Alesis M4 and M4C (M4 revision C) DSP ASIC

Designed and formerly produced by Alesis Semiconductor and then - Wavefront Semiconductor. The M4 and M4C is used in MidiVerb4, MicroVerb4, NanoVerb and probably also in some compact mixing consoles with effects.
Two main classic Alesis chip designers moved on to fund their own company: Spin Semiconductor.

While that chip is old (finished in 1993) and does not represent the extreme signal processing monsters league, it still has some interesting aspects - mostly as a good example of a system design simplification and integration. There's almost no information about it (or even insides of MidiVerb4) on the web, so I decided to put here some of the notes about it, that might also be useful for repairs. For DSP zealots I will also dump here some übersecret info on it's operation and algorithms.

M4 (p/n 2-27-0031) is used in MidiVerb 4 and early MicroVerb4 (rev.A and probably B as well)
M4C (p/n 2-27-0044) is used in MicroVerb 4 (rev.C and later), NanoVerb and seems also on later MidiVerb 4 units

As a somewhat oldschool design, chip is entirely 5V.

For every sample it executes 64 x 32bit instructions, each consisting of a several wcs microcode parts and a memory offset (which can be further offset by lfo). For each instruction cycle it needs eight clocks to work all the things including the multiplier - the design so optimized and clever that I'm very tempted to recreate one on a CPLD! I've made disassembler for it and other stuff that I will at some point make a separate page about.
 

Besides DSP stuff, M4 also has: An integrated ALE latch and drives the lower addresses for the CPU ROM Uses an 8-bit SRAM instead of DRAM Reset output (active low) to CPU OSC divider output for CPU (divides it's local oscillator by 2) Chip select decoder for CPU SRAM, LCD and its internal WCS Audio level detection for software bargraphs. Feature, which HISC and S-DISC misses a lot. Two hardware LFOs for very efficient chorus, flanger and pitch shift implementation

Besides DSP stuff, M4C also has: An integrated ALE latch and drives the lower addresses for the CPU ROM Has a 4-bit wide DRAM interface with refresh Double-input slow ADC for user interface panel knobs Reset output (active low) to CPU OSC divider output for CPU (divides it's local oscillator by 2) Internal VCO for samplerate slave sync operation Chip select decoder for SRAM, LCD and its internal WCS Audio level detection for software bargraphs. Feature, which HISC and S-DISC misses a lot. Two hardware LFOs for very efficient chorus, flanger and pitch shift implementation

For the M4C, memory transfer rate is about one third to one sixth in comparison to other manufacturer system designs of that day. However, it was a cost effective solution, to use only one DRAM chip and access memory by nibbles (thus making things four times slower). On newer board versions (after 2000), when old DRAM chips got phased out and Alesis had to switch to contemporary parts, things got even less pretty, as the M4C chip there actually uses only one fourth of the available bandwidth and amount of memory of the attached 16bit DRAM chip. The remaining 3/4 of the memory and speed is unused and therefore sadly wasted.

Nonetheless, chip is very capable and there are some wonderful sounding algos working on it. Thank You Keith, Frank and David for the amazing programs! :)
Any of it's effects DSP programs (even the magnificent reverb with swirl and filters, as well as combinations of chorus>delay>room) fit in just 64 instructions. One of the simplest things, the mono delay (that has two filters in it), fits in just 18 instructions. Auto Panner (that also has two filters in it) is even smaller - only 17 instructions. Instructions are 32bit wide, and, by the way, original assembler uses $00000303 as a NOP - a heartwarming "♥ ♥" when viewed in ASCII!

Page currently under development. Someday I hope to dump there my papers on system memory map, access timing, wcs microcode, bootup and realtime effects control.

Alesis M4 chip pinout

Alesis M4C (M4 revision C) chip pinout