urm is an implementation of a register machine based on the Unlimited Register Machine described by N.J. Cutland in Computability: An introduction to recursive function theory.
urm has 10 registers (marked R1 through R10) and supports four commands.
- Zero: Z(x) sets the entry in register x to 0.
- Successor: S(x) increments the entry in register x by 1.
- Transfer: T(x,y) replaces the entry in register y with the entry in register x.
- Jump: J(x,y,l) jumps to line l of the program iff the contents of registers x and y are equal.
The following program returns max(n-1,0) in R1, given input n in R1.
1. Z(2)
2. J(1,2,10)
3. Z(3)
4. S(2)
5. J(1,2,9)
6. S(2)
7. S(3)
8. J(1,1,5)
9. T(3,1)
10. HALT
(Line 10 can be left blank, or given any contents that are not a command. I write 'HALT' by convention).