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⌂ ∞ Aviator on the BBC Micro

Dashboard: UpdateIndicator (Part 11 of 15)

       Name: UpdateIndicator (Part 11 of 15)                         [Show more]
       Type: Subroutine
   Category: Dashboard
    Summary: Calculations for the artificial horizon (indicator 7)

    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file




 Calculations for the artificial horizon's line vector are all performed in
 the ArtificialHorizon routine, which calculates the starting coordinates and
 deltas for the vector line. This routine simply takes those results, sets the
 direction bits for the line, and moves the starting point so the line is
 centred on the artificial horizon indicator.



.uind20

                        \ If we get here then the indicator number in X is 7

 LDY #0                 \ Set Y = 0, to use as an argument to ArtificialHorizon

 STY K                  \ Set K = 0, to use as an argument to ArtificialHorizon

 STY R                  \ Set R = 0, to use as the first guess for the direction
                        \ of the horizon line (we will change it below if
                        \ required)

 JSR ArtificialHorizon  \ Call ArtificialHorizon with K = 0, Y = 0 to calculate
                        \ the x-coordinate of the line's starting point in A

 CLC                    \ Clear the C flag (this appears to have no effect)

 STA S                  \ Set H = S, so this sets the x-coordinate of the line's
                        \ starting point

 LDY #3                 \ Set Y = 3

 JSR ArtificialHorizon  \ Call ArtificialHorizon with K = 0, Y = 3 to calculate
                        \ the y-coordinate of the line's starting point in A

 STA H                  \ Set H = A, so this sets the y-coordinate of the line's
                        \ starting point

 LDY #0                 \ Set Y = 0, to use as an argument to ArtificialHorizon

 LDA #1                 \ Set K = 1, to use as an argument to ArtificialHorizon
 STA K

 JSR ArtificialHorizon  \ Call ArtificialHorizon with K = 1, Y = 0 to calculate
                        \ the x-delta of the line

 SEC                    \ Set A = A - S
 SBC S                  \       = A - x-coordinate of start

 BPL uind21             \ If A is positive, the sign of the x-delta is correct,
                        \ so jump to uind21

 STA T                  \ The returned x-delta is negative, so store it in T so
                        \ we can negate it below

 LDA #%10000000         \ Set bit 7 of R to indicate that the x-delta for the
 STA R                  \ line is negative and the y-delta is positive

 LDA #0                 \ Set A = 0 - T
 SEC                    \
 SBC T                  \ so the x-delta in A is now positive

.uind21

 CLC                    \ Set W = A + 1
 ADC #1                 \
 STA W                  \ so this sets the line's x-delta, making sure the line
                        \ is at least one pixel wide

 LDY #3                 \ Set Y = 3, to use as an argument to ArtificialHorizon

 JSR ArtificialHorizon  \ Call ArtificialHorizon with K = 1, Y = 3 to calculate
                        \ the y-delta of the line

 SEC                    \ Set A = A - H
 SBC H                  \       = A - y-coordinate of start

 BPL uind22             \ If A is positive, the sign of the y-delta is correct,
                        \ so jump to uind22

 STA T                  \ The returned y-delta is negative, so store it in T so
                        \ we can negate it below

 LDA #%01000000         \ Set bit 6 of R to indicate that the y-delta is
 ORA R                  \ negative, making sure to leave bit 7 as it is
 STA R

 LDA #0                 \ Set A = A - T
 SEC                    \
 SBC T                  \ so the y-delta in A is now positive

.uind22

 CLC                    \ Set G = A + 1
 ADC #1                 \
 STA G                  \ so this sets the line's y-delta, making sure the line
                        \ is at least 1 pixel tall

 LDA S                  \ Set S = S + 53
 CLC                    \
 ADC #53                \ so the start x-coordinate is moved to be relative to
 STA S                  \ the centre of the artificial horizon indicator, which
                        \ is at (53, 160 + 29)

 LDA H                  \ Set H = H - 29
 CLC                    \
 ADC #227               \ so the start y-coordinate is moved to be relative to
 STA H                  \ the centre of the artificial horizon indicator, which
                        \ is at (53, 160 + 29)

 JMP DrawIndicatorLine  \ Draw the new artificial horizon, returning from the
                        \ subroutine using a tail call