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Date: Fri, 23 Feb 2024 22:48:47 +0100
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Subject: Re: checking int2d amis appstring with djgpp
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Hi!

On 2024-02-23 22:11, Ozkan Sezer (sezeroz AT gmail DOT com) [via djgpp AT delorie DOT com] wrote:
> Hi everyone:
> 
> Does the following DJGPP port of SBEMU detection look correct?
> It is an adaptation from a WatcomC (32 bit flat) code in here:
> https://github.com/wbcbz7/sndlib-watcom/blob/master/sndlib.cpp#L41-L71
> 
> (I'm fairly rusty in gcc inline asm and djgpp address mapping, etc,
> thanks in advance.)
> 
>  int sndlib_sbemu_detect(void)
> {
>   __dpmi_raddr addr;
>   uint32_t r_addr;
>   char* appstring;
>   int mx;
> 
>   /* check for INT2D vector == NULL */
>   __dpmi_get_real_mode_interrupt_vector(0x2D, &addr);
>   r_addr = ((uint32_t)addr.segment<<4) + (uint32_t)addr.offset16;
>   if (!r_addr) return -1;
> 
>   /* scan all multiplexes of INT 2D */
>   for (mx = 0; mx < 256; mx++)
>   {
>     __asm__ __volatile__ ("movb %0,%%ah"::"m"(mx));
>     __asm__ __volatile__ (
>       "xorb %al, %al\n"
>       "int  $0x2d\n"
>       "cmpb $0xFF, %al\n" /* is this a free multiplex? */
>       "jz  _found\n"
>       "xorw %dx, %dx\n" /* it is, return NULL pointer */
>       "xorw %di, %di\n"
>      "_found:\n");
>     __asm__ __volatile__ ("movw %%dx,%0":"=m"(addr.segment));
>     __asm__ __volatile__ ("movw %%di,%0":"=m"(addr.offset16));

This doesn't look right - you can't assume gcc preserves registers
between asm blocks.

But this is a real-mode interrupt, no?  So you would have to do it via
__dpmi_int() here anyway, no asm involved.

> 
>     /* check for SBEMU application string */
>     r_addr = ((uint32_t)addr.segment<<4) + (uint32_t)addr.offset16;
>     if (!r_addr) continue;
>     appstring = (char *)real2ptr(r_addr);
>     if (memcmp(appstring + 8,"SBEMU",5) == 0)
>       return mx;
>   }
> 
>   return -1;
> }
> 
> 
> P.S.: real2ptr() in there actually returns
>     (void *) (real += __djgpp_conventional_base)
> after a successful __djgpp_nearptr_enable()
> 
> --
> O.S.