Forum OpenACS Q&A: Re: compiling OMSI PASCAL programs in DEC RT57XM monitor

Posted by Don Baccus on
PDP 11/780 is actually the 32-bit successor to the 16 bit PDP-11, technically it's the VAX 11/780. VAX stands for "Virtual Address eXtension".

There's a reason the MC 68000 assembly language looks much like the VAX's ... Gordon Bell's general register design and orthogonal instruction set for the 16-bit PDP-11 was the basis for the later VAX and also the 68K. The 68K was less orthogonal that either the PDP-11 or VAX. With these latter machines, operands could almost always be either memory or a register and the instruction set was very regular.

The PDP-11 is the only computer I'm aware of for which you can write a program which fills all memory with a copy of itself - running backwards!

mov -(pc),-(pc)

Where pc is the Program Counter, i.e. the register that points to the current instruction (actually points one word beyond as it is executing).

And of course the PDP-11 is why C has autoincrement and autodecrement operators ... they map directly onto the operand modes supported by the hardware.

Not only did Whitesmith's C compiler suck, so did the people at Whitesmith's.