MC68000 "size" (was Re: modern serial terminal)

Ethan Dicks ethan.dicks at usap.gov
Wed Nov 7 18:28:50 CST 2007


On Wed, Nov 07, 2007 at 03:42:16PM -0800, Eric J Korpela wrote:
> On Nov 7, 2007 1:26 PM, woodelf <bfranchuk at jetnet.ab.ca> wrote:
> > The 68K is 32 bit cpu
> 
> We'll have to agree to disagree on that.  MC68020 is the first 32 bit
> CPU in the 68K line.

As will you and I... the MC68020 is unarguably a 32-bit processor, but the
MC68000 is much more a 32-bit processor than a 16-bit processor. 

The external data bus _is_ 16 bits for the MC68000, but the internal
register model is 32 bits.  The address bus is 24 bits, not 32, but
a 16-bit processor like the 80286 or the PDP-11 has 16-bit address
registers, and can't access more than 64K without register segments or
an MMU or something similar.  The MC68000 needs no additional decoding
hardware or tricky software to address its full 16M address space (which
is the same size address bus as the VAX-11/750, which to all accounts
_is_ a 32-bit processor).

There's a reason I described the 68000 as a 16/32 processor earlier
today... internally it *is* 32 bits.  The bus interface is just reduced
to save on the pin count, etc.

Look at the size of the ALU and the accumulator(s)/data registers, then
come back to me.  

-ethan

-- 
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Ethan.Dicks at usap.gov            http://penguincentral.com/penguincentral.html



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