Re: What is an N-bit machine?

jgd@cix.co.uk (John Dallman) writes:

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.

360 had 32bit registers but addressing only used 24bits (16mbyte)
... except for 360/67 virtual memory mode which had 32bit addressing
(when got around to adding virtual memory to all 370s, it was only 24bit
addressing ... it wasn't until the 80s with 370/xa that 31bit addressing
was introduced). 360/67 also allowed for all (multiprocessor) CPUs to
address all channels (360/65 multiprocessor and later 370 multiprocessor
simulated multiprocessor I/O with multi-channel controllers connected to
different dedicated processor channels at the same address).

360/67
https://bitsavers.org/pdf/ibm/360/functional_characteristics/A27-2719-0_360-67_funcChar.pdf
https://bitsavers.org/pdf/ibm/360/functional_characteristics/GA27-2719-2_360-67_funcChar.pdf

Before 370/xa, MVS was getting so bloated that they did hack to 3033 for
64mbyte real memory ... still 24bit (real & virtual) instruction
addressing ... but they scavanged two unused bits in the virtual memory
16bit PTE, used to prefix the 12bit page numbers (4096 4096byte pages
... 16mbyte) for 14bit page numbers (16384 4096byte pages) aka
translating 24bit (virtual) addresses into 26bit (real) addresses
(64mbytes) ... pending 370/xa and 31bit

--
virtualization experience starting Jan1968, online at home since Mar1970