21 Nov
2007
21 Nov
'07
2 a.m.
Chuck Guzis wrote:
In 16-bit mode, I seem to recall that the 386SX was a
travesty of a
CPU; a 16 MHz SX ran nowhere near as fast as a comparably-clocked
286.
I find that incredibly hard to believe, looking at the instruction
timings for both processors. What are you basing this on? Memory timings?
The only thing I can think of that might substantiate this claim is when
you were working with 32-bit registers in 16-bit DOS. Each instruction
needed a 66h or 67h prefix opcode to indicate 32-bit. This was wasteful
on a 386sx because the 16-bit memory interface ate more time fetching
the additional opcodes than you were saving by using 32-bit registers
(!). A friend of mine wrote a 386 extender ("pmode", circa 1992)
specifically to get the full use of his 386sx because he was royally
cheezed when he learned of this.
Some 286 vendors made a big thing of
the fact that a 286 could execute 16-bit real mode code substantially
faster than the 386SX. For example:
http://www.intersil.com/data/an/an121.pdf
I don't think that's a fair comparison because that's not a system,
that's a CPU variant (80C286).
--
Jim Leonard (trixter at oldskool.org) http://www.oldskool.org/
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