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BBC Micro Elite

Maths (Arithmetic): DVID4 (6502SP version)

Name: DVID4 [View in context] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate (P R) = 256 * A / Q
Calculate the following division and remainder: P = A / Q R = remainder as a fraction of Q, where 1.0 = 255 Another way of saying the above is this: (P R) = 256 * A / Q This uses the same shift-and-subtract algorithm as TIS2, but this time we keep the remainder and the loop is unrolled. Returns: C flag The C flag is cleared
.DVID4 ASL A \ Shift A left and store in P (we will build the result STA P \ in P) LDA #0 \ Set A = 0 for us to build a remainder \ We now repeat the following five instruction block \ eight times, one for each bit in P. In the cassette \ and disc versions of Elite the following is done with \ a loop, but it is marginally faster to unroll the loop \ and have eight copies of the code, though it does take \ up a bit more memory (though that isn't a concern when \ you have a 6502 Second Processor) ROL A \ Shift A to the left CMP Q \ If A < Q skip the following subtraction BCC P%+4 SBC Q \ A >= Q, so set A = A - Q ROL P \ Shift P to the left, pulling the C flag into bit 0 ROL A \ Repeat for the second time CMP Q BCC P%+4 SBC Q ROL P ROL A \ Repeat for the third time CMP Q BCC P%+4 SBC Q ROL P ROL A \ Repeat for the fourth time CMP Q BCC P%+4 SBC Q ROL P ROL A \ Repeat for the fifth time CMP Q BCC P%+4 SBC Q ROL P ROL A \ Repeat for the sixth time CMP Q BCC P%+4 SBC Q ROL P ROL A \ Repeat for the seventh time CMP Q BCC P%+4 SBC Q ROL P ROL A \ Repeat for the eighth time CMP Q BCC P%+4 SBC Q ROL P LDX #0 \ Set X = 0 so this unrolled version of DVID4 also \ returns X = 0 JMP LL28+4 \ Jump to LL28+4 to convert the remainder in A into an \ integer representation of the fractional value A / Q, \ in R, where 1.0 = 255. LL28+4 always returns with the \ C flag cleared, and we return from the subroutine \ using a tail call