Editing One-instruction set computer (section)

=== Subtract and branch if negative ===
The {{mono|subneg}} instruction ("''subtract and branch if negative''"), also called {{mono|SBN}}, is defined similarly to {{mono|subleq}}:<ref name=caamp />{{rp|41,51–52}}

 '''Instruction''' <syntaxhighlight lang="nasm" inline>subneg a, b, c</syntaxhighlight>
     Mem[b] = Mem[b] - Mem[a]
     '''if''' (Mem[b] < 0)
         '''goto''' c

Conditional branching can be suppressed by setting the third operand equal to the address of the next instruction in sequence.  If the third operand is not written, this suppression is implied.

==== Synthesized instructions ====
It is possible to synthesize many types of higher-order instructions using only the {{mono|subneg}} instruction. For simplicity, only one synthesized instruction is shown here to illustrate the difference between {{mono|subleq}} and {{mono|subneg}}.

Unconditional branch:<ref name=caamp />{{rp|88–89}}

;{{mono|JMP c}}
:<syntaxhighlight lang="nasm">
  subneg POS, Z, c
</syntaxhighlight>

where {{mono|Z}} and {{mono|POS}} are locations previously set to contain 0 and a positive integer, respectively;

Unconditional branching is assured only if {{mono|Z}} initially contains 0 (or a value less than the integer stored in {{mono|POS}}). A follow-up instruction is required to clear {{mono|Z}} after the branching, assuming that the content of {{mono|Z}} must be maintained as 0.

==== subneg4 ====
A variant is also possible with four operands – subneg4.  The reversal of minuend and subtrahend eases implementation in hardware.  The non-destructive result simplifies the synthetic instructions.

 '''Instruction''' <syntaxhighlight lang="nasm" inline>subneg s, m, r, j</syntaxhighlight>
     ''(* subtrahend, minuend, result and jump addresses *)''
     Mem[r] = Mem[m] - Mem[s]
     '''if''' (Mem[r] < 0)
         '''goto''' j