PDP-11/83 w/FPU?

Noel Chiappa jnc at mercury.lcs.mit.edu
Thu Apr 18 09:56:18 CDT 2019

    > From: W2HX

    > i have a few CPUs available to me, a 11/23+, an 11/73 and I also have
    > available to me an 11/83
    > I would like to try to run as many different OS's as may interest me,
    > including some unixes as possible (bsd...etc).

Early Unixes in general will run on those machines - but not straight off the
tape (since they didn't exist then, and have quirks which aren't supported).

I've brought up V6 on a /23 (which must have the KTF11-A MMU chip); here:


are instructions on exactly what (minor) changes need to be made for it to

The /73 and /83 should be subsets of that, although you'll want to start with
m45.s, because those machines support the split-I-D MMU of the -11/45. (A /23
Unix binary would boot/run on them, if you don't feel like doing a special one
for them.) I haven't yet tried V6 on them; if you want me to, and do a
writeup, let me know. The /73 and /83 have LTC registers, so on those you
won't need the LTC hack.

Also, you may know this already, but if not, note that the /83 is a PMI:


machine, and _MUST_ be plugged into a Q/CD backplane _only_; plugging into
a standard Q/Q backplane will _damage_ it.

    > would I see any improvement in performance with the FPU compared to
    > without it? Or does the application running need to be something like
    > fortran to see any perceivable difference?

As someone noted, the /73 and /83 implement thefloating point instructions in
microcode, so the code can't tell if the optional FPJ11 FP hardware
accelerator is plugged in or not. In general, only on applications (the
language is not relevant) which are heavy users of FP would you see any

On the /23, with no KEF11-A FPU chip plugged in, there are no floating point
instructions at all, so any application which tries to use them will blow out
(although under V6 there's an emulator); see here:


and search for 'floating point' to see discussion of it).

    > From: Ethan Dicks

    > v5, v6, and v7 UNIX shouldn't require any sort of math hardware.

Don't know v5/v7 in detail, but AFAIK that's accurate. V6 can _support_
FP hardware on machines which have it, and is otherwised prepared to
emulate those instructions (see above).

    > From: Paul Koning

    > I think that was typically called "EAE" (extended arithmetic element),
    > a Unibus peripheral that implemented integer mul/div ... It only
    > applies to 11/20 and 11/05 since all the other machines have the
    > relevant instructions built into the CPU.

Also the -11/04 and -11/03 were both missing the EIS; the former could use
the EAE, for the latter the optional KEV11-A or KEV11-B microcode chips both
provide it.

    > From: Josh Dersch

    > The EAE was also an option on the 11/40.

Technically, on any UNIBUS machine; on the /40, the EIS (added instructions,
not the device model of the EAE) was available via an optional board in
the CPU.


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