Date: Fri, 14 Oct 1994  9:46:08 UTC+0200
From: Francesc Guasch-Ortiz <frankie AT si DOT upc DOT es>
Subject: short data types
To: djgpp AT sun DOT soe DOT clarkson DOT edu

Delivery-date: Tue, 11 Oct 1994 13:53:49 UTC+0200
From:       <cnc AT netcom19 DOT netcom DOT com>
To:         <djgpp AT sun DOT soe DOT clarkson DOT edu>
Message-ID: inbox:509
Subject:    Re: 32/16bit?

>X-Mailer: Mail User's Shell (7.2.5 10/14/92)



>This is incorrect.  If you refer to the Intel processor manuals for the i486
>and Pentium processors, you will find that when an instruction with 16-bit
>operands exists in a 32-bit segment, it requires an OPERAND SIZE prefix.
>The same is true for 32-bit instructions in a 16-bit segment.  This OPERAND
>SIZE prefix is 1 byte long and takes 1 clock cycle to decode on both i486
>and Pentium processors.  This means that for simple instructions like ADD
>or MOV which normally take 1 clock cycle, using the wrong bitness will DOUBLE
>the execution time of that instruction.  The upshot of all this is that for
>time-critical code, you should use 32-bit instructions in 32-bit segments and
>16-bit instructions in 16-bit segments.  Since djgpp executes all of its
>code in 32-bit segments, this means you should avoid 16-bit operands like
>the plague.  Avoid the "short" data type when reasonable.

>Christopher


that would explain why a program i made takes 30 seconds compiled under DJGPP
and 20 with other compiler that doesn't use protected mode. In that program
I make lots of calculations with BYTES. You mean if I replace every where i
put UNSIGNED CHAR for UNSIGNED INT i will get a better perfomance ?

When i get some time i'll do a couple of defines and i'll tell ya whats on !


frankie