From:	Paul Shirley <Paul AT foobar DOT co DOT uk DOT chocolat>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Weird problem
Date:	Wed, 19 Mar 1997 19:01:40 +0000
Organization:	wot? me?
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Distribution:	world
Message-ID:	<LRn33DAUgDMzEwli@foobar.co.uk>
References:	<33287185 DOT 892303 AT news DOT flash DOT net> <33287D83 DOT 63AD AT shoreland DOT com>
	<332aa0d0 DOT 1431214 AT ursa DOT smsu DOT edu> <5ggll4$6pb AT freenet-news DOT carleton DOT ca>
	<332c1c1c DOT 1840126 AT ursa DOT smsu DOT edu>
Reply-To:	Paul Shirley <junk AT defeating DOT email DOT address>
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To:	djgpp AT delorie DOT com
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In article <332c1c1c DOT 1840126 AT ursa DOT smsu DOT edu>, Tony O'Bryan
<aho450s AT nic DOT smsu DOT edu> writes
>In protected mode, the default register and word size are 32-bits.  In real
>mode, both are 16-bits.  The '386+ processors use the same code sequence to
>represent ax and eax.  They also use the exact same code for 32-bit overrides
>(from real mode) and 16-bit overrides (from protected mode).  So to access a
>16-bit memory address using ax from protected mode, an operand-size override is
>needed.  This override is 1 byte (66h) long and is included in the processor
>prefetch cycle.  It causes a delay from the processor's perspective (given that
>the processor can see delays 1 nanosecond long *grin*), but won't cause an
>appreciable delay for the user.

Wrong for a Pentium, which treats the prefix as a 1 clock unpairable
instruction. In the worst case (consecutive simple prefixed ops) this
can cause a 4* speed reduction!

The real problem is more subtle. gcc is inherently a 32 bit compiler,
and seems to have real problems understanding 'short's. It will
frequently generate code to set the upper half of registers even though
you and I know it makes no difference to the result!

---
Paul Shirley: shuffle chocolat before foobar for my real email address

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