.LL120 LDA XX15 \ Set R = x1_lo STA RName: LL120 Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate (Y X) = (S x1_lo) * XX12+2 or (S x1_lo) / XX12+2
Calculate the following: * If T = 0 (more vertical than horizontal), (Y X) = (S x1_lo) * XX12+2 * If T <> 0 (more horizontal than vertical), (Y X) = (S x1_lo) / XX12+2 giving (Y X) the opposite sign to the slope direction in XX12+3. Other entry points: LL122 Calculate (Y X) = (S R) * Q and set the sign to the opposite of the top byte on the stack
This variation is blank in the Disc (flight), Disc (docked), 6502 Second Processor, Master and Electron versions.
Cassette\.LL120 \ This label is commented out in the original source
JSR LL129 \ Call LL129 to do the following: \ \ Q = XX12+2 \ = line gradient \ \ A = S EOR XX12+3 \ = S EOR slope direction \ \ (S R) = |S R| \ \ So A contains the sign of S * slope direction PHA \ Store A on the stack so we can use it later LDX T \ If T is non-zero, so it's more horizontal than BNE LL121 \ vertical, jump down to LL121 to calculate this \ instead: \ \ (Y X) = (S R) / Q .LL122 \ The following calculates: \ \ (Y X) = (S R) * Q \ \ using the same shift-and-add algorithm that's \ documented in MULT1 LDA #0 \ Set A = 0 TAX \ Set (Y X) = 0 so we can start building the answer here TAY LSR S \ Shift (S R) to the right, so we extract bit 0 of (S R) ROR R \ into the C flag ASL Q \ Shift Q to the left, catching bit 7 in the C flag BCC LL126 \ If C (i.e. the next bit from Q) is clear, do not do \ the addition for this bit of Q, and instead skip to \ LL126 to just do the shifts .LL125 TXA \ Set (Y X) = (Y X) + (S R) CLC \ ADC R \ starting with the low bytes TAX TYA \ And then doing the high bytes ADC S TAY .LL126 LSR S \ Shift (S R) to the right ROR R ASL Q \ Shift Q to the left, catching bit 7 in the C flag BCS LL125 \ If C (i.e. the next bit from Q) is set, loop back to \ LL125 to do the addition for this bit of Q BNE LL126 \ If Q has not yet run out of set bits, loop back to \ LL126 to do the "shift" part of shift-and-add until \ we have done additions for all the set bits in Q, to \ give us our multiplication result PLA \ Restore A, which we calculated above, from the stack BPL LL133 \ If A is positive jump to LL133 to negate (Y X) and \ return from the subroutine using a tail call RTS \ Return from the subroutine