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BBC Micro Elite

Drawing lines: LL145 (Part 3 of 4)

Name: LL145 (Part 3 of 4) [View in context] Type: Subroutine Category: Drawing lines Summary: Clip line: Calculate the line's gradient
.LL115 TYA \ Store Y on the stack so we can preserve it through the PHA \ call to this subroutine LDA XX15+4 \ Set XX12+2 = x2_lo - x1_lo SEC SBC XX15 STA XX12+2 LDA XX15+5 \ Set XX12+3 = x2_hi - x1_hi SBC XX15+1 STA XX12+3 LDA XX12 \ Set XX12+4 = y2_lo - y1_lo SEC SBC XX15+2 STA XX12+4 LDA XX12+1 \ Set XX12+5 = y2_hi - y1_hi SBC XX15+3 STA XX12+5 \ So we now have: \ \ delta_x in XX12(3 2) \ delta_y in XX12(5 4) \ \ where the delta is (x1, y1) - (x2, y2)) EOR XX12+3 \ Set S = the sign of delta_x * the sign of delta_y, so STA S \ if bit 7 of S is set, the deltas have different signs LDA XX12+5 \ If delta_y_hi is positive, jump down to LL110 to skip BPL LL110 \ the following LDA #0 \ Otherwise flip the sign of delta_y to make it SEC \ positive, starting with the low bytes SBC XX12+4 STA XX12+4 LDA #0 \ And then doing the high bytes, so now: SBC XX12+5 \ STA XX12+5 \ XX12(5 4) = |delta_y| .LL110 LDA XX12+3 \ If delta_x_hi is positive, jump down to LL111 to skip BPL LL111 \ the following SEC \ Otherwise flip the sign of delta_x to make it LDA #0 \ positive, starting with the low bytes SBC XX12+2 STA XX12+2 LDA #0 \ And then doing the high bytes, so now: SBC XX12+3 \ \ (A XX12+2) = |delta_x| .LL111 \ We now keep halving |delta_x| and |delta_y| until \ both of them have zero in their high bytes TAX \ IF |delta_x_hi| is non-zero, skip the following BNE LL112 LDX XX12+5 \ If |delta_y_hi| = 0, jump down to LL113 (as both BEQ LL113 \ |delta_x_hi| and |delta_y_hi| are 0) .LL112 LSR A \ Halve the value of delta_x in (A XX12+2) ROR XX12+2 LSR XX12+5 \ Halve the value of delta_y XX12(5 4) ROR XX12+4 JMP LL111 \ Loop back to LL111 .LL113 \ By now, the high bytes of both |delta_x| and |delta_y| \ are zero STX T \ We know that X = 0 as that's what we tested with a BEQ \ above, so this sets T = 0 LDA XX12+2 \ If delta_x_lo < delta_y_lo, so our line is more CMP XX12+4 \ vertical than horizontal, jump to LL114 BCC LL114 STA Q \ Set Q = delta_x_lo LDA XX12+4 \ Set A = delta_y_lo JSR LL28 \ Call LL28 to calculate: \ \ R = 256 * A / Q \ = 256 * delta_y_lo / delta_x_lo JMP LL116 \ Jump to LL116, as we now have the line's gradient in R .LL114 LDA XX12+4 \ Set Q = delta_y_lo STA Q LDA XX12+2 \ Set A = delta_x_lo JSR LL28 \ Call LL28 to calculate: \ \ R = 256 * A / Q \ = 256 * delta_x_lo / delta_y_lo DEC T \ T was set to 0 above, so this sets T = &FF