PDP-11 Addressing Modes

[Allison J Parent]

<"In the PDP-11 family, all operand addressing is accomplished through
<the eight general purpose registers."
<
<If I understand correctly, even things like immediate operands and
<addresses are represented as an addressing mode of a register, namely
<the PC.  I think this is quite cool.

Same for stack relative access.

The PDP-11 archetecture was an example of CISC to the max for 16 bit 
machines, compared to most micros it has more and richers instruction
set, addressing modes and highlights what can be attained when all 
registers are general.  Added to a two address structure those registers 
and addressing modes make for flexibility and programming power.

...yes a PC relative jump could easily be done with an add r7!

<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).  The 
various chip versions of the PDP-11 (lsi11, T11, F11, J11) have achieved
performace exceeding many of the conteporary microprocessors in code
density and execution speed.  The PDP-11 and the C language are an 
excellent match, both for addressing modes and effienctcy of compilation.
It is also a good foundation for FORTH.

Terrible cpu, we should junk them all... ;-)   ...so I can collect them.

Allison



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