On Mon, Apr 22, 2024 at 2:30 PM Paul Koning via cctalk
<cctalk@classiccmp.org> wrote:
> Anyway, I would think such a small microprocessor could emulate a PDP-11 just 
> fine, and probably fast enough.  The issue isn't so much the instruction set 
> emulation but rather the electrical interface.  That's what would be needed 
> to be a drop-in replacement.  Ignoring the voltage levels, there's the matter 
> of implementing whatever the bus protocols are.

Emulating an F-11 chip or a J-11 chip is certainly possible with a
modern MCU, just need TTL-friendly I/O.  F-11 is 40-pins (and can have
additional instructions added by adding microcode ROMs next to the
CPU) and the J-11 is 64 pins on a fat chip carrier.

> Possibly an RP2040 (the engine in the Raspberry Pico) would serve for this, 
> with the PIO engines providing help with the low level signaling.  Sounds 
> like a fun exercise for the student.

Could be a good start but would still need level shifters.

J-11 runs at 15-18Mhz for an idea on how fast the bus implementation has to be.

-ethan

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