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

From: Jim Isbell <millenniumfalcon_at_cableone.net>
Date: Wed Sep 22 17:20:33 2004

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 - 17:20:33 BST