Re: What is an N-bit machine?

On Thu, 28 Nov 2024 15:31 +0000 (GMT Standard Time)
jgd@cix.co.uk (John Dallman) wrote:

In early computer designs, arithmetic registers were much longer than
addresses, the classic examples being machines with 36-bit words and
15- to 18-bit addresses.
 
Large logical address spaces started with the IBM 360, which had
32-bit arithmetic registers and 32-bit address registers. You
couldn't put 32-bits worth of physical memory in a machine for over a
decade after it appeared, but it was allowed for in the architecture.
 

S/360 normally referred as 24-bit address architecture.
S/370 normally referred as 31-bit address architecture.
I don't know technical reasons for it, but would think that they exist.

Nowadays, the bit-ness of a machine seems to be the *smaller* of the
arithmetic registers and the address space. This became true in the
early 1970s, as far as I can see, and the terminology became confused
around then. A few examples from that period:
 
Classic "8-bit" microprocessors, such as the 8080 or 6800 have 8-bit
arithmetic and 16-bit addressing.
 
The PDP-11 has 16-bit arithmetic and 16-bit addressing, plus
bank-switching.
 
The original 8086 has 16-bit arithmetic and a strange 20-bit
addressing scheme.
 
Modern architectures have arithmetic and address registers that are
the same size.
 
John