Xref: news2.mv.net comp.os.msdos.djgpp:5934
Newsgroups: comp.os.msdos.djgpp
From: Nick Collier <ntcollie AT midway DOT uchicago DOT edu>
Subject: Help: General Protection Fault
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Date: Fri, 12 Jul 1996 20:58:00 GMT
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To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

Hello all,

I'm trying to convert the polygon drawing code from M.Abrash's Zen 
of Graphics programming to work with djggp. The asm code chunk below is part
of the routing DrawHorizontalLineList which does the drawing to the
screen. It receives the line list structure:

struct HLineList {
   int Length;                /* # of horizontal lines */
   int YStart;                /* Y coordinate of topmost line */
   struct HLine * HLinePtr;   /* pointer to list of horz lines */
};

which contains:

struct HLine {
   int XStart; /* X coordinate of leftmost pixel in line */
   int XEnd;   /* X coordinate of rightmost pixel in line */
}; 


The members of the structure are referenced through offsets H_YSTART
etc. When I run the program it crashes with the following:

General Protection Fault at eip=00001c08
eax=00000303 ebx=28474348 ecx=00000000 edx=000a0140 esi=000000c7 edi=000a0140
ebp=0004ddd8 esp=0004ddbe cs=00a7 ds=00af es=00c7 fs=0087 gs=00c7 ss=00af
Call frame traceback EIPs:
  0x00001c08
  0x00001a12
  0x00001584
  0x00002177


0x00001c08 refers to the line marked below.

	movw	_video_seg,%ax
	movw	%ax,%es						/*point ES to display memory for REP STOS */

	movl	ARG1,%esi					/*point to the line list */
	movl	H_YSTART(%esi),%eax		/*point to the start of the first scan */
	imul	$320, %eax					/*line in which to draw */
	movl	%eax,%edx					/*ES:DX points to first scan line to draw */
	addl	$0xa0000, %edx
	movl	H_LPTR(%esi),%ebx			/*point to the XStart/XEnd descriptor */
											/*for the first (top) 
horizontal line */
	movl	H_LENGTH(%esi),%esi		/*number of scan lines to draw */
	andl	%esi,%esi					/*are there any lines to draw? */
	jz		fill_done					/*no, so done */
	movb	ARG2,%al						/*color with which to fill */
	movb	%al,%ah						/*duplicate color for STOSW */
fill_loop:		

**** ->	movl	X_START(%ebx),%edi		/*left edge of fill on this line */ <-
	movl	X_END(%ebx),%ecx			/*right edge of fill */
	subl	%edi,%ecx

X_START refers to the embedded HLine structure in the HLinelist. It is
defined through the following code:

	fprintf(f, "#define H_LENGTH        %ld\n",  offsetof(struct HLineList, Length));
	fprintf(f, "#define H_YSTART       %ld\n",  offsetof(struct HLineList, YStart));
	fprintf(f, "#define H_LPTR      %ld\n",  offsetof(struct HLineList, HLinePtr));
	fprintf(f, "#define X_START        %ld\n",  offsetof(struct HLine, XStart));
	fprintf(f, "#define X_END        %ld\n",  offsetof(struct HLine, XEnd));
	fprintf(f, "#define HL_SIZE        %ld\n",  sizeof(struct HLine));

(Thanks to the author of Allegro (Shawn Hargreaves?) for this code). The
file that this generates is then included. 

I figure the problem is either in seting up the offsets or with the asm
code. Anyway I hope this is enough information for suggestions as to a
solution.

TIA
Nick