Flight model: ApplyFlightModel (Part 7 of 7)

       Name: ApplyFlightModel (Part 7 of 7)                          [Show more]
       Type: Subroutine
   Category: Flight model
    Summary: Calculate fuel usage
  Deep dive: The flight model
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


 This routine calculates the following:

   * Calculate fuel usage, depending on the current thrust, and decrease the
     fuel level as required.


 LDA fuelLevel          \ If the fuel tank is empty, jump to fmod19 to turn the
 BEQ fmod19             \ engine off, as we just ran out of fuel

 LDA engineStatus       \ If the engine is off, jump to fmod20 to return from
 BEQ fmod20             \ the subroutine

                        \ Otherwise the engine is on and we are not yet out of
                        \ fuel, so we need to calculate the correct consumption
                        \ rate and deplete the fuel supplies

 LDA thrustHi           \ Set (R A) = (thrustHi thrustLo)
 STA R                  \           = thrust
 LDA thrustLo

 LDX #3                 \ We now shift (R A) right by four places, so set a
                        \ shift counter in X

.fmod18

 LSR R                  \ Set (R A) = (R A) / 2
 ROR A                  \           = thrust / 2

 DEX                    \ Decrement the shift counter

 BPL fmod18             \ Loop back until we have shifted (R A) four times

                        \ By now we have:
                        \
                        \   (R A) = thrust / 16
                        \
                        \ and because the maximum value of thrust is 1280, we
                        \ know R must be 0, so:
                        \
                        \   A = thrust / 16
                        \
                        \ We use this value of A as the amount of fuel used in
                        \ this iteration of the main loop, so the higher the
                        \ thrust, the more fuel is used
                        \
                        \ That said, we don't just take this off the fuel level
                        \ in fuelLevel - instead, we keep track of the fuel
                        \ used in two bytes, fuelUsedLo and fuelUsedHi
                        \
                        \ We add the fuel used in A to fuelUsedLo, and when
                        \ we've used 256 units and fuelLevelLo wraps around
                        \ from 255 to 0, we then add 4 to fuelUsedHi, and when
                        \ we've filled up fuelUsedHi and it wraps from 255 to 0,
                        \ that's when we take one unit of fuel off the main
                        \ fuel level in fuelLevel
                        \
                        \ In summary, to use up one unit of fuelLevel, we have
                        \ to use up 256 * 64 = 16384 units from the above
                        \ calculation

 CLC                    \ Set fuelUsedLo = fuelUsedLo + A
 ADC fuelUsedLo
 STA fuelUsedLo

 BCC fmod20             \ If the addition didn't overflow, jump to fmod20
                        \ to return from the subroutine

 LDA #4                 \ The addition overflowed, so set:
 CLC                    \
 ADC fuelUsedHi         \   fuelUsedHi = fuelUsedHi + 4
 STA fuelUsedHi

 BCC fmod20             \ If the addition didn't overflow, jump to fmod20
                        \ to return from the subroutine

                        \ The addition overflowed, so it's time to reduce our
                        \ fuel level

 LDA fuelLevel          \ If the fuel tank is already empty, jump to fmod19 to
 BEQ fmod19             \ turn the engine off, as we just ran out of fuel

 DEC fuelLevel          \ Otherwise decrement the fuel level by 1

 BNE fmod20             \ If the fuel tank is still not empty, jump to fmod20 to
                        \ return from the subroutine

.fmod19

 LDA #0                 \ The fuel tank is empty, so turn the engine off
 JSR SetEngine

.fmod20

 RTS                    \ Return from the subroutine