Help: PDP-11 instruction classification (again!)

johnh at johnh at
Wed May 10 11:35:53 AEST 2000

> lars brinkhoff wrote :-
> I'm adding PDP-11 support to GNU binutils, and I need help on
> classifying the instruction set.

I forgot to mention a critical point. Any PDP-11 runing Unix (except very
early versions and Miniunix), and certainly from Edition 6 onwards, MUST have
EIS. Even the Unix bootstraps used EIS. So binaries built for Unix, will run
on a 11/34/35/40/44/45/50/53/55/60/70/73/83/84/93/94 !!

Floating point is a bit more problematic. The kernel (see crevat latter) didn't
require it, but had to save FP status and registers on context switches.
Quite a few processors had the FPP as an option, and so there was FPP emulation
build into the kernel (conditionally). There were versions of the C compiler
that had code tables for the FIS, to suit the 11/35/40.

I have a distant memory, that I have seen FPP instructions used for some
integer arithmetic for speed. I cannot recall if it was in the kernel or
C libraries. It was conditional, and may have been in the latter BSD versions,
but I don't have the source code online.

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