Linux-kernel mailing list archive 1998-50,: NOP instruction

# NOP instruction

Chipzz (chipzz@Ace.ULYSSIS.Student.KULeuven.Ac.Be)
Tue, 15 Dec 1998 04:42:29 +0100 (CET)

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On Mon, 14 Dec 1998, Riley Williams wrote:

> From: Riley Williams <rhw@bigfoot.com>
> Subject: Re: [PATCH] i386 entry.S alignment tweak
>
> Hi Jamie.
>
> > BTW, they're not nop instructions these days: they're the largest
> > filler instructions gas can put in, so an alignment of a few bytes
> > needn't take any longer to execute than a single byte alignment.
>
> I remember seeing some assembly that had the following sequence in the
> middle thereof (not for gcc but you can probably decode it):
>
> LOCK
> SEG ES
> MOV DI,DI
>
> The first time I read through that, I wondered what on earth it was
> supposed to do, and only later did I realise it was effectively a
> rather verbose NOP instruction...

I've always wondered... A much used NOP "instruction" is
OR AX,AX (or similar);
But doesn't this, as well as MOV DI,DI, set the zero flag???
As a NOP instruction is supposed to do nothing, would this then be wrong?
(Nothing meaning not setting any flags?)

Hmmm... Where's the time from the good ol' Z80 when you HAD a real NOP
instruction? *Sigh* :-)

Chipzz AKA
Jan Van Buggenhout

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Chipzz@Ace.ULYSSIS.Student.KULeuven.Ac.Be
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