23 May
2005
23 May
'05
12:24 a.m.
On May 22 2005, 20:49, Jay West wrote:
or
Jim wrote....
> Location Content Op-code comment
> 001000 012700 mov #1,r0 load ro with 1
> 001002 000001
> 001004 006100 rol rotate r1 left
> 001006 012701 mov delay r1 load register r1 with delay
> 001010 007777 delay
> 001012 005301 dec r1 decrement register 1
> 001014 001376 bne -2 continue to decrement r1
until
r1=0
001016 000772 br -12 back to 001002 (dec r0)
This runs on my /45, but produces a steady light display, not a cylon "blinking" effect.
I'm not much of a Unibus expert but I seem to remember there's
something odd about what's displayed in the lights, and how you get it
to change.
It's been decades since I looked at '11
assembler. I'm
confused about something in the listing above. If the 012700 at
location
1000 is a load r0 with 1, I'm guessing the next
word (1002) is the
constant
to be loaded? So, why would the branch at location
1016 go back to
the data
at 1002? Obviously there's something I'm
missing.
The code is correct, the comments are correct, but the mnemonic
instructions are wrong :-)
Remember that what's counted in a BR instruction are words, not bytes,
but that when the instruction is interpreted, the PC has already been
incremented. So 777 is a "branch to self", and 776 is "branch back to
the previous instruction". 1376 is "branch back to the previous
instruction if Z is set", as at 1014. 772 does indeed branch back to
the ROL instruction, because it's "branch back 6". Looks like Jim
remembered that "branch back to the previous" is "br -2", thought
"two
bytes", and just doubled the six to twelve -- except that what he wrote
was octal 12 (there's no decimal point after it), or ten decimal :-)
Actually, if I'm being really picky, it should be "br .-2".
--
Pete Peter Turnbull
Network Manager
University of York
23 May
23 May
3:21 p.m.
I'm not much of a Unibus expert but I seem to
remember there's
something odd about what's displayed in the lights, and how you get it
to change.
THe easiest way on an 11/45 (or an 11/70?) is to store the value at the
address used for the switch register (777570 IIRC) and set the rotary
knob to the right of the data lights appropriately.
The first PDP11 program I ever wrote was something like
mov #0,r0 ; Initialise counter
loop: move r0.(#177570) ; Store counter in lights register
inc R0 ; increment the counter
reset ; wait a bit
jmp loop ; round again, jumos are easier to hand assemble
; than branches, but even so I got caught out
; the first time -- you use one more level of
; indirection than you might think!
-tony
24 May
24 May
6:40 a.m.
>>>> "Tony" == Tony Duell <ard
at p850ug1.demon.co.uk> writes:
> I'm not much of a Unibus expert but I seem to
remember there's
> something odd about what's displayed in the lights, and how you
> get it to change.
Tony> THe easiest way on an 11/45 (or an 11/70?) is to store the
Tony> value at the address used for the switch register (777570 IIRC)
Tony> and set the rotary knob to the right of the data lights
Tony> appropriately.
Definitely, if you have a machine with a display register (which is
how the write-only register at 777570 is referred to).
The "data paths" setting roughly matches what you get on an 11/40,
which doesn't have that switch. In that case, the lights display the
contents of R0 during a WAIT (and apparently also during a RESET).
The fact that this happens for WAIT is the basis of the "spinning
lights" idle code used in various operating systems (RSTS for one,
though not RT11 which uses the display register instead).
paul
7104
days inactive
7105
days old
2 comments
3 participants
participants (3)
-
ard＠p850ug1.demon.co.uk -
pete＠dunnington.u-net.com -
pkoning＠equallogic.com