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Date:	Thu, 23 Dec 2010 16:09:34 -0800 (PST)
From:	Pablo Marty <tigrepotrazosalvaje AT yahoo DOT com DOT ar>
Subject:	question of assembler
To:	djgpp AT delorie DOT com
MIME-Version:	1.0
Reply-To:	djgpp AT delorie DOT com

hi guys and girls
I'm still a the search of the VESA VBE protected mode bank switching functi=
on in order to run my SuperMarioBros videogame normally fast and with music=
 under DOSBox=20

this is the code I got from the DJGPP page about VESA 2.0
the compiler throws an error at the set_vesa_bank_pm() function (at the end=
 of the code)
In the last time I wrote to the list, Mr DJ Delorie told me that "you can't=
 clobber and use them", I suppose refering to the CPU registers. But he did=
n't tell me how to clobber them without using them .... I suppose that's wh=
at has to be done, because beside the code there is a coment that says "clo=
bber list ..."
Can someone tell me how to clobber the list of registers correctly? I'm sur=
e all of you (including Rugxulo) will enjoy my game when it runs Ok and wit=
h sound under DOSBox
And, by the way, i can't understand how there is an eliminating error in th=
e code in the page of such great and important programming platform .... bu=
t, .... will is the important thing
Thank you
Pablo Marty
Bs As - Argentina=20

=A0
//***********************************************************************
=A0   typedef struct VESA_PM_INFO
   {
      unsigned short setWindow            __attribute__ ((packed));
      unsigned short setDisplayStart      __attribute__ ((packed));
      unsigned short setPalette           __attribute__ ((packed));
      unsigned short IOPrivInfo           __attribute__ ((packed));
   } VESA_PM_INFO;


   VESA_PM_INFO *vesa_pm_info;

   void *pm_bank_switcher;


   int get_vesa_pm_functions()
   {
      __dpmi_regs r;

      /* check that the driver is at least VBE version 2.0 */
      if (vesa_info.VESAVersion < 0x200)=20
=09 return -1;

      /* call the VESA function */
      r.x.ax =3D 0x4F0A;
      r.x.bx =3D 0;
      __dpmi_int(0x10, &r);
      if (r.h.ah)
=09 return -1;

      /* allocate space for the code stubs */
      vesa_pm_info =3D malloc(r.x.cx);

      /* copy the code into our address space */
      dosmemget(r.x.es*16+r.x.di, r.x.cx, vesa_pm_info);

      /* store a pointer to the bank switch routine */
      pm_bank_switcher =3D (void *)((char *)vesa_pm_info +=20
=09=09=09=09  vesa_pm_info->setWindow);

      return 0;
   }

This code will give you a pointer to the protected mode bank switching func=
tion, but you cannot call this directly from C because it uses a special re=
gister based argument passing convention. A little bit of inline asm is nee=
ded to make sure the parameters go into the correct registers, eg:   void s=
et_vesa_bank_pm(int bank_number)
   {
      asm (
=09 " call *%0 "
      :                             /* no outputs */

      : "r" (pm_bank_switcher),     /* function pointer in any register */
=09"b" (0),                   /* set %ebx to zero */
=09"d" (bank_number)          /* bank number in %edx */

      : "%eax",                     /* clobber list (we have no way of */
=09"%ebx",                    /* knowing which registers the VESA */
=09"%ecx",                    /* code is going to change, so we */
=09"%edx",                    /* have to assume the worst and list */
=09"%esi",                    /* them all here) */
=09"%edi"
      );
   }
=0A=0A=0A      

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