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Elite on the BBC Micro

Version analysis of TT15

This code appears in the following versions (click to see it in the source code):

Code variations between these versions are shown below.

Name: TT15 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.

Code variation 1 of 8A variation in the comments only

This variation is blank in the Cassette, Disc (flight), Disc (docked), Master and Electron versions.

6502SP

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

Code variation 2 of 8A variation in the comments only

This variation is blank in the Cassette, Disc (flight), Disc (docked), Master and Electron versions.

6502SP

Other entry points: TT15b Draw the crosshairs in the current colour
.TT15

Code variation 3 of 8Related to the screen mode

This variation is blank in the Cassette, Disc (flight), Disc (docked), Master and Electron versions.

6502SP

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)

Code variation 4 of 8A variation in the labels only

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, Docked, Electron

6502SP, Master

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
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

Code variation 5 of 8Related to the Master version

In the Master version, the horizontal crosshair doesn't overlap the left border of the Short-range Chart, while it does in the other versions.

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, Docked, 6502SP, Electron

Master

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
BIT QQ11 \ If bit 7 of QQ11 is set, then this this is the BMI TT84 \ Short-range Chart, so jump to TT84 BCC P%+6 \ If the above subtraction underflowed, then A is \ positive, so skip the next two instructions CMP #2 \ If A >= 2, skip the next instruction BCS TT84 LDA #2 \ The subtraction underflowed or A < 2, so set A to 2 \ 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

Code variation 6 of 8A variation in the labels only

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, Docked, Electron

Master

6502SP

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)
BCS P%+6 \ If the above addition overflowed, skip the following \ two instructions to set A = 254 CMP #254 \ The addition didn't overflow, so if A < 254, jump to BCC TT85 \ TT85 LDA #254 \ Set A = 254, so the crosshairs don't spill out of the \ right of the screen .TT85
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

Code variation 7 of 8Related to Elite's use of the Tube

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, Docked, Electron

Master

6502SP

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
JSR HLOIN3 \ Call HLOIN3 to draw a line from (X1, Y1) to (X2, Y1)
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

Code variation 8 of 8Related to the Master version

The bottom border of the Long-range Chart is one pixel lower down the screen in the Master version than in the other versions, so crosshair-clipping code is slightly different too.

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, Docked, 6502SP, Electron

Master

LDA #151 \ Otherwise this is the Long-range Chart, so we need to \ clip the crosshairs at a maximum y-coordinate of 151
LDA #152 \ Otherwise this is the Long-range Chart, so we need to \ clip the crosshairs at a maximum y-coordinate of 152
.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

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