PDP-11 Addressing Modes

[Johnny Billquist]

On Wed, 15 Apr 1998, Pete Turnbull wrote:

> On Apr 14, 23:53, Allison J Parent wrote:
> > <What do people here on the list think of the flexibility and
> > <generality of the PDP-11's addressing modes?  Is this a well thought
> > <out architecture in your view?  How are the PDP-11's addressing modes
> > <better or worse than those of other processors, past and present?
> >
> > Personally I consider it a high point in 16 bit computing and one that
> > is a standard of comparison.  VAX carried this to the 32bit realm.  I
> > know of few 16 bit microprocessors that are as capable as the PDP-11
> > and as fast (the ti9900 was good but slow, Z8000 was close).
> 
> Don't forget the 68000.  Motorola deliberately adopted a lot of similar design
> features for the 68K; there's a very interesting design paper still available
> called "Design Philosophy Behind Motorola's 68000", publication no.AR208.  The
> same sort of instruction/address-mode orthogonality as found in the PDP11, is
> one of the big features.

You got to be kidding?!?

<FLAME ON>
The 68K is a miserable beast at the best of times.
Separated address and data registers, PC is a special register, some
addressing modes are not allowed in some instructions, some manipulations
can only be done on data register, not address registers, immediate mode
is just an assembler fake, it's actually another instruction, the
semantics of some instructions differ depending on what type of arguments
you use, writing PIC can be a real pain unless you have the 68K20. The
list is long and sad.

The 68K is what happens if you take a good design (PDP-11) and mungle up
every part of the design. It's like if they never really understood why
the PDP-11 was done they way it was, and copied the parts they though
nifty and continued with adding their own strange ideas on top of it.
<FLAME OFF>

Having said all this, it's still a nice thing compared to Intel stuff, I
guess. :-) (But I've only programmed the Z80...)

	Johnny

Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt at update.uu.se           ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol


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