short answer to technical question?

[Berna Massingill]

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