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⌂ ∞ ▽ Revs on the BBC Micro

Keyboard: ProcessDrivingKeys (Part 3 of 6)

       Name: ProcessDrivingKeys (Part 3 of 6)                        [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Process joystick brake and throttle

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




.keys13

 LDA leaveTrackTimer    \ If leaveTrackTimer is non-zero then the leave track
 BNE keys19             \ timer is counting down, so jump to keys19 to skip the
                        \ whole brake/throttle section

 BIT configJoystick     \ If bit 7 of configJoystick is clear then the joystick
 BPL keys15             \ is not configured, so jump to keys15 to check for key
                        \ presses for the brake/throttle

 LDX #2                 \ Read the joystick y-axis into A and X, clearing the
 JSR GetADCChannel      \ C flag if A < 10, setting A to the high byte, and
                        \ setting X to 1 if the stick is up, or 0 if the stick
                        \ is down

 BCC keys19             \ If A < 10 then the joystick is pretty close to the
                        \ centre, so jump to keys19 so we don't register any
                        \ throttle or brake activity

 STA T                  \ Set T = A / 2
 LSR T

 ASL A                  \ Set A = A * 2

 ADC T                  \ Set A = A + T
                        \       = A * 2 + A / 2
                        \       = 2.5 * A

 BCS keys14             \ If the addition overflowed, then the joystick has
                        \ moved a long way from the centre, so jump to keys14 to
                        \ apply the brakes or throttle at full power

 CMP #250               \ If A < 250, jump to keys20 to store A in throttleBrake
 BCC keys20             \ and X in throttleBrakeState, so we store the amount of
                        \ brakes or throttle to apply

.keys14

 CPX #0                 \ If X = 0 then the joystick y-axis is positive (down),
 BEQ keys18             \ so jump to keys18 to apply the brakes

 BNE keys16             \ Otherwise the joystick y-axis is negative (up), so
                        \ jump to keys16 to increase the throttle (this BNE is
                        \ effectively a JMP as we already know X is non-zero)