Skip to content
Permalink
master
Switch branches/tags

Name already in use

A tag already exists with the provided branch name. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Are you sure you want to create this branch?

subleq/README.md

Go to file
 
 
Cannot retrieve contributors at this time
SUBLEQ SIC/OISC Example:
42 lines (34 sloc) 1.38 KB
Raw Blame
Open in GitHub Desktop

SUBLEQ SIC/OISC

This is a simple interpreter for a single-insutruction computer.

The single instruction is SUBLEQ (subtract and branch if less-than-or-equal-to 0)

Example: subleq a b c

  • Subtract value in memory address a from value in memory address b
  • store result in memory address b
  • if the value is less than or equal to 0, jump to c
  • Programs are made of groups of 3 values (a, b, c)
  • Code and data share the same memory
  • A jump to -1 terminates
  • A jump to -2 just moves forward to the next group of 3 (essentially means "no jump", just continue)
  • Writing to address -1 outputs an ascii value

Example:

If we have a piece of subleq code: 9 10 3 11 9 0 12 12 0 14 100 -1 0

It would be executed like this:

9 10 3, 11 9 0, 12 12 0, 14 100 -1, 0
execute at 0
9 10 3, 11 9 0, 12 12 0, 14 86 -1, 0
execute at 3 (not jumping, just moving on)
9 10 3, 11 9 0, 12 12 0, 15 86 -1, 0
execute at 6 (not jumping, just moving on)
9 10 3, 11 9 0, 12 12 0, 15 86 -1, 0
execute at 0 (jumped!)
9 10 3, 11 9 0, 12 12 0, 15 71 -1, 0
execute at 3 (not jumping, just moving on)
9 10 3, 11 9 0, 12 12 0, 16 71 -1, 0
execute at 6 (not jumping, just moving on)
9 10 3, 11 9 0, 12 12 0, 16 71 -1, 0
execute at 0 (jumped!)
9 10 3, 11 9 0, 12 12 0, 16 55 -1, 0
...and so on

Note that any instruction that doesn't jump is automatically followed by a jump forward by 3 to get to the next set of three values.