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BBC Micro Elite

Drawing lines: TT15 (6502SP version)

Name: TT15 [View in context] Type: Subroutine Category: Drawing lines Summary: Draw a set of crosshairs
For all views except the Short-range Chart, the centre is drawn 24 pixels to the right of the y-coordinate given. The crosshairs are drawn in colour 3, which is white in the chart view and cyan elsewhere. We can draw them in the current colour by calling the TT15b entry point. Arguments: QQ19 The pixel x-coordinate of the centre of the crosshairs QQ19+1 The pixel y-coordinate of the centre of the crosshairs QQ19+2 The size of the crosshairs Other entry points: TT15b Draw the crosshairs in the current colour
.TT15 LDA #CYAN \ Send a #SETCOL CYAN command to the I/O processor to JSR DOCOL \ switch to colour 3, which is white in the chart view \ and cyan elsewhere .TT15b LDA #24 \ Set A to 24, which we will use as the minimum \ screen indent for the crosshairs (i.e. the minimum \ distance from the top-left corner of the screen) LDX QQ11 \ If the current view is not the Short-range Chart, BPL TT178 \ which is the only view with bit 7 set, then jump to \ TT178 to skip the following instruction LDA #0 \ This is the Short-range Chart, so set A to 0, so the \ crosshairs can go right up against the screen edges .TT178 STA QQ19+5 \ Set QQ19+5 to A, which now contains the correct indent \ for this view LDA QQ19 \ Set A = crosshairs x-coordinate - crosshairs size SEC \ to get the x-coordinate of the left edge of the SBC QQ19+2 \ crosshairs BCS TT84 \ If the above subtraction didn't underflow, then A is \ positive, so skip the next instruction LDA #0 \ The subtraction underflowed, so set A to 0 so the \ crosshairs don't spill out of the left of the screen .TT84 \ In the following, the authors have used XX15 for \ temporary storage. XX15 shares location with X1, Y1, \ X2 and Y2, so in the following, you can consider \ the variables like this: \ \ XX15 is the same as X1 \ XX15+1 is the same as Y1 \ XX15+2 is the same as X2 \ XX15+3 is the same as Y2 \ \ Presumably this routine was written at a different \ time to the line-drawing routine, before the two \ workspaces were merged to save space STA XX15 \ Set XX15 (X1) = A (the x-coordinate of the left edge \ of the crosshairs) LDA QQ19 \ Set A = crosshairs x-coordinate + crosshairs size CLC \ to get the x-coordinate of the right edge of the ADC QQ19+2 \ crosshairs BCC TT85 \ If the above addition didn't overflow, then A is \ correct, so jump to TT85 to skip the next instruction LDA #255 \ The addition overflowed, so set A to 255 so the \ crosshairs don't spill out of the right of the screen \ (as 255 is the x-coordinate of the rightmost pixel \ on-screen) .TT85 STA XX15+2 \ Set XX15+2 (X2) = A (the x-coordinate of the right \ edge of the crosshairs) LDA QQ19+1 \ Set XX15+1 (Y1) = crosshairs y-coordinate + indent CLC \ to get the y-coordinate of the centre of the ADC QQ19+5 \ crosshairs STA XX15+1 STA XX15+3 \ Set XX15+3 (Y2) = crosshairs y-coordinate + indent JSR LL30 \ Draw a line from (X1, Y1) to (X2, Y2), where Y1 = Y2, \ which will draw from the left edge of the crosshairs \ to the right edge, through the centre of the \ crosshairs LDA QQ19+1 \ Set A = crosshairs y-coordinate - crosshairs size SEC \ to get the y-coordinate of the top edge of the SBC QQ19+2 \ crosshairs BCS TT86 \ If the above subtraction didn't underflow, then A is \ correct, so skip the next instruction LDA #0 \ The subtraction underflowed, so set A to 0 so the \ crosshairs don't spill out of the top of the screen .TT86 CLC \ Set XX15+1 (Y1) = A + indent to get the y-coordinate ADC QQ19+5 \ of the top edge of the indented crosshairs STA XX15+1 LDA QQ19+1 \ Set A = crosshairs y-coordinate + crosshairs size CLC \ + indent to get the y-coordinate of the bottom edge ADC QQ19+2 \ of the indented crosshairs ADC QQ19+5 CMP #152 \ If A < 152 then skip the following, as the crosshairs BCC TT87 \ won't spill out of the bottom of the screen LDX QQ11 \ A >= 152, so we need to check whether this will fit in \ this view, so fetch the view number BMI TT87 \ If this is the Short-range Chart then the y-coordinate \ is fine, so skip to TT87 LDA #151 \ Otherwise this is the Long-range Chart, so we need to \ clip the crosshairs at a maximum y-coordinate of 151 .TT87 STA XX15+3 \ Set XX15+3 (Y2) = A (the y-coordinate of the bottom \ edge of the crosshairs) LDA QQ19 \ Set XX15 (X1) = the x-coordinate of the centre of the STA XX15 \ crosshairs STA XX15+2 \ Set XX15+2 (X2) = the x-coordinate of the centre of \ the crosshairs JMP LL30 \ Draw a vertical line (X1, Y1) to (X2, Y2), which will \ draw from the top edge of the crosshairs to the bottom \ edge, through the centre of the crosshairs, returning \ from the subroutine using a tail call