bit-widths, was Re: HP Laserjet ..again....

From: Lyle Bickley <>
Date: Wed Sep 22 17:49:50 2004

The IBM 7090 was the first machine I worked on - but I am familiar with the
IBM 7070.

The IBM 7070 was modeled after the IBM 650 - so the 7070 was a bi-quinary (2
out of 5) encoded machine. There were 3 accumulators and 99 index registers.
Memory locations were used for the accumulators (9991, 9992 and 9993), the
program counter was 9995. The index registers were memory locations 1-99.

Technically memory ran from 0-9999, but maximum memory was specified as 9900
words - as that was approximately what was available as "standard" (non
register) memory.

Memory words were 10 decimal digits plus sign


On Wednesday 22 September 2004 15:20, Jim Isbell wrote:
> The IBM 7070, The first computer I worked on, had 10 bits per byte and
> 9K of memory. The 10 bits must have been a hold over from the decimal
> system. I have no idea why there was 9K of memory.
> Tom Jennings wrote:
> >>If
> >>you hold the word width constant, yes, you are right. But that is not
> >>what I was talking about. In many early computers, the data buss and
> >>the word width were the same.
> >
> >... and many did not. The 'byte' as a convention for talking about
> >memory is just that, a convention, and fails miserably on machines whose
> >major casual metric is not a multiple of 8 bits. Many, many machines
> >were built on a multiple of 6 bits because that's how many it took to
> >define a character.
> >
> >For machines which have some architectural feature > 8 but modulo 8 ==
> >0, 32- and 64-bit wide memory and paths could be byte-addressed. I don't
> >know for sure, but I would imagine there are 6-bit-character-addressable
> >instruction sets too.
> >
> >Until more or less when CPUs fit entirely within silicon, there was no
> >hard and true correlation between the bit-widths of busses, registers
> >and paths; this was because constructing those things cost actual money
> >and scaling of silicon didn't exist. Lots of machines have different
> >width regs/accumulator, memory, index regs, program counters, arithmetic
> >units, etc.
> >
> >(My LGP-21 is a good example: 32-bit accumulator, 31-bit memory, 12-bit
> >program counter, double-32 product reg, 4- or 6-bit I/O.)
> >
> >(Nothing in a Microchip Inc PICxxxx except the register files is
> >byte-width!)
> >
> >For non-multiple-of-8 machines, the 'byte' is not relevant generally.
> >
> >Boy with hammer: everything looks like a nail.

Lyle Bickley
Bickley Consulting West Inc.
"Black holes are where God is dividing by zero"
Received on Wed Sep 22 2004 - 17:49:50 BST

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