On Sun, Aug 03, 1997 at 04:03:26AM -0400, Mike wrote:
> At 05:41 PM 8/3/97 +1000, you wrote:

> I seem to recall that the 'true' 32 bit code was slower and occupied more
> space than 'selectively' preparing the appropriate segment registers, doing

32 bit code tends to be smaller (because of better addressing modes) - if you
avoid doing any segment register loads, 16 bit code could execute as
fast as 32 - it's unlikely to be faster except in very exceptional
circumstances. In practice, any 16 bit Windows application will use
segment register loads, which really kills performance (each segment
register load actually loads several hidden registers from the segment
table).

> can't see how you could write an OS which ONLY had 32 bit instructions on
> the 486/Pentium processor - there must be some 8/16 bit instruction - at

Of course. The 8 bit addressing is still available in 32 bit mode, and
has no performance penalty, but trying to use 16 bit index registers is
costly, and offers no benefit (since the 386 and up have 32 bit data
buses anyway).

> least in terms of word size - is there a 32bit IO space as well as memory

Again, of course the address space is 32 bit - the external data bus
requires that. But there's no problem accessing data 8 bits at a time, it's
just you get a 32 bit address put out (though the external hardware may see
less than 32 bits).

--
Clyde Smith-Stubbs  |HI-TECH Software,      |Email: clyde@htsoft.com
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