This is my successor project to Sonne8, a discrete micro-controller built using about one-hundred 74HC series logic chips, some CMOS memory and passive-components only. It runs about 1 million instructions per second on a four-layer PCB.
A demonstration video in the other repo shows the controller board with a companion I/O-board I designed stacked on top of it.
In the video, it loads a program for multiplying two numbers from a serial EEPROM running a native SPI implementation. It then computes the result and displays it on the I/O boards 7-segment display.
The new project over here will help me explore a specific, stable version of the CPU a little more and write software for it, before developing more hardware.
The schematics have been stripped of everything I thought wasn't essential, so it's a bit easier to read. In other words, although it's possible to load the schematics file into KiCad, its intended use is only for reference.
The C file is also for reference. It shows the intended workings of the CPU. For a more detailed description of this microcontroller read "spec.md".
The component count is now higher than for the prototype, because in this project I've implemented the ALU (top and top-right in schematics) in discrete logic, where it used to be just look-up tables in a big PROM.
There is now a command-line tool (my) for exploring Myth. It includes an assembler. The tool is documented in the specification file.
Here is an example of a native multiplication routine which multiplies the two 8-bit numbers in the accumulator, leaving the accumulator with the 16-bit result (high-order in A). The listing is the output of the assembler using my -la.
ADDR: OBJCODE: LIN: SOURCE:
0001
0002 (Multiply 4x7)
0000: 84 04 84 07 80 03 0003 fa 4, fa 7 - fc >MUL8. ; Sets A to 7, X to 4 and calls mul8
0006: 8C 06 0004 @idle fj <idle ; my-tool stops at 64k cycles
0005 ; Check the result using: "my -p"
0006
0007 ; Multiply A times X, result in A:X
0008 ; A:X accumulator acts as a two-element push-down stack when writing to A
0009 ; Both regs are the implied ALU operands, primary result in A, secondary in X
0010
0011 ; Using page 3: "my"-tool uses p1 for persisting regs, p2 as IO-buffer
0012
0013 3@MUL8
0300: 68 0014 a1 (Save multiplicand into L1 - turns into low order result)
0301: 17 6B 0015 xa a4 (Save multiplier into L4)
0303: 84 00 69 0016 fa 0, a2 (Set high-order result to 0, keep in L2)
0306: 85 07 0017 fd 7 (Initialise loop counter, 8 bits to process)
0018 @loop
0019 fa b0000_0001, 1a AND (Check LSB of multiplicand)
030C: 8E 12 0020 fz >a (Skip if zero)
030E: 63 61 1D 69 0021 4a 2a ADDC a2 (Add multiplier to high order result)
0022 @a
0023 1a SHR, a1 (Shift low-order result right)
0315: 61 1B 69 0024 2a SHR, a2 (Shift high-order result right, LSB saved to X)
0318: 17 60 15 68 0025 xa 1a IOR, a1 (Carry high-order LSB into low-order MSB)
031C: 8B 08 0026 fw <loop
031E: 60 0027 1a (Push low-order result)
031F: 61 0028 2a (Push high-order result)
0320: 05 0029 RTS
Source Code 
of the multiplication routine in Sublime text with syntax highlighting.