28 Apr
2020
28 Apr
'20
1:56 a.m.
Hiya!
Got these two pdp11s, one an 11/23 (Ultrix-11 3.1) and the other an 11/84
(2.11BSD)
On the Ultrix machine, I can enter an assembly language program, assemble
it and run it fine.
amnesiac# cat hello.s
mov $1,r0
sys 4
a
6
sys 1
a: <Hello\n amnesiac# od hello
0000000 000407 000022 000000 000000 000014 000000 000000 000000
0000020 012700 000001 104404 000014 000006 104401 062510 066154
0000040 005157 000000 000000 000000 000002 000000 000000 000000
0000060 000000 000000 000141 000000 000000 000000 000002 000014
0000100
amnesiac# ./hello
Hello
amnesiac#
But on the BSD machine, the exact same source program assembles differently
and crashes with Illegal instruction when I run it.
Anyone know what I'm doing wrong?
thx
jake
cat hello.s
mov $1,r0
sys 4
a
6
sys 1
a: <Hello\n od a.out
0000000 000407 000022 000000 000000
000010 000000 000000 000000
0000020 012700 000001 104404 000014 000006 104401 062510 066154
0000040 005157 000000 000000 000000 000002 000000 000000 000000
0000060 000000 000000 000000 000004 000002 000014 000000 000006
0000100 000141
0000102
./a.out
Illegal instruction (core dumped)
28 Apr
28 Apr
1:03 p.m.
Yes, you aren’t programming 2.11 BSD correctly.
Your examples are the older UNIX syscalls (your programs work correctly in Version 6 by
the way as well).
In 2.11 BSD, all the arguments for the syscalls are inline (i.e., none are passed in
registers. This appears to be the beginnings of making the kernel protable across
architectures.
The systent table no longer has separate fields for args in registers and not in registers
and the code in sys/pdp/trap.c doesn’t look at the registers anymore.
On Apr 27, 2020, at 9:56 PM, Jacob Ritorto
<jacob.ritorto(a)gmail.com> wrote:
mov $1,r0
sys 4
a
6
sys 1
Proper code now should be:
sys 4
1
a
6
sys 1
0
Note your previous code used to just return 6 from the program (the return value of
write).
29 Apr
29 Apr
12:17 a.m.
On Tue, Apr 28, 2020 at 9:03 AM Ronald Natalie <ron(a)ronnatalie.com> wrote:
Yes, you aren’t programming 2.11 BSD correctly.
Wow, I'd hoped it was that. Thank you so much! I spent way too much time
fiddling incorrectly.
Was an example I'd cobbled together from my college textbook I've been
going back through, _Assembly_Language_for_the_PDP-11_RT-RSX-UNIX_ (c)1981
Kapps and Stafford. We didn't have UNIX for the class so never ran into
this.
Your examples are the older UNIX syscalls (your
programs work correctly in
Version 6 by the way as well).
In 2.11 BSD, all the arguments for the syscalls are inline (i.e., none are
passed in registers. This appears to be the beginnings of making the
kernel protable across architectures.
The systent table no longer has separate fields for args in registers and
not in registers and the code in sys/pdp/trap.c doesn’t look at the
registers anymore.
I wonder if the differences are written up somewhere. I did try to look
for more documentation but came up short. Must've been quite
well-ingrained in programmers' minds in the day.
Proper code now should be:
sys 4
1
a
6
sys 1
0
Note your previous code used to just return 6 from the program (the return
value of write).
Ah, so passing exit code as an arg to sys 1. Cool. Thanks again!
12:54 a.m.
Yes, the calling sequence changes were in the back of my mind, but
fortunately the TUHS source archives allowed me to easily look at the
kernel source to see how the arguments were passed. Somewhere between
2.8 and 2.11 they changed it. Again, 2.11 was one fo the first attempts
to formalize a true "multi platform" kernel rather than just copying over
the kernel and reworking it for a new machine from a seperate source
"tree."
Yes, your code takes the return of the write system call and uses it as
the exit code. Not that it makes too much difference. My 2.11 version
of your code passes a zero explicitly.
Amusingly, speaking of college courses. I had early on joined the UNIX
systems programming team at JHU and had also done some custom work for
various PDP-11 sites on campus (DOS/BATCH, RT-11, etc...). PDP-11
assembler was something I knew well. The head of the EE department told
me he'd be very disappointed if I actually signed up for his PDP-11
assembler programming course my senior year. Oddly, this caused some
consternation with the faculty committee approving my graduation as I
hadn't taken it and it was required.
Our department UNIX machine as a PDP-11/45 running a high modified V6
kernel. I also got one of the early 11/23's and we brought up the same
software on that. A year after I graduated, I was attending a DEC
announcement on the T-11 (I think the first single chip PDP-11). The
speaker says it had all the instructions with the exception of MARK. Me
and one other guy are going, "What? No MARK instruction?" MARK was
the most useless instruction concocted and it wouldn't even work on an
executable that was set up split I/D.
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3 comments
3 participants
participants (3)
-
Jacob Ritorto -
ron＠ronnatalie.com -
Ronald Natalie