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BBC Micro Elite

Maths (Arithmetic): LL120

Name: LL120 [View in context] 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
.LL120 LDA XX15 \ Set R = x1_lo STA R \.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