[GreenKeys] Model 33 with my PDP-11

[Harold Hallikainen]

On Tue, January 2, 2024 1:01 pm, Ethan Blanton via GreenKeys wrote:
> Eric Moore wrote:
>> https://youtu.be/frMwsDTjmAs
>>
>> The line feed either slips or fails to feed, I am hoping another clean
>> and
>> lube helps.
>
 Really nice! I ran the 33 on a PDP-8 with punched tape. The first "glass
terminal" I ran was the Lear Siegler ADM-1A. I did some stuff with the
LSI-11, programming in assembly. I really liked the instruction set of
the PDP/LSI-11. As I recall, the instructions were like the following:

3 bit octal instruction
3 bit octal source register
3 bit octal source mode
3 bit octal destination register
3 bit octal destination mode

Modes were something like register direct (read or write the register),
register indirect (read or write memory address in the register), register
indirect postincrement (read or write memory address in register, then
increment the register), register indirect preincrement (same as previous,
but increment register before using it as memory address), and pre and
post decrement register indirect.

Register 6 was the stack pointer. The stack would build down using
predecrement and post increment.

Register 7 was the program counter. A really clever way of handling
literal arguments was used. The source was register 7 post increment. The
CPU read the instruction and incremented the program counter (just as it
would for any instruction, to point to the next). The contents of memory
now pointed to (the literal argument) were then moved to the destination.
Then, since this was post decrement, the program counter would be
incremented again to point to the next instruction.

I always thought this was EXTREMELY clever!

Thanks for posting the video!

Harold
https://w6iwi.org