[TUHS] Happy birthday, PDP-8!

Noel Chiappa jnc at mercury.lcs.mit.edu
Tue Mar 28 12:38:51 AEST 2017

    > From: Johnny Billquist

    > the PDP-11 have the means of doing this as well....  If anyone ever
    > wondered about the strangeness of the JSR instruction of the PDP-11, it
    > is precisely because of this.
    > ...
    > I doubt Unix ever used this, but maybe someone know of some obscure 
    > inner kernel code that do. :-)

Actually Unix does use JSR with a non-PC register to hold the return address
very extensively; but it also uses the 'saved PC points to the argument'
technique; although only in a limited way. (Well, there may have been some
user-mode commands that were not in C that used it, I don't know about that.)

First, the 'PC points to arguments': the device interrrupts use that. All
device interrupt vectors point to code that looks like:

	jsr r0, _call

where iservice() is the interrupt service routine. call: is a common
assembler-language routine that calls iservice(); the return from there goes
to later down in call:, which does the return from interrupt.

Use of a non-PC return address register is used in every C routine; to save
space, there is only one copy of the linkage code that sets up the stack
frame; PDP-11 C, by convention, uses R5 for the frame pointer. So that common
code (csv) is called with a:

     jsr r5, csv

which saves the old FP on the stack; CSV does the rest of the work, and jumps
back to the calling routine, at the address in R5 when csv: is entered. (There's
a similar routine, cret:, to discard the frame, but it's 'called' with a plain


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