From: Michael Krause <mhkrause AT umich DOT edu>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Slow MOV opcode
Date: Sun, 14 Dec 1997 22:01:58 -0800
Organization: University of Michigan
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Message-ID: <3494C7D5.9305B6CA@umich.edu>
References: <01bd08e7$dfe77f00$10fbffc2 AT fsl22 DOT novo DOT dk>
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Flemming Stig Larsen wrote:

<snip>

> This unuseable loop:
>    __asm__ __volatile__ (
>    "1:movb %%al, (%%edi)
>       incl %%edi
>       incl %%eax
>       decl %%ecx
>       jnz 1b"
>       :  : "ecx" (60000), "D" (video_buffer) : "ecx", "eax", "edi" );
>
> seemed to take about 19 - 20 clocks per cycle ! ouch!!
>
> while this:
>    __asm__ __volatile__ (
>    "1:incl %%edi
>       incl %%eax
>       decl %%ecx
>       jnz 1b"
>       :  : "ecx" (60000), "D" (video_buffer) : "ecx", "eax", "edi" );
>
> only took about 2 clocks p. cycle !!!
>      (must be the pairing!)
>
> **WHY** is that mov opcode so slow? can anyone explain that to me?
> **WHAT** can i do to make it faster?????
>
> Help me
>
>  -Lars  (reach me at:   flemming DOT larsen AT private DOT dk )

You're writing to video memory.  Video memory accesses can take from two
to ten times longer than regular memory accesses.  Try to point edi to
something else instead, and time it.

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