🚀 go-pugleaf

Val Kartchner writes:
> I've heard that it is possible to implement all programming languages with
> seven instructions.  (Possible does not mean practicle.)  Does anyone know
> what these instructions are?  Is it possible to do all operations in less
> than seven instructions?

You can get away with four instructions in the following (rather
inefficient) Turing-Machine-equivalent architecture.  Allow yourself a
number of registers, each of which can hold large enough numbers for
what you want to do.  Then, you only need:

	Z(register):        Zero a register
	S(register):        Increment a register (`S' for `successor')
	T(r1, r2):          Transfer value from r1 to r2
	J(r1, r2, address): Jump to address if r1 and r2 hold the same
			    value.

In fact, you can get away without the T instruction, because T(r1, r2)
is the same as:

		Z(r2)
	loop:	J(r1, r2, finished)
		I(r2)
		J(r1, r1, loop)

You wouldn't want to program it, but I guess it's fairly easy to reason
mathematically about it.

Pete.