its own stack segment.  This allows
you to guarantee enough stack space, but more importantly, it allows
DS=SS which is what the compiler expects in tiny model.  Unfortunately,
however, the floating point emulator doesn't work in a foreign stack
segment.  Do not use this option unless you are using a math coprocessor.

2. External function uses Mercury's stack segment.  This is more efficient,
but requires you to write your function in tiny model without using DS=SS,
which Borland C tiny model normally assumes.  Most simple functions in tiny
model don't actually make use of DS=SS, but this option is dangerous unless
you are familiar with the way Turbo C generates code.

bcc -c -mt! -w -ox01 -DOpt=1 -Z -O x0
bcc -c -mt! -w -ox02 -DOpt=2 -Z -O x0

x01.obj uses stack swap
x02.obj uses Mercury stack

See the examples, GCD.C and ASINH.C, for instructions on how to link.
Using Option 1 requires linking with F87.OBJ.

If using Turbo, compile with "tcc -c -mt -w ...".
*/


#pragma inline

/* TC is not too happy about this, but it does the right thing. */
#pragma warn -asm
asm DGROUP	group	_TEXT,_DATA,_BSS
#pragma warn .asm

pascal xmain(double far *);

/*
To handle tiny model properly, must have CS=DS=SS.
Simple programs, eg not passing addresses of local variables, don't need SS.

*/

#ifndef	Opt
#error Must define Opt=1 or Opt=2.
#endif

#if	Opt == 1
#define	SwapSS	1
#endif

#if	Opt == 2
#define	SwapSS	0
#endif

#if	!SwapSS

/* Opt = 2 */

static void far pascal start(double *a)
{

	_ES = _SI = _DS;
	_DS = _CS;

	xmain((double _es *) a);

	_DS = _SI;
	return;
}

#else

/* Opt = 1 */

/* make external, so user can set it? */
#define StackWords	1000
int stack[StackWords] = { 0 };

/*
Must preserve bp, si, di, ds, ss.
Called with ds = ss.
*/

static void far pascal start(double *a)
{
	_BX = (unsigned int) a;
asm	mov	cx, sp
asm	mov	dx, ss

	_AX = (unsigned int) (&stack[StackWords]);

/* switch to local stack */
asm	push	cs
asm	pop	ss
asm	mov	sp, ax

/* save prior ss, sp */
asm	push	dx
asm	push	cx

/* set ds=cs */
asm	push	cs
asm	pop	ds

/* call xmain */
asm	mov	es, dx
	xmain((double _es *) _BX);

/* restore ss, sp, ds=ss */
asm	pop	cx
asm	pop	dx
asm	mov	ss, dx
asm	mov	sp, cx
asm	mov	ds, dx

	return;
}

#endif