Chips that changed the world

[Brent Hilpert]

Tony Duell wrote:
>
> Andrew Burton wrote:
> > Thanks for the link, that was a very interesting read.
> > However, isn't there a mistake in the Motorola 68000 description. Doesn't i=
> > t use 32-bit addressing and not 24-bit?
> 
> The addres _registers_ were 32 bits wide, but only 24 address lines were
> brought off-chip. There are actually 23 address pins (A1...A23), becuase
> A0 is determined from the states of UDS/ and LDS/

Keeping in mind as well, that at the the time of the 68000 release ca. '79/80,
16MB=2^24 was a lot of memory. Going beyond 16-bit physical addressing was
warranted in terms of market/economics, the same could not be said for 32-bit
physical addressing for a microprocessor system. 24 bits was a practical
goldilocks solution.

The 32-bit initial architecture provided for future upgrade/compatibility, and
was addressed (pun) in later versions.

(Didn't the IBM 360 architecture vs. implementation take the same upgrade path?)