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Path: utzoo!linus!philabs!pwa-b!mmintl!franka
From: franka@mmintl.UUCP (Frank Adams)
Newsgroups: net.arch
Subject: Re: Re: Re: RISC (really on multiplication d
Message-ID: <537@mmintl.UUCP>
Date: Thu, 25-Jul-85 15:33:05 EDT
Article-I.D.: mmintl.537
Posted: Thu Jul 25 15:33:05 1985
Date-Received: Sun, 28-Jul-85 05:25:37 EDT
References: <149@mips.UUCP> <600005@pbear.UUCP> <493@mmintl.UUCP> <233@weitek.UUCP> <1202@sjuvax.UUCP>
Reply-To: franka@mmintl.UUCP (Frank Adams)
Organization: Multimate International, E. Hartford, CT
Lines: 26
Summary: 8086 does not


In article <1202@sjuvax.UUCP> jss@sjuvax.UUCP (J. Shapiro) writes:
>> > ... one multiplies two words and wants a word result.
>> > But hardware multiplies invariably generate a two word result, leaving the
>> > high-order word to be allowed for and/or disposed of.
>> 
>> The Decsystem-10 & 20 has an instruction IMUL which does exactly what you
>> want.
>
>For comparison and elucidation, so do:
>
>	VAX
>	PDP-11
>	National 32016 and family
>	Motorola 68000, 68020, 6800, 6809, ...
>	Zilog Z8000
>	Z80
>	Intel 8086, 80186, 82086
>

All right, I shouldn't have said *invariably*.  The PDP-10 is a good
machine.  But while I am not sufficiently familiar with the others in
this list, the 8086 does *not* have such an instruction.  You can multiply
two 8-bit quantities to get a 16-bit result, or two 16-bit quantities
to get a 32-bit result.  The MUL/IMUL distinction on the 8086 is for
signed vs unsigned operands.