[TUHS] Subroutine calling conventions - was Re: Happy birthday, PDP-8!

Toby Thain toby at telegraphics.com.au
Mon Mar 27 23:45:37 AEST 2017

On 2017-03-26 8:31 PM, Noel Chiappa wrote:
>     > From: Dave Horsfall
>     > And as for subroutine calls on the -8, let's not go there...  As I dimly
>     > recall, it planted the return address into the first word of the called
>     > routine and jumped to the second instruction; to return, you did an
>     > indirect jump to the first word.
> That do be correct.
> That style of subroutine call goes back a _long_ way. IIRC, Whirlwind used
> that kind of linkage (alas, I've misplaced my copy of the Whirlwind
> instruction manual, sigh - a real tresure).

This link arrived in my twitter feed this morning.


> ISTVR there was something about the way Whirlwind did it that made it clear
> how it came to be the way it was - IIRC, the last instruction in the
> subroutine was normally a 'jump to literal' (i.e. a constant, in the
> instruction), and the Whirlwind 'jump to subroutine' stored the return address
> in a register; there was a special instruction (normally the first one in any
> subroutine) that stored the low-order N bits of that register in the literal
> field of the last instruction: i.e. self-modifying code.
> The PDP-6 (of which the PDP-10 was a clone) was on the border of that period;
> it had both types of subroutine linkage (store the return in the destination,
> and jump to dest+1; and also push the return on the stack).
> 	Noel

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