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From: kds@mipos3.UUCP (Ken Shoemaker ~)
Newsgroups: comp.sys.m68k,comp.sys.intel
Subject: Re: Recent Motorola ad seen in Byte
Message-ID: <639@mipos3.UUCP>
Date: Mon, 4-May-87 16:00:33 EDT
Article-I.D.: mipos3.639
Posted: Mon May  4 16:00:33 1987
Date-Received: Tue, 5-May-87 06:37:11 EDT
References: <8705030305.AA18195@cory.Berkeley.EDU>
Reply-To: kds@mipos3.UUCP (Ken Shoemaker ~)
Organization: Intel, Santa Clara, CA
Lines: 23
Xref: mnetor comp.sys.m68k:444 comp.sys.intel:220

and, of course, the win of having special registers for certain operations
is that operations with those registers can be made faster than in normal
circumstances.  For example, in the 386 (and 286) there is special hardware
in the bus interface unit that is used in the string move instruction to
effect string transfers at the bus bandwidth.  Trying to do this in a general
instruction loop would be difficult in that it would require looking at a
couple of instructions and decoding them as a group.

Also, with respect to specifying operands, the decoding of the instruction
itself is an important part of the execution of the instruction.  In the
8086 there isn't a way to specify any more than two different operands, and
if you specify two operands, one of them is a register.  Thus, for a string
move instruction, you'd be at a loss to try to specify all the operands
you need explicitly unless you increase the complexity of the instruction
decode.
-- 
The above views are personal.

...and they whisper and they chatter, but it really doesn't matter.

Ken Shoemaker, Microprocessor Design, Intel Corp., Santa Clara, California
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