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Elite on the BBC Micro

Version analysis of EDGES

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

Code variations between these versions are shown below.

Name: EDGES Type: Subroutine Category: Drawing lines Summary: Draw a horizontal line given a centre and a half-width
Set X1 and X2 to the x-coordinates of the ends of the horizontal line with centre x-coordinate YY(1 0), and length A in either direction from the centre (so a total line length of 2 * A). In other words, this line: X1 YY(1 0) X2 +-----------------+-----------------+ <- A -> <- A -> The resulting line gets clipped to the edges of the screen, if needed. If the calculation doesn't overflow, we return with the C flag clear, otherwise the C flag gets set to indicate failure and the Y-th LSO entry gets set to 0. Arguments: A The half-length of the line YY(1 0) The centre x-coordinate Returns: C flag Clear if the line fits on-screen, set if it doesn't X1, X2 The x-coordinates of the clipped line LSO+Y If the line doesn't fit, LSO+Y is set to 0 Y Y is preserved

Code variation 1 of 5A variation in the comments only

This variation is blank in the Cassette, Disc (flight), 6502 Second Processor and Master versions.

Docked

Other entry points: PL44 Clear the C flag and return from the subroutine
.EDGES STA T \ Set T to the line's half-length in argument A CLC \ We now calculate: ADC YY \ STA X2 \ (A X2) = YY(1 0) + A \ \ to set X2 to the x-coordinate of the right end of the \ line, starting with the low bytes LDA YY+1 \ And then adding the high bytes ADC #0 BMI ED1 \ If the addition is negative then the calculation has \ overflowed, so jump to ED1 to return a failure BEQ P%+6 \ If the high byte A from the result is 0, skip the \ next two instructions, as the result already fits on \ the screen

Code variation 2 of 5Specific to an individual platform

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

Cassette, Flight, Docked, 6502SP

Master

LDA #254 \ The high byte is positive and non-zero, so we went STA X2 \ past the right edge of the screen, so clip X2 to the \ x-coordinate of the right edge of the screen
LDA #255 \ The high byte is positive and non-zero, so we went STA X2 \ past the right edge of the screen, so clip X2 to the \ x-coordinate of the right edge of the screen
 LDA YY                 \ We now calculate:
 SEC                    \
 SBC T                  \   (A X1) = YY(1 0) - argument A
 STA X1                 \
                        \ to set X1 to the x-coordinate of the left end of the
                        \ line, starting with the low bytes

 LDA YY+1               \ And then subtracting the high bytes
 SBC #0

 BNE ED3                \ If the high byte subtraction is non-zero, then skip
                        \ to ED3

 CLC                    \ Otherwise the high byte of the subtraction was zero,
                        \ so the line fits on-screen and we clear the C flag to
                        \ indicate success

 RTS                    \ Return from the subroutine

.ED3

 BPL ED1                \ If the addition is positive then the calculation has
                        \ underflowed, so jump to ED1 to return a failure

Code variation 3 of 5Specific to an individual platform

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

Cassette, Flight, Docked, 6502SP

Master

LDA #2 \ The high byte is negative and non-zero, so we went STA X1 \ past the left edge of the screen, so clip X1 to the \ y-coordinate of the left edge of the screen
LDA #0 \ The high byte is negative and non-zero, so we went STA X1 \ past the left edge of the screen, so clip X1 to the \ y-coordinate of the left edge of the screen

Code variation 4 of 5A variation in the labels only

This variation is blank in the Cassette, Disc (flight), 6502 Second Processor and Master versions.

Docked

.PL44
 CLC                    \ The line does fit on-screen, so clear the C flag to
                        \ indicate success

 RTS                    \ Return from the subroutine

.ED1

 LDA #0                 \ Set the Y-th byte of the LSO block to 0
 STA LSO,Y

Code variation 5 of 5Minor and very low-impact

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

Cassette, Flight, 6502SP, Master

Docked

SEC \ The line does not fit on the screen, so set the C flag \ to indicate this result RTS \ Return from the subroutine
\ The line does not fit on the screen, so fall through \ into PL21 to set the C flag to indicate this result
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