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Implementing a modulo counter

When you need a counter that counts modulo N, this can be implemented using an LSUB instruction. An LSUB subtracts two numbers and a “borrow”, and produces the result and a borrow. Supply as input parameters:

• A zero borrow (any address that points to a word or short will do)
• The number N
• The value of the counter

The LSUB instruction produces

• An outgoing borrow - if this is “zero” then the counter has wrapped
• The wrapped counter, only use this if the borrow is zero.

This implements both the comparison and the computation of the wrapped counter, provided that the counter is between N and 2N-1.

An example program:

  ldc    r1, 0x9842            // Zero borrow, any even number will do
  ldc    r5, 10                // Value of N
  ldc    r3, 0                 // The counter

  ...
  add    r3, r3, 3
  lsub   r0, r2, r3, r5, r1
  bt     r0, noWrap
  or     r3, r2, r2            // Copy r2 to r3; can be avoided by
noWrap:                        // judicious register renaming
  ...