Re: What is an N-bit machine?

According to Michael S  <already5chosen@yahoo.com>:

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.

S/360 had 24 bit addresses and 32 bit registers. When doing address arithmetic
the high 8 bits of the register were ignored. That turned out to be a really bad
decision since a few instructions and a lot of programming conventions stored
other stuff in that high byte, causing severe pain a few years later when
memories got bigger than 16 meg. The kludge in S/370 was to use the high bit as
a flag, 0 meant 24 bit addressing, 1 meant 31 bit addressing. That worked
reasonably well although they came up with yet more kludges to let programs
switch among multiple 31-bit address spaces. They finally bit the bullet in 2000
when they announced zSeries with 64 bit addresses and registers.

These days I'd say the relevant N is the size of arithmetic registers but a
lot of marketers appear to disagree with me.

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