2026-09-23
What happened
I came back to the MMT-8 simulator to wire up MIDI, and MIDI itself turned out to be the easy part: a cycle-counted UART in the hardware layer, an ALSA sequencer bridge, and the simulated MMT-8 showed up in aconnect -l like any other device. The hard part was what happened next — pressing PLAY spat out phantom note events and stopping a recording left the display reading SELECT PART 250. Tracing IRAM writes instruction by instruction led to the emu8051 core itself: MOV direct,@Ri had its operands swapped, so a MOV DPL,@R0 in the firmware was quietly overwriting the per-track pointer table, and a differential test against the MCS-51 spec turned up three more silent bugs (auxiliary carry, XCHD, DA A carry). With those fixed, the original 1987 firmware passes its own RAM, EPROM and MIDI diagnostics, records a phrase from a keyboard, and plays it back out with MIDI clock — a real Alesis MMT-8 running as a Linux MIDI device.
Along the way the headless mode grew scripted button presses and an LCD change log, which let me press all 48 key-matrix positions and read the firmware's reaction off the display to recover the real button map (43 of 45 buttons, EDIT and NAME still hiding).
Files touched
- README.md — simulator section, key matrix, LED latch bits, file layout
- sim/README.md — rewritten: usage, MIDI, options, self-test, emulator patches
- sim/mmt8_hw.c, sim/mmt8_hw.h — 8051 UART emulation (TX/RX, TI/RI, FIFOs)
- sim/mmt8_midi.c, sim/mmt8_midi.h — ALSA sequencer MIDI IN / MIDI OUT ports
- sim/main.c — CLI options, headless mode, scripted presses, LCD log, loopback, screenshot
- sim/mmt8_gui.c — verified button matrix, status-latch LEDs, software renderer fallback
- sim/opcodes.c — emu8051 fixes: MOV direct,@Ri, AC flag, XCHD, DA A
- sim/tests/cputest.c, sim/Makefile — differential CPU core test (
make test)
Tweet draft
Got MIDI working on my Alesis MMT-8 simulator: the original 1987 firmware now runs as an ALSA MIDI device on Linux, records from a keyboard and plays back with clock. The real fight was four silent bugs in the 8051 emulator core, found by watching one byte of RAM change. [link]
commit: ad07112 · screenshot: captured (sim --screenshot)