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From: dricejb@drilex.UUCP (Craig Jackson drilex1)
Newsgroups: comp.arch
Subject: Re: 48-bit computers
Message-ID: <24963@drilex.UUCP>
Date: 21 Mar 91 04:31:18 GMT
References: <6684@hplabsz.HP.COM> <1991Mar14.054359.27231@sma2.uucp> <1991Mar18.220827.27542@unislc.uucp> <1126@spim.mips.COM>
Organization: DRI/McGraw-Hill, Lexington, MA
Lines: 26

In article <1126@spim.mips.COM> mash@mips.com (John Mashey) writes:
>In article <1991Mar18.220827.27542@unislc.uucp> ram@unislc.uucp (Ram Madduluri) writes:
>
>>Unisys A-Series machines are 48bit machines and are currently available
>>in the market....
>
>Is that true? I thought the A-series was 51 bits or more, upward-compatible
>from B6500, etc.  Did these go back to the 48 used by B5500?

The A-Series, like the B6500 before it, has 48-bit words visible in the
applications architecture.  E-mode level Beta and Gamma machines also have
4 tag bits with each word; the B6500 had 3.  All of the machines had
various other bits associated with each word for parity, SECDEC, etc.

If one wishes, one can consider the A-Series a 52-bit architecture from
an operating systems viewpoint.  However, the tag bits cannot take on
arbitrary values--the meanings are defined by the hardware.

From the view of applications other than compilers and linkers, it is
a 48-bit machine.  The tag bits are not addressable.  (If they were, the
machine would not be secure--the A-Series (and the B6500) have no
hardware distinction between kernel and user mode.)
-- 
Craig Jackson
dricejb@drilex.dri.mgh.com
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