Skip to navigation

Maths (Arithmetic): DVID4

[BBC Micro disc version, Flight]

Name: DVID4 [Show more] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate (P R) = 256 * A / Q Deep dive: Shift-and-subtract division
Context: See this subroutine in context in the source code Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * DOEXP calls DVID4 * SPS2 calls DVID4

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.
Returns: C flag The C flag is cleared
.DVID4 LDX #8 \ Set a counter in X to count the 8 bits in A 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 .DVL4 ROL A \ Shift A to the left BCS DV8 \ If the C flag is set (i.e. bit 7 of A was set) then \ skip straight to the subtraction CMP Q \ If A < Q skip the following subtraction BCC DV5 .DV8 SBC Q \ A >= Q, so set A = A - Q SEC \ Set the C flag, so that P gets a 1 shifted into bit 0 .DV5 ROL P \ Shift P to the left, pulling the C flag into bit 0 DEX \ Decrement the loop counter BNE DVL4 \ Loop back for the next bit until we have done all 8 \ bits of P 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