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BBC Micro Elite

Drawing lines: TT15

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. 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
.TT15 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 P%+4 \ which is the only view with bit 7 set, then 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 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 P%+4 \ If the above addition didn't overflow, then A is \ correct, so 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) 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 JSR HLOIN \ Draw a horizontal line from (X1, Y1) to (X2, Y1), \ 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