Re: IIgs RAM card Question

[dempson@actrix.gen.nz (David Empson)]

Roy and/or Janet Miller <millers@inetnebr.com> wrote:

> When Intel designed the 8086 (which, as I'm sure you know, is older than
> the 8088)

Yes, I did know that.

> they wanted to make it as similar to the 8080 as possible so
> that programmers would have a less steep learning curve.

Makes sense.

> Motorola, not having the same success with the 6800 series as Intel did
> with the 8080 series, didn't make the 68000 as similar to the 6800 as
> Intel did with 8086.

I suspect (without any detailed investigation, particularly as far as
dates are concerned) that the evolution path was:

6800 -> 6809 -> 68000

My reasoning: the 6809 is similar to the 6800 architecture, but adds
more advanced addressing modes (indirection, post indexing, several
indexing methods) and a few more registers.  The 68000 has a similar
(but more powerful) set of addressing modes, with a more generalised
register architecture (numbered address and data registers), and changes
to 32 bits internally, of course.

Another possibilty is that the 68000 and 6809 were developed in
parallel, with the 68000 being a fresh design and the 6809 trying to
shoehorn a reasonable subset of its features into an 8/16-bit micro.

There are other 6800 derivatives, such as the 6801, which led to the
68HC11 (very popular in embedded devices) and later chips.

> Which, so I've read, is why the 680X0 is better than the 80X86. What are
> your thoughts opinions on that?

The 68000 is certainly a cleaner processor architecture than the 8086 -
linear address space, for starters.  Apparently IBM was originally
considering the 68000 for its IBM PC, but decided on the 8088, probably
for cost reasons more than anything else (only having to use 8-bit wide
memory instead of 16-bit wide, and thinking that "noone needs more than
1 MB address space").  Yeah, right. :-)

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P O Box 27-103, Wellington, New Zealand