Variable Word length CPU

[stevew]

"Dwight K. Elvey" <dwight at ca2h0430.amd.com> Wrote:
>Hi
>My thought is to make a variable word length processor using
>a single bit ALU. Of course, I've been thinking in terms of a relay
>based machine.
>Dwight

Hi Dwight,

Having helped design a variable word length computer (B1000 series machines 
(B1955, B1965), I'm of the opinion that it isn't a useful construct.  
Burroughs justified the idea by saying they ran "microcode" on the hardware. 
It was really a basic machine language just like any other computer.  The 
only real difference was that all the compilers compiled down to intermediate 
languages which were run on interpreters.  It didn't yield very good 
performance even though the hardware wasn't that slow for it's era.

There are a couple of interesting side effects of the choices they made.  They 
needed to have a variable word length rotate capability, and also chose to 
address memory on bit boundaries. If you had a 13 bit word length, then 
memory delivered 13 bit elements to you (wasting alot of bandwidth in the 
process since it was organized by 32...)

You also need the ability to pull off the carry bit from any particular point 
in the ALU.

If I were the Burroughs architect, I wouldn't have gone to those extremes 
knowing what I know now.

Steve Wilson