🚀 go-pugleaf

I am trying to program a graphics application with BC++ 3.1 and
Phar Lap 286 Dos Extender Lite.  I need to write directly to the
video memory, but can't seem to do this successfully.  Perhaps
someone could help me solve this problem.

Before using the Dos extender I was writing directly to segment
A000h in VGA mode 13h.  Now I'm calling DosMapRealSeg () to 
create a selector to A000h, but when I use that selector in an
assembly routine to write to the screen, I get a general protection
fault.

The function DosMapRealSeg () seems to be returning a legitimate
selector (22CFh), but I get a fault at this line in the ASM code:

  mov  es:[di], al  ;ES contains the selector to A000h, di the offset

Can anyone offer any suggestions to solve this problem? Attached
below is the C code with inline assembly and the error message.

Thanks in advance,

Scott Lewis
cslewis@hydra.unm.edu
----

#include <dos.h>
#include <conio.h>
#include "vga.h"

#ifdef DOSX286
#include <phapi.h>
#endif

int drawPixel (int x, int y, int color);

main ()
{
int i;
extern unsigned short vga_seg;         // vga_seg is initialized with
                                       // DosMapRealSeg (), in another
vgaInit (0x0013);                      // module, which contains
                                       // vgaInit () and vgaClose ().
for (i=0; i<320; i++)
    drawPixel (i, 100, 4);
getch ();

vgaClose ();
return 0;
}

drawPixel (int x, int y, int color)
{
extern unsigned short vga_seg;

asm    mov ax, vga_seg
asm    mov es, ax
asm    mov ax, 320
asm    mul y                           // The fault is occuring in this
asm    add ax, y                       // routine, at the last line,
asm    mov di, ax                      // I believe.
asm    mov ax, color
asm    mov es:[di], al

return 0;
}

When run, the code gives this error message:

Fatal error 286.3330: General protection fault detected.
PID=0001  TID=0001  SID=0001  ERRORCODE=0000
AX=0004  BX=0017  CX=0013  DX=0000  SI=0000  DI=7D64  BP=0FE2
CS:IP=024F:004C   DS=0267  ES=22CF  SS:SP=026F:0FE0   FLAGS=2202

-- 
---------------------
Scott Lewis
cslewis@vesta.unm.edu
---------------------