From: "Ron Grunwald" <RGRUNWAL AT wasp DOT cs DOT cowan DOT edu DOT au>
Organization:  Edith Cowan University
To: djgpp AT sun DOT soe DOT clarkson DOT edu, Marc Coupar <mc AT dstn24 DOT dct DOT ac DOT uk>
Date:          Tue, 12 Dec 1995 11:17:56 GMT+800
Subject:       Re: blitting to vga problem
Reply-To: r DOT grunwald AT cowan DOT edu DOT au

> Hi,
> I've been having a little trouble with the following routine which should
> blitcopy a 64k buffer to vga mem (used in mode 13h).  I have set it to 
> copy dwords, but this indiscriminately crashes my machine.  The confusing
> thing is that when I change it to copy the buffer with just word writes,
> it works fine, although obviously a bit slower.
> 
> The only thing I can think of is it might be my video card - a poor 16 bit
> ISA Trident TVGA.  I believe there can be problems with 32 bit writes to
> a 16 bit card.  Can anybody shed some light on this ?

This is true but only applies if you're writing to the VGA 
processor's registers directly with an outportl() call for instance.
However, in your case a memory block is effectively copied from one
mem. location to another with no direct interaction with the VGA chip.


> 
> note: I'm using gcc v1 with the latest maintenance release.
>       0xd0000000 is where gcc maps the screen segment to.
> 
> void BlitBufferToVga (int * buffer) {  
>         asm(".align 4                 ");
>         asm("movl  %0,%%esi           ":: "g" (buffer) : "%esi");            
                        ^^^^^
That's an interesting syntax you're using. Can you enlighten me on
how it works?

>         asm("movl  $0xd0000000,%%edi  "::              : "%edi");   
>         asm("movl  $16000,%%ecx       "::              : "%ecx");             
>         asm("cld                      ");                        
>         asm("rep                      ");                        
>         asm("movsl (%%esi),(%%edi)    "::              : "%esi","%edi");       
> }
> 
> PS: I know that an unrolled loop version will be faster, and I will do that
>     as soon as I understand what the problem with this version is.
> 
> I would be grateful if any help was emailed rather than posted.
> 
> Thanks
> 
> Marc
> mc AT dstn24 DOT dct DOT ac DOT uk
> 
> 
> 

 ********************************************************************
 | Author.............. Ron Grunwald                                | 
 | Internet............ r DOT grunwald AT cowan DOT edu DOT au                     |
 | Phone............... (09)273 8027 or (09)273 8468                |
 |------------------------------------------------------------------|
 | Department.......... Computer Operations and Systems Management  |
 | Division/Faculty.... Information Technology                      | 
 | Institute........... Edith Cowan University, Churchlands         | 
 | Location............ Perth, Western Australia                    |
 ********************************************************************
 "I don't have any solution but I certainly admire the problem!"