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From: hansen@mips.UUCP (Craig Hansen)
Newsgroups: net.arch
Subject: Re: Re: Addressing modes
Message-ID: <379@mips.UUCP>
Date: Thu, 6-Mar-86 00:29:45 EST
Article-I.D.: mips.379
Posted: Thu Mar  6 00:29:45 1986
Date-Received: Mon, 10-Mar-86 08:36:05 EST
References: <187@anwar.UUCP> <1441@gitpyr.UUCP> <890@umn-cs.UUCP> <1468@gitpyr.UUCP> <307@ethz.UUCP>
Organization: MIPS Computer Systems, Sunnyvale, CA
Lines: 59

> For those who have never seen a PDP-8 instruction set, here it is
> The instructions were, then (some of the opcode names may be wrong,
> it's been a long time...):
> 	TAD		Two's Complement Add ((acc) <- acc+(EA))
> 	SCA		Store and Clear Accumulator ((EA)<-(acc), (acc)<-0)
> 	ANA		And Accumulator ((acc)<-(EA)&(acc))
> 	ISZ		Increment & Skip on Zero ((EA)<-(EA)+1, skip if zero)
> 	JMP		Jump (PC)<-EA
> 	JSR		Jump to (EA)<-(PC),(PC)<-EA+1

More correctly, these should be:

	AND		And
	TAD		Two's Complement Add
	ISZ		Increment and Skip if zero
	DCA		Deposit and clear accumulator
	JMP		Jump
	JSR		Jump to subroutine

> Mind you, I wouldn't want to program one in assembler though :-) .
> -- 
> Peter Lamb	({seismo|decvax}!mcvax!cernvax!ethz!prl)
> Institut fur Informatik
> ETH-Zentrum
> 8092 Zurich

Actually, I found the machine really enjoyable to program in assembler.  It
was a instruction set that one could really learn completely, down to
knowing exactly what the assembler could generate. It was common practice to
locate the targets of AND instructions into memory so that the AND
instruction was a useful character constant.  Using the "microcoded" operate
instructions, you could generate any of the constants 0,1,2,3,4,6,-1,-2,-3
in a single instruction. One of the more unique programs written
for the machine was a two-voice music generator that generated
its output on an FM reciever placed close to the front panel of the machine!
Try that on your favorite multi-user, virtual memory computer!

Because of the small page sizes, and the difficulty of generating
relocatable code, I never encountered a really practical compiler for the
PDP-8.  Most so-called "compilers" generated pseudo-code, or virtual machine
code, that was interpreted.

In the realm of exploring the extremes of the price/performance
relationship, DEC build the PDP-8/S, which was a bit-serial implementation
of the PDP-8!

In later years, the PDP-8 line suffered from "creeping featurism." For some
reason, designers added an instruction which would swap the high and
low-order 6 bits of the 12-bit accumulator, called "byte swap."

I've got a lot of old code for the eight, but without a DECtape drive or a
paper tape reader, I can't make too much use of it.  Sigh.
I could run a software emulator for the eight in real time nowadays,
but it would never generate the same pattern of electrical noise
on an FM radio....

Craig Hansen
MIPS Computer Systems
...{decwrl,glacier}!mips!hansen