> On Nov 4, 2024, at 12:34 AM, ben via cctalk <cctalk@classiccmp.org> wrote:
>
> On 2024-11-03 9:37 p.m., Tony Duell via cctalk wrote:
>> On Mon, Nov 4, 2024 at 4:06 AM Mychaela Falconia via cctalk
>> <cctalk@classiccmp.org> wrote:
>>>
>>> Paul Koning wrote:
>>>
>>>> 1. Show a one-word PDP-11 program that writes all of memory, in reverse
>>>> order.
>>>
>>> MOV -(PC),-(PC)
>> Does that work on all models of PDP11?
>> I had an idea that (as in C) the order of the decrements and reads was
>> not specified by the instruction set definition and that on some
>> machines (the 11/45?) it doubly decrements PC before reading or
>> storing.
>> -tony
>
> Where there any user instructions between models that gave strange behavior
> on the wrong machine?
Yes, and they are documented in the PDP-11 architecture manual, in an appendix.
But it cases that are machine dependent are more exotic, things like mixing a
register reference with an auto inc/dec of the same register. The magic move
will work on any of them.
On the "one word clear", I missed a detail. I pointed out you get to
initialize the registers. You can also initiate one memory word. In other
words: choose two words in memory, and registers contents, such that execution
will give you a memory full of zeroes and a halted machine.
paul
