From:	Nate Eldredge <neldredge AT hmc DOT edu>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: rep stosb generates a gpf:-(
Date:	Fri, 17 Sep 1999 13:00:43 -0700
Organization:	Harvey Mudd College
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"Santosh H." wrote:
> 
> Hi,
>   I was trying today to port some of my graphics functions I'd written for
> borland C to DJGPP(why prot mode u knucklehead!sorry:-)).Right , So i have
> a procedure which i converted from TASM to NASM syntax.the procedure is as
> below
> 
> _Drawhorizline:
>    push ebp
>    mov ebp,esp
>    push es
>    push ds
> 
>    mov es,_VideoSegmentSelector ;which I get by push word 0a000h
>                                                call ___dpmi_segment_to_descriptor
>                                                mov [_Videosegmentselector],ax
>    mov di,0                ;0 just for eg
>    mov cx,80
>    rep stosb              "BANG!"<-generates a gpf

You're in 32-bit mode now, but you're only loading the low 16 bits of
edi.  The high half still contains garbage.  The same goes for ecx.  You
want to do:

mov edi, 0
mov ecx, 80

Btw, "rep stosd" will be on the order of 4 times faster, since it moves
4 bytes at a time instead of 1.  You'll just have to duplicate the value
to be stored (from al) 4 times in eax.  This is left as an exercise for
the reader. :)

-- 

Nate Eldredge
neldredge AT hmc DOT edu

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