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⌂ ∞ Elite on the BBC Micro and NES

Dashboard: DrawPitchRollBars

[NES version, Bank 0]

       Name: DrawPitchRollBars                                       [Show more]
       Type: Subroutine
   Category: Dashboard
    Summary: Update the pitch and roll bars on the dashboard

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DrawSpaceViewInNMI calls DrawPitchRollBars




 The roll indicator uses sprite 11 and the pitch indicator uses sprite 12.



.DrawPitchRollBars

                        ; We start by drawing the roll indicator

 LDA JSTX               ; Set SC2 = 216 + ~JSTX / 8
 EOR #$FF               ;         = 216 - (JSTX + 1) / 8
 LSR A                  ;
 LSR A                  ; We use this as the x-coordinate of the roll indicator
 LSR A                  ; as the centre of the indicator is at x-coordinate 232
 CLC                    ; and JSTX / 8 is in the range 0 to 31, with a value of
 ADC #216               ; 16 in the middle of the indicator, so SC2 is in the
 STA SC2                ; range 216 to 247 with a middle point of 231 (this
                        ; isn't 232 because of the + 1 in the calculation above)

 LDY #29                ; Set Y = 29 so we draw the indicator sprite on pixel
                        ; y-coordinate 170 + 29 = 199 (which is the y-coordinate
                        ; of the roll indicator)

 LDA #11                ; Set A = 11 so we draw the indicator using sprite 11

 JSR piro1              ; Call piro1 below to draw the roll indicator

                        ; We now draw the pitch indicator

 LDA JSTY               ; Set SC2 = 216 + JSTY / 8
 LSR A                  ;
 LSR A                  ; We use this as the x-coordinate of the pitch indicator
 LSR A                  ; as the centre of the indicator is at x-coordinate 232
 CLC                    ; and JSTY / 8 is in the range 0 to 31, with a value of
 ADC #216               ; 16 in the middle of the indicator, so SC2 is in the
 STA SC2                ; range 216 to 247 with a middle point of 232

 LDY #37                ; Set Y = 37 so we draw the indicator sprite on pixel
                        ; y-coordinate 170 + 37 = 207 (which is the y-coordinate
                        ; of the pitch indicator)

 LDA #12                ; Set A = 11 so we draw the indicator using sprite 12

.piro1

 ASL A                  ; Set X = A * 4
 ASL A                  ;
 TAX                    ; So we can use X as an index into the sprite buffer to
                        ; update the sprite data for sprite A (as each sprite
                        ; has four bytes in the buffer)

 LDA SC2                ; Set the x-coordinate of the relevant indicator's
 SEC                    ; sprite to SC2 - 4, so the centre of the eight-pixel
 SBC #4                 ; wide sprite is on x-coordinate SC2
 STA xSprite0,X

 TYA                    ; Set the y-coordinate of the relevant indicator's
 CLC                    ; sprite to 170 + Y, so it appears on the correct row
 ADC #170+YPAL
 STA ySprite0,X

 RTS                    ; Return from the subroutine