.DVID4 \LDX #8 \ This instruction is commented out in the original \ source 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 \ This label is commented out in the original source \ 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 STA widget \ This contains the code from the LL28+4 routine, so TAX \ this section is exactly equivalent to a JMP LL28+4 BEQ LLfix22 \ call, but is slightly faster as it's been inlined LDA logL,X \ (so it converts the remainder in A into an integer LDX Q \ representation of the fractional value A / Q, in R, SEC \ where 1.0 = 255, and it also clears the C flag SBC logL,X LDX widget LDA log,X LDX Q SBC log,X BCS LL222 TAX LDA alogh,X .LLfix22 STA R \ This is also part of the inline LL28+4 routine RTS .LL222 LDA #255 \ This is also part of the inline LL28+4 routine STA R RTSName: DVID4 [Show more] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate (P R) = 256 * A / Q Deep dive: Shift-and-subtract divisionContext: 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 and the loop is unrolled.
Returns: C flag The C flag is cleared
[X]
Label LL222 is local to this routine
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Label LLfix22 is local to this routine
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Variable alogh (category: Maths (Arithmetic))
Binary antilogarithm table
[X]
Variable log (category: Maths (Arithmetic))
Binary logarithm table (high byte)
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Variable logL (category: Maths (Arithmetic))
Binary logarithm table (low byte)