--- On Tue, 12/1/09, Doug McNutt <[email protected]> wrote:
> The 68 k series had instructions that, if not present in
> the hardware, would generate "A_line" traps that would
> interrupt to run user-provided code buried somewhere in the
> operating system.
>
> I doubt that the OS did much to emulate a real PMMU but
> it's quite possible that Apple's virtual memory did. The
> floating point instructions were certainly emulated by
> A_line traps.
The PMMU is required to use virtual memory. PMMU = Paged Memory Management
Unit. The 68030 is basically a 68020 with onboard PMMU. The 68040 is basically
a 68030 with onboard FPU, except in the case of the 68LC040 or 68EC040 where
the LC has no FPU and the EC is pretty much a hotted up 020 with the extra
instructions etc of the 040 series.
> Actually the PMMU socket on the Mac-II was filled with some
> cross connections and nothing else. It was the UNIX types
> who actually installed a chip.
That'd be the "dummy" AMU chip. Can't run virtual memory on a Mac II with an
AMU.
Is it possible to add a PMMU to the original LC? (The only Mac other than the
Mac II to use the 68020 CPU.)
DayStar's CPU upgrades and the later IIci PDS adapter for the Mac II plugged
into the CPU, FPU and AMU/PMMU sockets and thus required no control panel to
activate the upgraded CPU and FPU. The control panel could still be used with
the PowerMath switch to redirect certain SANE math function calls to the FPU.
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