Quoted from the author's initial email/posting:
The accumulator is subtracted from the memory location and the next instruction is skipped if there was a borrow. The result is stored in both the accumulator and the memory location. The PC is memory location 0.
The original work on this package was done by Mike Albaugh of Atari Games.
By 'borrow' this assumes that the memory location was smaller than the accumulator.
Temp is a temporary memory location.
To set x to the value of y minus z (There may be a faster way.):
How to do something
Reference