short answer to technical question?
Berna Massingill
bmassing at cs.trinity.edu
Thu Nov 17 00:46:00 UTC 2005
On Wed, Nov 16, 2005 at 02:26:12PM -0600, Nix, Robert P. wrote:
>> Actually, in this case 32 bit and 64 bit refer not to the natural
>> word size, but to the address size. i.e. a 32 bit machine uses a 32
>> bit address for memory, and cannot address as much memory as a 64
>> bit machine, using 64 bit addresses, can.
>>
>> In the case of the IBM mainframe computers, the natural word used to
>> be 32 bits, but they were 31 bit machines, as the top bit of the
>> word was reserved for something other than an address bit. The new
>> mainframes are 64 bit machines, using all 64 bits of an address to
>> address memory. They're still 32 bit words though; they just happen
>> now to use two of them for an address.
There was also an earlier era (late 1970s and before) in which these
machines used 32-bit words but only 24-bit addresses, reserving an
entire byte of each word for other purposes. The transition from
24-bit addressing to 31-bit addressing was -- messy? a massive
kludge? -- since for reasons of upward compatibility IBM ended up
providing support for a while (possibly still??) for two addressing
modes.
There's probably a lesson here for future architects, something along
the same lines as not saying in public that 640K ought to be enough
for anyone. :-)?
>>
>> (Actually, the smallest thing the mainframe would deal with for a
>> long time (early 1980's forward) has been 64 bits, or a double-word.
>> The main change has been the use of larger addresses.)
>>
-- blm
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