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⌂ ∞ ▽ Elite on the 6502

Moving: MVEIT (Part 8 of 9)

[NES version, Bank 0]

       Name: MVEIT (Part 8 of 9)                                     [Show more]
       Type: Subroutine
   Category: Moving
    Summary: Move current ship: Rotate ship about itself by its own pitch/roll
  Deep dive: Orientation vectors
             Pitching and rolling by a fixed angle

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




 This routine has multiple stages. This stage does the following:

   * If the ship we are processing is rolling or pitching itself, rotate it and
     apply damping if required



 LDA INWK+30            ; Fetch the ship's pitch counter and extract the sign
 AND #%10000000         ; into RAT2
 STA RAT2

 LDA INWK+30            ; Fetch the ship's pitch counter and extract the value
 AND #%01111111         ; without the sign bit into A

 BEQ MV8                ; If the pitch counter is 0, then jump to MV8 to skip
                        ; the following, as the ship is not pitching

 CMP #%01111111         ; If bits 0-6 are set in the pitch counter (i.e. the
                        ; ship's pitch is not damping down), then the C flag
                        ; will be set by this instruction

 SBC #0                 ; Set A = A - 0 - (1 - C), so if we are damping then we
                        ; reduce A by 1, otherwise it is unchanged

 ORA RAT2               ; Change bit 7 of A to the sign we saved in RAT2, so
                        ; the updated pitch counter in A retains its sign

 STA INWK+30            ; Store the updated pitch counter in byte #30

 LDX #15                ; Rotate (roofv_x, nosev_x) by a small angle (pitch)
 LDY #9
 JSR MVS5

 LDX #17                ; Rotate (roofv_y, nosev_y) by a small angle (pitch)
 LDY #11
 JSR MVS5

 LDX #19                ; Rotate (roofv_z, nosev_z) by a small angle (pitch)
 LDY #13
 JSR MVS5

.MV8

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 LDA INWK+29            ; Fetch the ship's roll counter and extract the sign
 AND #%10000000         ; into RAT2
 STA RAT2

 LDA INWK+29            ; Fetch the ship's roll counter and extract the value
 AND #%01111111         ; without the sign bit into A

 BEQ MV5                ; If the roll counter is 0, then jump to MV5 to skip the
                        ; following, as the ship is not rolling

 CMP #%01111111         ; If bits 0-6 are set in the roll counter (i.e. the
                        ; ship's roll is not damping down), then the C flag
                        ; will be set by this instruction

 SBC #0                 ; Set A = A - 0 - (1 - C), so if we are damping then we
                        ; reduce A by 1, otherwise it is unchanged

 ORA RAT2               ; Change bit 7 of A to the sign we saved in RAT2, so
                        ; the updated roll counter in A retains its sign

 STA INWK+29            ; Store the updated pitch counter in byte #29

 LDX #15                ; Rotate (roofv_x, sidev_x) by a small angle (roll)
 LDY #21
 JSR MVS5

 LDX #17                ; Rotate (roofv_y, sidev_y) by a small angle (roll)
 LDY #23
 JSR MVS5

 LDX #19                ; Rotate (roofv_z, sidev_z) by a small angle (roll)
 LDY #25
 JSR MVS5