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Maths (Arithmetic): LL61

[6502 Second Processor version]

Name: LL61 [Show more] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate (U R) = 256 * A / Q
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * LL9 (Part 8 of 12) calls LL61

Calculate the following, where A >= Q: (U R) = 256 * A / Q This is a sister routine to LL28, which does the division when A < Q.
.LL61 LDX Q \ If Q = 0, jump down to LL84 to return a division BEQ LL84 \ error \ The LL28 routine returns A / Q, but only if A < Q. In \ our case A >= Q, but we still want to use the LL28 \ routine, so we halve A until it's less than Q, call \ the division routine, and then double A by the same \ number of times LDX #0 \ Set X = 0 to count the number of times we halve A .LL63 LSR A \ Halve A by shifting right INX \ Increment X CMP Q \ If A >= Q, loop back to LL63 to halve it again BCS LL63 STX S \ Otherwise store the number of times we halved A in S JSR LL28 \ Call LL28 to calculate: \ \ R = 256 * A / Q \ \ which we can do now as A < Q LDX S \ Otherwise restore the number of times we halved A \ above into X LDA R \ Set A = our division result .LL64 ASL A \ Double (U A) by shifting left ROL U BMI LL84 \ If bit 7 of U is set, the doubling has overflowed, so \ jump to LL84 to return a division error DEX \ Decrement X BNE LL64 \ If X is not yet zero then we haven't done as many \ doublings as we did halvings earlier, so loop back for \ another doubling STA R \ Store the low byte of the division result in R RTS \ Return from the subroutine .LL84 LDA #50 \ If we get here then either we tried to divide by 0, or STA R \ the result overflowed, so we set U and R to 50 STA U RTS \ Return from the subroutine