Flight model: SetEngine

       Name: SetEngine                                               [Show more]
       Type: Subroutine
   Category: Flight model
    Summary: Set the engine status
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ApplyFlightModel (Part 7 of 7) calls SetEngine
             * MainLoop (Part 11 of 15) calls SetEngine
             * ProcessLanding (Part 6 of 7) calls SetEngine


 Arguments:

   A                    The new status of the engine:

                          * 0 = engine off

                          * 1 = engine on


.SetEngine

 CMP engineStatus       \ If the value of engineStatus is already the same as A,
 BEQ seng3              \ jump to seng3 to return from the subroutine

 STA engineStatus       \ Update the value of engineStatus to the new status in
                        \ A

 TAX                    \ Either turn the engine sound off (if A = 0) or turn it
 JSR ToggleEngineSound  \ on (if A is non-zero)

 LDA forceFactor+5      \ Set A to the force factor for zLiftDrag

 LDX engineStatus       \ If the engine is now on, jump to seng1
 BNE seng1

 CLC                    \ Set A = A + 20
 ADC #20                \
                        \ so having the engine off increases drag

 BNE seng2              \ Jump to seng2 (this BNE is effectively a JMP as A is
                        \ never zero)

.seng1

 SEC                    \ Set A = A - 20
 SBC #20                \
                        \ so having the engine on decreases drag

.seng2

 STA forceFactor+5      \ Update the force factor for zLiftDrag, so it is
                        \ incremented by 20 when the engine is off, and reduced
                        \ by 20 when the engine is on, i.e. having the engine
                        \ off increases drag

.seng3

 RTS                    \ Return from the subroutine