[TUHS] Happy birthday, PDP-8!

Noel Chiappa jnc at mercury.lcs.mit.edu
Mon Mar 27 10:31:05 AEST 2017


    > 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).

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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