Sujet : What is an N-bit machine? De : jgd (at) *nospam* cix.co.uk (John Dallman) Groupes :comp.arch Date : 28. Nov 2024, 16:31:28 Autres entêtes Organisation : A noiseless patient Spider Message-ID :<memo.20241128153105.12904U@jgd.cix.co.uk>
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.
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.