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MUXLEQ

THE ORIGINAL MUXLEQ.

A faster SUBLEQ variant called MUXLEQ. SUBLEQ is a single instruction machine, this is a two instruction machine. By just adding one instruction is becomes possible to speed up the system dramatically whilst also reducing the program size. The machine is not much more complex, and it would still be trivial to implement the system in hardware.

SUBLEQ programs can (usually) run unaltered on the MUXLEQ CPU.

MUXLEQ runs a Forth interpreter, a programming language, as a test program. To run, type:

make run

This requires a C compiler and make to be installed.

The pseudo code for this SUBLEQ variant, MUXLEQ, is:

while pc >= 0:
	a = m[pc + 0]
	b = m[pc + 1]
	c = m[pc + 2]
	pc = pc + 3
	if a == -1:
		m[b] = get_byte()
	else if b == -1:
		put_byte(m[a])
	else if c != -1 and c < 0:
		m[b] = (m[a] & ~m[c]) | (m[b] & m[c])
	else:
		r = m[b] - m[a]
		if r <= 0:
			pc = c
		m[b] = r

Removing the line else if c != -1 and c < 0: along with the clause turns this variant back into a SUBLEQ machine.

Possible variants to pack as much functionality as possible in would include:

  • A pure single instruction muxleq variant, this would have the problem that shifting bits in either direction would be expensive, however it would be an interesting comparison to SUBLEQ.
  • Bit reversal, the resulting multiplexed value could have its bits reversed. This could be folded into the mux instruction.
  • Right shift, even if only by one place.
  • Comparison, the result of comparing m[a] and m[b] could be stored in m[a], useful comparisons would be is zero?, signed or unsigned less than or greater than. Any of those five would be useful.
  • The paper https://janders.eecg.utoronto.ca/pdfs/esl.pdf "Subleq(?): An Area-Efficient Two-Instruction-Set Computer" extends SUBLEQ in a different way using bit-reversal that should lend itself to a hardware implementation that uses minimal extra resources as the structure of the new instruction is very similar SUBLEQ. If the c operand is negative it computes r = reverse(reverse(m[b]) - reverse(m[a])). This turns the less than or equal to zero branch into a branch on evenness, allowing a quicker right shift to be implemented using minimal resources. reverse reverses all bits in a cell.

This variant greatly speeds up loading, storing and the bitwise operations, even with minimal effort. There are a few features missing from this MUXLEQ variant (such as the "self-interpreter") and as such have been disabled. No doubt if more effort was expended many more performance gains could be made.

The old SUBLEQ only version of the Forth interpreter can be run with:

make old

This is useful for speed tests.

This project could contain alternative SUBLEQ instructions and improvements in the future, altleq might be a better name for the project.

gforth can be used to build the images from either muxleq.fth or subleq.fth, but the Forth systems are self-hosting so you can use them to build the new images if you modify either Forth source file.

References