Infocom on PDP-11

[Paul Koning]

>>>>> "Allison" == Allison  <ajp166 at bellatlantic.net> writes:

 Allison> Correct.  But a stack makes some forms of programming
 Allison> easier.

Sure, just like CISC instructions make some things seem easier.  (They
actually shift the burden to a different spot.)

 >> The IBM 360/370 series doesn't have a stack, and some of its
 >> restrictions are vaguely PDP-8 like.  Nevertheless, GCC supports C
 >> (and C++) quite nicely on those machines.

 Allison> IBM360 like a PDP-8????  Explain please.

Small directly addressable range (128 words for the 8, 4k for the
360) is what I was thinking about.

 >> For that matter, Algol had local variables long before C was
 >> invented, and as you pointed out, there's an Algol for the PDP-8.
 >> (Then again, that's not a true compiler -- it compiles to an
 >> intermediate form that looks very much like a subset of the
 >> Burroughs 5500 instruction set.)

 Allison> Using an IL was a way of making the complier easier I'd
 Allison> guess.  Compiler deign has become more sophisticated since.

I believe the main reason is to make the code smaller.  P-code
compilers have been used on the PDP-11 (the Algol compiler was ported
there, and Fortran-4 was P-code).  Basic-Plus is P-code of course, and
so is Forth.  Then there is UCSD Pascal.

 >> Finally, CDC 6000s don't have a stack either, but the first Pascal
 >> compiler ran on that machine.  Implementing a stack on a non-stack
 >> machine (or non-stack language like Fortran-II) is a nice
 >> elementary Exercise for the Student.

 Allison> Be very careful what you call a stack or not.  The PDP-8
 Allison> (Straight 8) has no hardware stack but, it has autoindex
 Allison> registers that are very handy for stack implmentation. It's
 Allison> also not hard to store a return address elsewhere to
 Allison> implement subroutine recursion.  Later PDP-8a and the 6120
 Allison> chip versions had a real hardware stack added. The stacks
 Allison> were implemented using IOTs so it was possible to add them
 Allison> to any -8. The lack of a return stack doesn't mean there
 Allison> isn't a set of addressing modes to implement a software
 Allison> stack from all the minis and micros I've seen.

Sure.  But my point is that, even when you have NO help from the
processor instruction set, you can do a block oriented language.  The
Cyber is about the most unhelpful architecture you can find: no stack,
no autoincrement registers, function call works by writing the return
address into the first word of the function, etc.  And sure enough,
Wirth bitched a lot about that, but the compiler works, anyway.  In
fact, it worked well enough that the first VMS Pascal compiler was a
quick hackjob on the Cyber compiler -- you can still see the Cyber
code patterns when you look at the VAX machine code that VAX Pascal
V1.0 generates -- which is quite hilarious.

     paul