27 Mar
2017
27 Mar
'17
1:45 p.m.
On 2017-03-26 8:31 PM, Noel Chiappa wrote:
This link arrived in my twitter feed this morning.
https://people.cs.clemson.edu/~mark/subroutines.html
--T
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