Xref:	news2.mv.net comp.os.msdos.djgpp:7008
Newsgroups:	comp.os.msdos.djgpp
From:	Nick Collier <ntcollie AT midway DOT uchicago DOT edu>
Subject:	asm loading register problem
Message-ID:	<320A42DC.7CB5@midway.uchicago.edu>
Sender:	news AT midway DOT uchicago DOT edu (News Administrator)
Organization:	University of Chicago -- Academic Computing Services
Mime-Version:	1.0
Date:	Thu, 8 Aug 1996 19:41:16 GMT
Lines:	82
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

Hey,

I'm trying to convert the polygon scannning code from M. Abrash's Zen of
Graphics programming to work with DJGPP, but am having problems with the
ASM version of ScanEdge.

When I run a program that uses it, I get the following:

General Protection Fault at eip=00001d38
eax=00000000 ebx=000000c7 ecx=00000000 edx=00000001 esi=00000000
edi=c0118004
ebp=0004e5a0 esp=0004e582 cs=00a7 ds=00af es=00af fs=0087 gs=00bf
ss=00af
Call frame traceback EIPs:
  0x00001d38
  0x000019ee
  0x000015ab
  0x000022df

eax, ebx, edx are all correct, that is, they contain the values assigned
by the ASM code. The following is the edited code with the problem line
marked:

movl    ARG7, %edi
        movl    (%edi), %edi
        cmpl    $1, ARG5

        jz      HLinePtrSet

        addl    X_END, %edi
HLinePtrSet:
        movl    ARG4, %ebx
        subl    ARG2, %ebx              #edge height
        jle     ToScanEdgeExit          #guard against 0-length & horz
        movl    %ebx,-4(%ebp)           #Local Variable Height = Y2 - Y1
        subl    %ecx,%ecx               #assume ErrorTerm starts at 0
                                        # were moving right as we draw)
        movl    $1, %edx                #assume AdvanceAmt = 1 (move
        movl    ARG3, %eax
        subl    ARG1, %eax              #DeltaX = X2 - X1
        jz      IsVertical              #its a vertical edge--special

<snip>

IsVertical:
        movl    ARG1, %eax              #starting (and only) X
                                        #coordinate
        subl    ARG6, %ebx              #loop count = Height - SkipFirst
        jz      ScanEdgeExit            #no scan lines left after
                                        #skipping 1st
VerticalLoop:

-> ******movl   %eax, (%edi)    <-**********************
        addl    HL_SIZE, %edi           #point to the next HLine struc
        decl    %ebx                    #count off this scan line
        jnz     VerticalLoop
       jmp     ScanEdgeDone

The first polygon is vertical so the above contains all the relevant
executed code.

ARG7 refers to a double pointer - **EdgePtPtr - hence the double
dereference. eax refers to the starting x coord. which is 0 in this
case. ebx refers to the loop count which is 199, that is, the height of
the screen as the first polygon is a screen size square.

Why can't (%edi) be loaded with the value of %eax?

I compiled the C version with -S and the output is similar to this ASM
code. Loads, in this case, (%esi) with the value of %eax.

Help is appreciated of course.

Tia,
Nick

p.s. Once I get enough converted to be useful, I'll post it somewhere.

-- 
-------------------------
Nick Collier				5237 S. Greenwood #2
ntcollie AT midway DOT uchicago DOT edu		Chicago, IL 60615

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