Dashboard: ArtificialHorizon

       Name: ArtificialHorizon                                       [Show more]
       Type: Subroutine
   Category: Dashboard
    Summary: Vector line calculation for the artificial horizon on indicator 7
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * UpdateIndicator (Part 11 of 15) calls ArtificialHorizon


 The commentary in this routine is a work in progress.

 matrix4Lo, matrix4Lo+2, matrix4Lo+3, matrix4Lo+4 are only used to provide
 signs in bit 0:
   Negative if bit 0 is set, positive if clear

 matrix4Hi, matrix4Hi+3 provide values for T calculations
 matrix4Hi+2, matrix4Hi+4 provide values for U calculations

 matrix4Hi+3 = current roll orientation, 0-&FF for 0 to 45 degrees
 matrix4Lo+3 = direction of roll

 Y = 0, K = 0:
   T = (matrix4Lo matrix4Hi) / 4
   U = -(matrix4Lo+2 matrix4Hi+2) / 4
   Return (T + U) / 8 with the sign bits retained = x-coord of start

 Y = 0, K = 1:
   T = -(matrix4Lo matrix4Hi) / 4
   U = -(matrix4Lo+2 matrix4Hi+2) / 4
   Return (T + U) / 8 with the sign bits retained = y-coord of start

 Y = 3, K = 0:
   T = (matrix4Lo+3 matrix4Hi+3) / 4
   U = -(matrix4Lo+4 matrix4Hi+4) / 4
   Return (T + U) / 8 with the sign bits retained = x-delta

 Y = 3, K = 1:
   T = -(matrix4Lo+3 matrix4Hi+3) / 4
   U = -(matrix4Lo+4 matrix4Hi+4) / 4
   Return (T + U) / 8 with the sign bits retained = y-delta

 The line is returned relative to the origin (0, 0), so that's as if the centre
 of the artificial horizon indicator were at (0, 0). This means that the deltas
 that are calculated are the equivalent to the end point of the line. The line
 itself gets moved to the location of the on-screen indicator in part 11 of
 UpdateIndicator.


 Arguments:

   K                    The axis to calculate:

                          * 0 = x-axis

                          * 1 = y-axis

   Y                    The value to calculate:

                          * 0 = coordinate of starting point

                          * 3 = deltas (i.e. coordinates of end point)


 Returns:

   A                    Depending on the values of K and Y:

                          * K = 0, Y = 0: returns the x-coordinate of the
                                          artificial horizon's starting point

                          * K = 0, Y = 3: returns the y-coordinate of the
                                          artificial horizon's starting point

                          * K = 1, Y = 0: returns the x-delta of the artificial
                                          horizon

                          * K = 1, Y = 3: returns the y-delta of the artificial
                                          horizon


.ArtificialHorizon

 LDA matrix4Hi,Y        \ Set A = matrix4Hi (Y = 0) or matrix4Hi+3 (Y = 3)

 LSR A                  \ Set A = A / 4
 LSR A

 CPY #0                 \ If Y = 3, halve A again, so A = A / 8
 BNE arhi1
 LSR A

.arhi1

 STA T                  \ Set T = A, so T = A / 4 or A / 8

 LDA matrix4Lo,Y        \ Set A = matrix4Lo (Y = 0) or matrix4Lo+3 (Y = 3)

 EOR K                  \ If K = 1, flip bit 0 of A

 AND #1                 \ If bit 0 of A is zero, jump to arhi2 to skip the
 BEQ arhi2              \ following

 LDA #0                 \ Set T = 0 - T
 SEC
 SBC T
 STA T

.arhi2

 LDA matrix4Hi+2,Y      \ Set A = matrix4Hi+2 (Y = 0) or matrix4Hi+4 (Y = 3)

 LSR A                  \ Set A = A / 4
 LSR A

 CPY #0                 \ If Y = 3, halve A again, so A = A / 8
 BNE arhi3
 LSR A

.arhi3

 STA U                  \ Set U = A, so U = A / 4 or A / 8

 LDA matrix4Lo+2,Y      \ Set A = matrix4Lo+2 (Y = 0) or matrix4Lo+4 (Y = 3)

 CPY #0                 \ If Y = 0, flip bit 0 of A
 BNE arhi4
 EOR #1

.arhi4

 AND #1                 \ If bit 0 of A is zero, jump to arhi5 to skip the
 BEQ arhi5              \ following

 LDA #0                 \ Set U = 0 - U
 SEC
 SBC U
 STA U

.arhi5

 CLC                    \ A = T + U
 LDA T
 ADC U

 BMI arhi6

 LSR A                  \ A = A / 8
 LSR A
 LSR A

 ADC #0                 \ Round up the A/8 division

 RTS                    \ Return from the subroutine

.arhi6

 SEC                    \ A = A / 8 + with bits 5-7 set
 ROR A
 SEC
 ROR A
 SEC
 ROR A

 ADC #0                 \ Round up the A / 8 division

 RTS                    \ Return from the subroutine