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Date:	Sat, 22 May 2010 15:41:48 -0700 (PDT)
From:	Pablo Marty <tigrepotrazosalvaje AT yahoo DOT com DOT ar>
Subject:	my videogame in DJGPP and VBE
To:	djgpp AT delorie DOT com
MIME-Version:	1.0
Reply-To:	djgpp AT delorie DOT com

Dear Rugxulo and other members

Hi, I tried my game again with DOSBox and it ran quit faster and now the mu=
sic sounded correctly.=20
Now I'm looking for the VESA or VBE mode with no bank switching (linear fra=
mebuffer) in order to make my game (Super Mario, I'm a great fan ... also S=
onic the hedgehog) normally fast.=20
Obviously I googled something like "SVGA DJGPP with no bank switching"
And I got this from: http://www.delorie.com/djgpp/doc/ug/graphics/vbe20.htm=
l=20

/************/
   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 set_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"
      );
   }

This routine is an exact drop-in replacement for the set_vesa_bank() functi=
on described in the previous chapter, but will run several hundred times fa=
ster!

VBE 2.0 also provides the ability to use a linear framebuffer mode in which=
 the entire video memory can accessed as a single block at some location ot=
her than the standard 0xA0000, which gets rid of the need for bank switchin=
g altogether. This is both the fastest and the easiest way to program SVGA =
graphics, but unfortunately you can't count on it being supported by all ha=
rdware. Even if the card has a VBE 2.0 driver, many older boards don't supp=
ort linear framebuffers at all, and a few of the more recent ones can only =
use linear addressing in certain resolutions.

Setting a linear framebuffer mode is extremely simple. After calling the fi=
nd_vesa_mode() function, check that bit 7 of mode_info.ModeAttributes is se=
t, to make sure that linear addressing is possible in this mode. Assuming t=
hat it is supported, when you call function 0x4F02 to select the mode you s=
hould put (mode_number | 0x4000) into the BX register, instead of just mode=
_number, and you will have a linear framebuffer!

The video memory is located at the physical address specified by the mode_i=
nfo.PhysBasePtr field, but you must map this area into your address space b=
efore you can access it. This can be done with the code:

   __dpmi_meminfo mapping;
   int selector;

   /* map into linear memory */
   mapping.address =3D mode_info.PhysBasePtr;
   mapping.size =3D vesa_info.TotalMemory << 16;
   if (__dpmi_physical_address_mapping(&mapping) !=3D 0)
      return -1;

   /* allocate an LDT descriptor to access the linear region */
   selector =3D __dpmi_allocate_ldt_descriptors(1);
   if (selector < 0) {
      __dpmi_free_physical_address_mapping(&mapping);
      return -1;
   }

   /* set the descriptor location and size */
   __dpmi_set_segment_base_address(selector, mapping.address);
   __dpmi_set_segment_limit(selector, mapping.size-1);

You can now write to any part of the screen using the selector that we just=
 created, and without any need for bank switching, eg:

   void linear_putpixel(int x, int y, int color)
   {
      _farpokeb(selector, y*640+x, color);
   }

Finally, at the end of your program you should free the video memory mappin=
g with the code:

   __dpmi_free_physical_address_mapping(&mapping);
   __dpmi_free_ldt_descriptor(selector);
/***************/

The compiler detected that these functions try to access fields of the VESA=
_PM_INFO struct that are not declared.=20
I remmember having seen in other page a code like this and the definition o=
f VESA_PM_INFO was also absent
Can you tell me where find this code correct or how to fix this?=20
Also I fixed almost all bugs in my program and added music with play_midi()=
; in brief, when I get it some better I'll ask you to send it again.
Thank you
Pablo=20
Bs As Argentina
=0A=0A=0A      

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