This Sentence is False (DEC-20)

Richard Welty weltyrp at rpics.UUCP
Thu Oct 3 12:07:20 AEST 1985

> > On a Dec-20, you have a huge number of strange instructions, including :
> ...
> > skipn : skip never
> > jumpn : jump never
> I always assumed these instructions 'existed' out of dedication to
> orthogonality in the instruction set.  The DEC 10/20 seemed to have
> about a bizzilion instructions that boiled down to NOP.  The advantage
> was that it was easy to learn the mnemonics, since they were all
> constructed in a regular fashion.  It was also very easy to create and
> examine object code:  all the instructions are the same size, and the
> scheme for modifying operand types is also very regular.  Stanford had

One more reason why they existed:  When the PDP-6 was developed in the early
sixties, DEC was a small company trying to compete in a big world.  Allowing
the massive number of nops was a way to keep the hardware small and simple,
and thus fast and cheap -- it is harder not to have such instructions.
				Rich Welty

	(I am both a part-time grad student at RPI and a full-time
	 employee of a local CAE firm, and opinions expressed herein
	 have nothing to do with anything at all)

	CSNet:   weltyrp at rpi
	ArpaNet: weltyrp.rpi at csnet-relay
	UUCP:  seismo!rpics!weltyrp

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