Path: utzoo!news-server.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!ucbvax!agate!darkstar!felix!haynes
From: haynes@felix.ucsc.edu (99700000)
Newsgroups: comp.arch
Subject: Re: 48-bit computers
Message-ID: <13215@darkstar.ucsc.edu>
Date: 9 Mar 91 05:52:15 GMT
References: <666@spim.mips.COM>
Sender: usenet@darkstar.ucsc.edu
Organization: University of California, Santa Cruz Open Access Computing
Lines: 19


In article <666@spim.mips.COM> mash@mips.com (John Mashey) writes:
>In the B6500 and later, they added 3 more bits for tags, which seemed to
>me to be cleaner, as it left the fairly clean number 48.

Well, yes and no.  With the 48-bit B5500 you could write words to tape as
8 six-bit characters.  With the 51-bit B6500 and later you could write words
to tape as 8 six-bit characters by omitting the tags (or 6 eight-bit
characters with 9-track tape) but now you've got the tag bits to do something
with.  So you need two different tape formats, one for pure data without
the tags, and one that preserves the tags.

Not that the problem is new or unique to the B6500.  It goes back at least
to the IBM 1401, where you could write six-bit characters on tape but if
you wanted to preserve word marks you had 7-bit characters to deal with.
And the G.E. (later Honeywell) machines have 36 bit words and 9-bit bytes
because 9 is a divisor of 36 and 8 isn't.  So again you can write tapes
that hold all the bits of a word, or that hold 8-bit ASCII data with the
ninth bit of each byte omitted.