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From: baxter@zola.ics.uci.edu (Ira Baxter)
Newsgroups: comp.arch
Subject: Odd word size?  Word size?  Word?  Definitely Odd.
Summary: Folk seem to have forgotten word mark machines
Message-ID: <270CB430.21156@ics.uci.edu>
Date: 5 Oct 90 16:26:24 GMT
Lines: 75
Nntp-Posting-Host: zola.ics.uci.edu
bcc: baxter

There has been recent query on machines with word sizes that
were not a power of two, related to algorithms that used binary addressing.

It has been a long time since I touched an IBM 1620 or 1401, and my
memory is a bit fuzzy on the details.  Real oldtimers, feel free to
correct the fine details.  The 1401 was quite similar in style to the 1620,
so I'll skip describing it.

If I remember right, these machines violate not only the notion
of power of two word sizes, but even the notion of word size.

The IBM 1620 had a 5-bit "digit", which could represent, using 4 bits,
a decimal digit or some special codes, one of which was called a
record mark.  The 5th bit was called a "mark" bit.  Addressing
granularity was to the "digit" level.

There were no registers (take THAT! mr. optimal register allocator)
save some peculiar regions in lower memory that acted like index
registers; all operations were memory to memory.

One defined an arithmetic data
object by using specifying the (decimal!!) address of the least
significant digit.  The arithmetic data object extended from there
thru more significant digits (at successively lower addresses) until a
digit was found with the mark bit set.  One defined a numeric field at
runtime by applying a "SETMARK" instruction at addresses representing
Most Significant Digits of areas to be used as numbers, although I
think the assembler would arrange this for you automatically.  It was
fun to "clear core" (by MOVing a single digit number without mark bit
to its own address+1) and then enter an add-to-self instruction; this
put the machine into an infinite loop adding the digits of the
enormous number defined by the complete absence of mark bits.

String-class objects worked the opposite way; the lowest address digit
was specified, and the object ended when a digit containing a record
mark code was found.  I think characters were formed by taking pairs
of digits.  Instructions were 12 digits, a 2 digit opcode, and 2,
5-digit addresses; I think the MSD of the opcode had to have the mark bit set.
I forget precisely what happened when you gave an
odd-sized string to instructions that printed strings on the console,
printer or card punch (Who needs PRINTF?  The hardware did it
directly!), but the 1620 had lots of entertaining ways of aborting
your program, including the infamous "CHECK" light, which froze the
machine whenever a malformed instruction, or numeric operand
(containing a record mark) was encountered.

Earlier 1620s had a multiply instruction which worked by looking
up multipler/multiplicand digit pairs in the multiplication table
in lower memory.   This table had to be loaded correctly before
you used multiply.  There are apocryphal stories of folk loading
the table in such a way that multiplies worked in octal rather
than the standard decimal.

I don't remember any boolean operations such as AND or XOR.
They'd be a bit out place in a decimal machine, certainly out
of character.

It was amazing easy to enter raw machine code.  You could
literally type it in on the console terminal or enter via card
punch.  I spent many enjoyable hours doing this (forgive me,
this was the machine on which I learned assembly.  The 360
was total surprise!).

Anyway, to summarize: You can't talk about word size when you don't
have the notion of word.  If you insist on a digit as "word size", I
can't imagine an odder size than 5 bits.  Addresses were all 5 digits
and decimal.

I don't think I'd try to run an algorithm on this machine that
depended fundamentally on powers of two related to word size.

I suspect this machine would put a "C" programmer into a rubber room.

--
Ira Baxter