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

From: Tom Jennings <tomj_at_wps.com>
Date: Wed Sep 22 18:21:19 2004

I'm honored to be around so many 2nd-gen machine users!

Clearly the 'byte' discussion applies to binary machines. I assume the
unit for decimal machines is simply a 'digit'?


On Wed, 2004-09-22 at 15:49, 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 - 18:21:19 BST