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Elite on the BBC Micro

Maths (Arithmetic): DV42 [Electron version]

Name: DV42 [Show more] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate (P R) = 256 * DELTA / z_hi
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * STARS1 calls DV42 * STARS6 calls DV42

Calculate the following division and remainder: P = DELTA / (the Y-th stardust particle's z_hi coordinate) R = remainder as a fraction of A, where 1.0 = 255 Another way of saying the above is this: (P R) = 256 * DELTA / z_hi DELTA is a value between 1 and 40, and the minimum z_hi is 16 (dust particles are removed at lower values than this), so this means P is between 0 and 2 (as 40 / 16 = 2.5, so the maximum result is P = 2 and R = 128. This uses the same shift-and-subtract algorithm as TIS2, but this time we keep the remainder. Arguments: Y The number of the stardust particle to process Returns: C flag The C flag is cleared
.DV42 LDA SZ,Y \ Fetch the Y-th dust particle's z_hi coordinate into A \ Fall through into DV41 to do: \ \ (P R) = 256 * DELTA / A \ = 256 * DELTA / Y-th stardust particle's z_hi