Editing One-instruction set computer (section)

=== Reverse subtract and skip if  borrow ===
In a ''reverse subtract and skip if borrow'' (RSSB) instruction, the [[Accumulator (computing)|accumulator]] is subtracted from the memory location and the next instruction is skipped if there was a borrow (memory location was smaller than the accumulator). The result is stored in both the accumulator and the memory location. The [[program counter]] is mapped to memory location 0. The accumulator is mapped to memory location 1.<ref name=caamp />

 '''Instruction''' <syntaxhighlight lang="nasm" inline>rssb x</syntaxhighlight>
     ACCUM = Mem[x] - ACCUM
     Mem[x] = ACCUM
     '''if''' (ACCUM < 0)
         '''goto''' PC + 2

==== Example ====
To set x to the value of y minus z:

<syntaxhighlight lang="asm">
# First, move z to the destination location x.
  RSSB temp # Three instructions required to clear acc, temp [See Note 1]
  RSSB temp
  RSSB temp
  RSSB x    # Two instructions clear acc, x, since acc is already clear
  RSSB x
  RSSB y    # Load y into acc: no borrow
  RSSB temp # Store -y into acc, temp: always borrow and skip
  RSSB temp # Skipped
  RSSB x    # Store y into x, acc
# Second, perform the operation.
  RSSB temp # Three instructions required to clear acc, temp
  RSSB temp
  RSSB temp
  RSSB z    # Load z
  RSSB x    # x = y - z [See Note 2]
</syntaxhighlight>
* [Note 1] If the value stored at "temp" is initially a negative value and the instruction that executed right before the first "RSSB temp" in this routine borrowed, then four "RSSB temp" instructions will be required for the routine to work.
* [Note 2] If the value stored at "z" is initially a negative value then the final "RSSB x" will be skipped and thus the routine will not work.