[TUHS] Happy birthday, PDP-8!

Johnny Billquist bqt at update.uu.se
Wed Mar 29 08:10:26 AEST 2017

On 2017-03-28 09:37, jnc at mercury.lcs.mit.edu (Noel Chiappa) wrote:

>     > 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
> 	_iservice
> 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.

Ah. Thanks for that. I hadn't dug into those parts, but that's the kind 
of place where I might have suspected it might have been, if anywhere.

> 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
> jmp.)

Hah! Thinking about it, I actually knew that calling style, but didn't 
reflect on it, as you're not passing any arguments in the instruction 
stream in that situation.
But it's indeed not using the PC as the register in the call, so I guess 
it should count in some way. :-)


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