[Simh] PDP-11 MUL / DIV Bug

[Ludwig TAUER]

Hi folks!

Seems I have found a bug in the PDP-11 emulating the
MUL instruction.

The current code takes the specified register as source
instead of the register pair R and R+1 if the register is even.

The DIV instruction is also invalid, but the other way around:
if R is odd, R is duplicated into the high half of the 32 bit int.

The attached patch will correct the problems.

Hope that helps

Ludwig Tauer

[Paul Koning]

Post by Ludwig TAUER
Hi folks!
Seems I have found a bug in the PDP-11 emulating the
MUL instruction.
The current code takes the specified register as source
instead of the register pair R and R+1 if the register is even.

The current code is correct: mul takes 16 bit inputs, and delivers a 32 bit result if the register is even, the low 16 bits if it is odd.

Post by Ludwig TAUER
if R is odd, R is duplicated into the high half of the 32 bit int.

Odd R for div is a strange thing to do. I'm not sure what it is supposed to do, the manuals are not at all clear. What do you believe is correct, and what document says so?

paul

[Ludwig TAUER]

Post by Paul Koning

Post by Ludwig TAUER
Hi folks!
Seems I have found a bug in the PDP-11 emulating the
MUL instruction.
The current code takes the specified register as source
instead of the register pair R and R+1 if the register is even.

The current code is correct: mul takes 16 bit inputs, and delivers a 32
bit result if the register is even, the low 16 bits if it is odd.

OK. Your point. Sorry, memory is fading...

Post by Paul Koning

Post by Ludwig TAUER
if R is odd, R is duplicated into the high half of the 32 bit int.

Odd R for div is a strange thing to do. I'm not sure what it is supposed
to do, the manuals are not at all clear. What do you believe is
correct, and what document says so?

Just reviewed PDP-11 Processor Handbook pdp11/04/34a/44/S0/70, 1979
freshly and indeed there was no mention of it. Again you are right.

I remembered using it that way when I ran out of registers and
stack space. But as it turns out to never trust your memories.

Sorry for the noise!

Ludwig TAUER