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

From: Jim Isbell <millenniumfalcon_at_cableone.net>
Date: Wed Sep 22 18:52:06 2004

I worked on the 7070 in 1959/60 time frame. It was a wonder of science
at the time. I remember looking at the new frame of core memory that
we had just added and imagining that if I stuck a screw driver into it
and twisted it around, that the computer might scream.

Lyle Bickley wrote:

>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
>
>Lyle
>
>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.
>>>
>>>
>
>
>
Received on Wed Sep 22 2004 - 18:52:06 BST