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From: hansen@mips.UUCP (Craig Hansen)
Newsgroups: net.arch
Subject: Re: PDP-8
Message-ID: <391@mips.UUCP>
Date: Wed, 12-Mar-86 01:31:24 EST
Article-I.D.: mips.391
Posted: Wed Mar 12 01:31:24 1986
Date-Received: Fri, 14-Mar-86 07:30:50 EST
References: <187@anwar.UUCP> <1441@gitpyr.UUCP> <890@umn-cs.UUCP> <1468@gitpyr.UUCP> <307@ethz.UUCP> <2007@peora.UUCP> <749@well.UUCP>
Organization: MIPS Computer Systems, Sunnyvale, CA
Lines: 34

> I never quite reconciled (sp?) myself with the addressing scheme of the 
> machine.  For instance, if I did not have PAL8 to do the hard stuff I
> would be *more* brain-damaged than I am now :-)
> All memory references outside a 128 word range were memory indirect, which
> took up one word out of the 128 and (you get the drift).

> My other fun is something I *never* figured out.  Maybe somebody in
> netland would educate me as to the finer points.  None of my programs
> ever exceeded the basic 4K core size.
> I have not ever got the hang of how those funny IOT instructions for
> extra memory worked (I saw a lot of "CIF CDF 10" instructions in the
> disassembly of the CCL).
> J. R. (May the farce be with you) Stoner, Esq.

If you relied on PAL8 to generate off-page references for you, I'm not
surprised that you never got the hang of EMA addressing.  CDF causes all
INDIRECT references to fetch the referenced data from the specified field.
Direct references, and indirect pointers are always retrieve from the
current instruction field.  CIF causes the next JMP or JMS instruction to
change the instruction field.

CIF CDF together (remember that the low-order three bits or an IOT cause up
to three separate IOP pulses on the early eights) was a common way to branch
to subroutine in another field.  The JMS would store the low-order 12 bits
of the return address, and the value of the current data field when the
subroutine is entered is both the field to get immediate data (placed after
the JMS) and the field to return to.

As you can guess by this point, letting PAL8 generate off-page links
in this environment is dangerous at best.

Craig Hansen
MIPS Computer Systems
...decwrl!glacier!mips!hansen