Meaning of "architecture width" - Re: 68K Macs with MacOS 7.5 still in production use...

[Chuck Guzis]

On 09/15/2016 11:18 PM, ben wrote:
> On 9/15/2016 6:56 PM, Chuck Guzis wrote:
>
>> So what's the width of an IBM 1620?
>
> I don't expect to fit in a standard rack... runs.
> Ben.
> PS: I think it is 12 bits, to offset the slow core speed
> if I am thinking of the right machine.

Nope, that can't be.  1620 is a decimal machine (normally) and while the
memory interface is 12 bits, two of those bits are parity bits, so you
can't count those.

That leads to "bytes" of 10 bits, two of which are flag/sign bits, so
they don't really enter into computation.  That is, the smallest unit on
which one can do arithmetic is two decimal digits,  with one flag bit
serving as field demarcation and the other serving as sign.

While you might call this a 10 bit machine, that's misleading.  There
are special bit patterns that mean things (such as 8-2 bit, which means
a record mark, but the special combinations must be read, as there's no
way to generate or test for them programatically.

There are no visible registers; i.e., there's no way to query the
P-counter directly.

My point is that there are many machines that defy simple labels.
Harvard architecture machines can be sioppery--take the humble PIC it
its simplest form--12 bit instruction words that may also contain data,
but 8 bit computation and data registers.

I'm aware of supercomputers with 512 bit memory paths that, for example,
can compute 128 bits per cycle, but have 64-bit GP registers.

--Chuck