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⌂ ∞ ▽ Elite on the 6502

Moving: MVEIT (Part 4 of 9)

[NES version, Bank 0]

       Name: MVEIT (Part 4 of 9)                                     [Show more]
       Type: Subroutine
   Category: Moving
    Summary: Move current ship: Apply acceleration to ship's speed as a one-off

    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:

   * Apply acceleration to the ship's speed (if acceleration is non-zero),
     and then zero the acceleration as it's a one-off change



 LDA INWK+27            ; Set A = the ship's speed in byte #24 + the ship's
 CLC                    ; acceleration in byte #28
 ADC INWK+28

 BPL P%+4               ; If the result is positive, skip the following
                        ; instruction

 LDA #0                 ; Set A to 0 to stop the speed from going negative

 STA INWK+27            ; Store the updated speed in byte #27

 LDY #15                ; Set A to byte #15 from the current ship blueprint,
 JSR GetShipBlueprint   ; which contains the ship's maximum speed

 CMP INWK+27            ; If A >= the ship's current speed, skip the following
 BCS P%+4               ; instruction as the speed is already in the correct
                        ; range

 STA INWK+27            ; Otherwise store the maximum speed in byte #27

 LDA #0                 ; We have added the ship's acceleration, so we now set
 STA INWK+28            ; it back to 0 in byte #28, as it's a one-off change