12 May
2005
12 May
'05
1:30 a.m.
On Wed, 11 May 2005 Allison <ajp166 at bellatlantic.net> wrote:
Not to mention the fact that on the PDP-10 there actually was the
instructions PUSHJ and POPJ, which probably was the inspiration for the
macros on a PDP-8.
The PDP-10 is a fun architecture. You can do subroutine calls in all the
paradigms available. Stack? Sure. Return address at the start of the
routine? Sure. Return address in a register? No problem.
Subject: What
does PUSHJ do? (was Re: was "C on the PDP-8 ... ")
From: spc at conman.org (Sean 'Captain Napalm' Conner)
Date: Wed, 11 May 2005 17:43:49 -0400 (EDT)
To: cctalk at classiccmp.org
It was thus said that the Great Brad Parker once stated:
It's a PAL macro for push and jump and there is a return and pop.
heh. I remember puzzling at the the "pushj"'s and "popj"'s
in the FOCAL
source code. This was before I had seen a pdp-11 or dec-10.
What does PUSHJ actually do? I remember reading about it in Steven Levy's
Hackers, but never got what was so special about it.
Come from the fact that PDP-8 put the return address
at the called location and the next
location is executed.
MAIN ; do something
JMS I,FOO
; more something
FOO ; return stored here
Add I,BLAH
JMP I,FOO
In PDP-8 the I in the addressing is use the contents at the address
pointed to.
Not a good example. It should really be JMS FOO, and at the label FOO you
need to place a 0, which is overwritten by the return address. (What
assembler do you usually use, btw? Comments come after a slash... :-) )
Now if you want recursion
You call a standard call and return routine (COSMAC 1802 requires this too).
The routine gets the target address(work routine) and the return adddress
(caller) and saves the return and dispatches to the work routine. The
reverse is done to get back to the originating caller.
Yup.
In the PDP-8A and 6120 there are IOTs to push/pop the
ACC on the stack
and and also there are a set of push/pops for the PC register.
Eh? No. The PDP-8/a don't have any stack IOT. Same set as the 8/e. The
only difference is how some illegal combinations of OPR instructions act.
The CPU deals with almost no IOTs itself. You then have the KK8A and
DKC-8AA which handles most IOTs of the machine which are considered to be
built in.
However the PDP-8 programmer is less likely to use a
stack than a
flavor of computed jump or call. PDP-8 is good candidate for an
state machine coding.
Of course, since there isn't any stack in hardware.
Johnny
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt at update.uu.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol