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BBC Micro Elite

Shift-and-subtract division

The main algorithm behind Elite's many division routines

References: TIS1, TIS2, DVID4

Elite implements division in routines like TIS2 using the shift-and-subtract algorithm. This is similar in concept to the shift-and-add algorithm used to implement multiplication in routines like MULT1, but it's essentially the reverse of that algorithm.

In the same way that shift-and-add implements a binary version of the manual long multiplication process, shift-and-subtract implements long division. We shift bits out of the left end of the number being divided (A), subtracting the largest possible multiple of the divisor (Q) after each shift; each bit of A where we can subtract Q gives a 1 the answer to the division, otherwise it gives a 0.

In pseudo-code, the algorithm to calculate T = P / Q (with remainder A) looks like this:

  T = 0
  A = 0
  for x = 7 to 0
    A = A << 1
    A(bit 0) = P(bit x)
    if A >= Q then
      A = A − Q
      T(bit x) = 1

This is the algorithm implemented in TIS2, except we save space (and make things much more confusing) by using A for both the number being divided and the remainder, building the answer in T instead of P, and using set bits in T to implement the loop counter. The basic idea of shifting and subtracting is the same, though.