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From: kds@intelca.UUCP (Ken Shoemaker)
Newsgroups: net.micro.pc
Subject: Re: Re: Clock Interrupt Screw-Up?
Message-ID: <116@intelca.UUCP>
Date: Tue, 8-Oct-85 00:21:09 EDT
Article-I.D.: intelca.116
Posted: Tue Oct  8 00:21:09 1985
Date-Received: Sat, 12-Oct-85 19:06:36 EDT
References: <822@gitpyr.UUCP> <201@mips.UUCP>
Organization: Intel, Santa Clara, Ca.
Lines: 49

> Without seeing the code, this is purely a guess ...
> 
> Sounds alot like you've been bitten by the "multiple prefixes on a string-op"
> feature of the 8086/8.  Let's say, for example, you code:
> 
> 		rep movs   ES:byte ptr[DI],CS:[SI]
> 
> to move a string.  "rep" is a prefix to the "movs" instruction, but so is
> the "CS:" seg-reg override byte.  As long as there's no interruption, this
> works just fine.  If a rupt occurs in the middle of the move though, watch
> out!
> 
> .........................
> 
> Anyway, if you find any such beasties, just stick a "cli/sti" pair around
> the offender, and maybe send a "Thank you" note to those wonderful folk who
> brought you segmentation in the first place :-).  Hope this helps.
> 
> BTW, anyone know if this stunning example of architectural innovation is still
> with us in the 286?
> 
> /kim
> 
> "Anything free is worth what you pay for it."  --  Lazarus Long
> 
> -- 
> 
> UUCP:  {decvax,ucbvax,ihnp4}!decwrl!mips!kim
> DDD:   415-960-1200
> USPS:  MIPS Computer Systems Inc,  1330 Charleston Rd,  Mt View, CA 94043

Actually, the "normal" order of prefixes puts the repeat prefix before the
segment override prefix, so that what happens then is that you don't move
enough words or whatever.  In this case, the fix is even easier: you just
check the cx register after the string move and loop if it isn't zero.
I agree that this isn't the best of all possible worlds, and this little
feature isn't also featured on the 186 or the 286: they iret you to the
first prefix, so the string move picks up correctly where it left off.
What this has to do with segmentation, I'll never know...
-- 
...and I'm sure it wouldn't interest anybody outside of a small circle
of friends...

Ken Shoemaker, Microprocessor Design for a large, Silicon Valley firm

{pur-ee,hplabs,amd,scgvaxd,dual,qantel}!intelca!kds
	
---the above views are personal.  They may not represent those of the
	employer of its submitter.