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BBC Micro Elite

Drawing circles: BLINE (6502SP version)

Name: BLINE [View in context] Type: Subroutine Category: Drawing circles Summary: Draw a circle segment and add it to the ball line heap Deep dive: The ball line heap
Draw a single segment of a circle by adding the point to the ball line heap, so it can be sent to the I/O processor for drawing once the whole circle has been added to the heap. Arguments: CNT The number of this segment STP The step size for the circle K6(1 0) The x-coordinate of the new point on the circle, as a screen coordinate (T X) The y-coordinate of the new point on the circle, as an offset from the centre of the circle FLAG Set to &FF for the first call, so it sets up the first point in the heap but waits until the second call before drawing anything (as we need two points, i.e. two calls, before we can draw a line) K The circle's radius K3(1 0) Pixel x-coordinate of the centre of the circle K4(1 0) Pixel y-coordinate of the centre of the circle SWAP If non-zero, we swap (X1, Y1) and (X2, Y2) Returns: CNT CNT is updated to CNT + STP A The new value of CNT FLAG Set to 0
.BLINE TXA \ Set K6(3 2) = (T X) + K4(1 0) ADC K4 \ = y-coord of centre + y-coord of new point STA K6+2 \ LDA K4+1 \ so K6(3 2) now contains the y-coordinate of the new ADC T \ point on the circle but as a screen coordinate, to go STA K6+3 \ along with the screen y-coordinate in K6(1 0) LDA FLAG \ If FLAG = 0, jump down to BL1 BEQ BL1 INC FLAG \ Flag is &FF so this is the first call to BLINE, so \ increment FLAG to set it to 0, as then the next time \ we call BLINE it can draw the first line, from this \ point to the next BEQ BL5 \ This is the first call to BLINE, so we don't need to \ to copy the previous point to XX15 as there isn't one, \ so we jump to BL5 to tidy up and return from the \ subroutine (this BEQ is effectively a JMP, as we just \ incremented FLAG to 0) .BL1 LDA K5 \ Set XX15 = K5 = x_lo of previous point STA XX15 LDA K5+1 \ Set XX15+1 = K5+1 = x_hi of previous point STA XX15+1 LDA K5+2 \ Set XX15+2 = K5+2 = y_lo of previous point STA XX15+2 LDA K5+3 \ Set XX15+3 = K5+3 = y_hi of previous point STA XX15+3 LDA K6 \ Set XX15+4 = x_lo of new point STA XX15+4 LDA K6+1 \ Set XX15+5 = x_hi of new point STA XX15+5 LDA K6+2 \ Set XX12 = y_lo of new point STA XX12 LDA K6+3 \ Set XX12+1 = y_hi of new point STA XX12+1 JSR LL145 \ Call LL145 to see if the new line segment needs to be \ clipped to fit on-screen, returning the clipped line's \ end-points in (X1, Y1) and (X2, Y2) BCS BL5 \ If the C flag is set then the line is not visible on \ screen anyway, so jump to BL5, to avoid drawing and \ storing this line LDY LSP \ Set Y = LSP LDA X1 \ Store X1 in the LSP-th byte of LSX2 STA LSX2,Y LDA Y1 \ Store Y1 in the LSP-th byte of LSY2 STA LSY2,Y INY \ Increment Y to point to the next byte in LSX2/LSY2 LDA X2 \ Store X2 in the LSP-th byte of LSX2 STA LSX2,Y LDA Y2 \ Store Y2 in the LSP-th byte of LSX2 STA LSY2,Y INY \ Increment Y to point to the next byte in LSX2/LSY2 STY LSP \ Update LSP to point to the same as Y .BL5 LDA K6 \ Copy the data for this step point from K6(3 2 1 0) STA K5 \ into K5(3 2 1 0), for use in the next call to BLINE: LDA K6+1 \ STA K5+1 \ * K5(1 0) = screen x-coordinate of this point LDA K6+2 \ STA K5+2 \ * K5(3 2) = screen y-coordinate of this point LDA K6+3 \ STA K5+3 \ They now become the "previous point" in the next call LDA CNT \ Set CNT = CNT + STP CLC ADC STP STA CNT RTS \ Return from the subroutine