Re: What Is The Difference Between 68000 Chip And 80x86 Chip?

[pausch@saaf.se (Paul Schlyter)]

In article <20011203212320.22386.00001047@mb-md.aol.com>,
Michael J. Mahon <mjmahon@aol.com> wrote:
 
> Perhaps the most significant difference between Motorola and
> Intel processors is in the way that memory is addressed.  The
> 68xxx processor line offers linear addressing, meaning that all
> addressible memory can be named by counting up an address,
> without discontinuities.  On the other hand, x86 processors use
> segmented addressing, in which a pointer can only directly address
> a limited segment of the address space, and another "segment"
> register must be changed to move to another segment.  This model
> finally gave way with the 386, which offfered support for linear
> addressing, as well.
 
All Intel x86 chips supported linear addressing in the same way: by
setting all segment registers to the same value.  The news with the
386 was that the address space grew from 16 bits to 32 bits.  Thus
you could do linear addressing also on the 286 and 8086, but then
you'd have to remain within a 64K address space, which was too
restrictive: people did prefer segmented addressing over an address
space limited to 64K.
 
> But the difference created very different views of software
> creation on the two different processors, some of which persist
> to this day.  Needless to say, programmers are not overly
> fond of segments.  ;-)
 
Segments are more convenient than the (at that time) alternative of
overcoming the 64K address limits: bank switching.  That's _really_
inconvenient.
 
-- 
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