14 Mar
2022
14 Mar
'22
10:56 p.m.
From: "Nelson H. F. Beebe"
there is a nice compact history of the PDP-11 and
early Unix at this
new article:
Not a bad artcle, but it had a number of minor errors, which I found irritating.
They should have gotten an expert to proof it.
Here are some:
registers could access any other register - as well as memory or direct
data - using one of the six different addressing modes
I think 'instruction' may have been meant for the first "register";
as
written it makes on sense. (Unlike the PDP-10, in the PDP-11, a register
can't address another register.) Oh, and there are eight addressing modes.
use of labels instead of hard-coded addresses makes it easier to
program and makes the code relocatable in memory
I'm not sure whether the writer knows the difference between 'PC-relative
addressing' (a PDP-11 feature; code that will run at any address, without
_any_ changes to the binary) and 'relocatable binary (which uses labels, as he
describes). - but is not PDP-11 specific.
The program counter can be accessed like any of the other registers ... The
program counter is intended to support jump, branching, and other control
flow instructions.
Uh, no. It's not clear _exactly_ what was meant here, but... having the PC as
a general register is for PC-relative addressing of operands, immediate data,
and absolute addressing of operands. Jumps, branches, etc don't use the fact
that the PC is a general register.
These are the key advantages of assembly programming over high-level
languages - assembler code always runs faster and uses less memory.
Not really, with modern compilers.
The number above represents -2.
A commenter caught this one.
Just use a $ to represent a decimal number
In UNIX assembler, "$" means 'immediate data'; a trailing
"." means decimal.
And a tip of the hatly hat for getting this:
Although the ++ and - operators in C are equivalents of DEC and INC
instructions, they were inspired by an addressing mode in the PDP-7.
right.
Noel
15 Mar
15 Mar
midnight
New subject: [tuhs] PDP-11 and early Unix
On Mon, Mar 14, 2022 at 12:59 PM Noel Chiappa <jnc(a)mercury.lcs.mit.edu>
wrote:
Not a bad artcle, but it had a number of minor errors, which I found
irritating.
They should have gotten an expert to proof it.
Sure - but usually these are way more bad than good. Usually way too
much urban legend, so I'm thrilled to see the 11 kept alive.
And a tip of the hatly hat for getting this:
Although the ++ and - operators in C are equivalents of DEC and INC
instructions, they were inspired by an addressing mode in the PDP-7.
As I said, the authors seem to have done a little homework, although I was
under the impression ??Ken?? had created them for B independently (which,
of course, was first on the PDP-7). The ++/-- for increment and decrement
was just a nice notation since they are such a common occurrence in real
code, it's handy to have. But, I did not think the PDP-7 ISA includes
addressing modes in the same manner as the 11. I'm probably confused, but
I thought PDP-7 is a very simple instruction (and small) with an AC,
Link/Indirection and a PC - it reminded me of the PDP-8 more than anything
else [although, IIRC it could do both 1's complement and 2's - depending
the type of AND instruction].
1:03 a.m.
New subject: [tuhs] PDP-11 and early Unix
On 14/03/22, Clem Cole wrote:
The PDP-7 has auto-increment registers at 10-17 that are incremented
when addressed indirectly.
I don't think it's even possible that the B runtime used them, because
of the implied indirection, which is logically separate in B.
They might still have been an inspiration, but i didn't get that
impression from the copious discussion of that topic.
aap
And a tip of the hatly hat for getting this:
Although the ++ and - operators in C are equivalents of DEC and INC
instructions, they were inspired by an addressing mode in the PDP-7.
As I said, the authors seem to have done a little homework, although I was
under the impression ??Ken?? had created them for B independently (which,
of course, was first on the PDP-7). The ++/-- for increment and decrement
was just a nice notation since they are such a common occurrence in real
code, it's handy to have. But, I did not think the PDP-7 ISA includes
addressing modes in the same manner as the 11. I'm probably confused, but
I thought PDP-7 is a very simple instruction (and small) with an AC,
Link/Indirection and a PC - it reminded me of the PDP-8 more than anything
else [although, IIRC it could do both 1's complement and 2's - depending
the type of AND instruction].
796
days inactive
796
days old
2 comments
3 participants
participants (3)
-
Angelo Papenhoff -
Clem Cole -
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