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

Lyle Bickley lbickley at
Sat Sep 17 14:03:20 CDT 2016

On Sat, 17 Sep 2016 11:38:13 -0700
Guy Sotomayor Jr <ggs at> wrote:

> > On Sep 17, 2016, at 10:10 AM, Chuck Guzis <cclist at> wrote:
> > 
> > On 09/17/2016 09:23 AM, Eric Smith wrote:
> >   
> >> I don't know what the width of the TMS9900 ALU is, but I'm pretty
> >> sure it's not bit-serial, as an add instruction only takes 14 clock
> >> cycles, including four memory cycles. I'd be very surprised if the
> >> ALU isn't either 8 or 16 bits, though 4 might be possible.
> >> 
> >> Possibly someone is confused by the bit-serial "CRU' I/O space,
> >> but that is unrelated to the ALU width.  
> > 
> > Could very well be--I'm just going by other's appraisals of the
> > architecture.
> > 
> > But there were some "8 bit" MPUs with bit-serial ALUs, so the
> > question is still valid.
> >   
> Why?  What does the width of the ALU have to do with the “bitness” of
> the architecture?  If the programmer’s view is 8-bits (or 16 or 23,
> or ??), what does it matter (other than performance) what the width
> of the internal data paths or ALU are?
> It’s interesting from an implementation point of view but not really
> anything else.
> In a previous email, I mentioned the IBM 360 as an example of a
> 32-bit machine (architecture) that had 8, 16 and 32 bit internal data
> paths and I don’t think anyone would suggest that the 360 models that
> did not have 32-bit data paths wasn’t a 32-bit “machine”.
> The same could be said for the PDP-8/s.  That’s a bit serial machine
> but it is a member of the PDP-8 family.  Would you call it a 1-bit
> machine or a 12-bit machine?
> That’s why (in my previous email) I made the distinction between
> architecture and implementation.  The reference “machine” (which I’ve
> intentionally used here) is somewhat ambiguous and I tend to use
> architecture or implementation when I want to be specific.
> TTFN - Guy

Since I have a running PDP-8/S as an example, I want to back up what Guy
has said.

The User Manual for the PDP-8/S says: "The PDP-8/S is a one-address,
fixed word length, serial computer using a word length of 12-bits plus
parity and two's complement arithmetic."

So the architecture is 12-bits and the implementation is serial.

However, just to muck things up - it is really implemented both as a
serial and parallel machine! The logic is one-bit serial - and the
memory is 12-bit parallel. The serial logic runs on one clock - and the
memory on another clock. It's one of the things that makes debugging a
PDP-8/S and "interesting" experience ;)

73      AF6WS
Bickley Consulting West Inc.

"Black holes are where God is dividing by zero"

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