Messages in this thread |  | | Date | Wed, 19 Sep 2001 00:06:26 -0400 | From | Brian Gerst <> | Subject | Re: [Q] Implementation of spin_lock on i386: why "rep;nop" ? |
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Jamie Lokier wrote: > > Alan Cox wrote: > > > The "rep;nop" line looks dubious, since the IA-32 programmer's manual from > > > Intel (year 2001) mentions that the behaviour of REP is undefined when it > > > is not used with string opcodes. BTW, according to the same manual, REP is > > > supposed to modify ecx, but it looks like is is not the case here... which > > > is fortunate, since ecx is never saved. :-) > > > > rep nop is a pentium IV operation. Its retroactively after testing defined > > to be portable and ok. > > Are we sure that the value of ECX doesn't matter on a 386? Or does it > count down doing nops ECX times on a 386?
Older processors ignore the rep prefix when used with non-string opcodes. %ecx should not be affected.
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