Skip to navigation

Aviator on the BBC Micro

Flight model: ApplyFlightModel (Part 5 of 7)

Name: ApplyFlightModel (Part 5 of 7) [Show more] Type: Subroutine Category: Flight model Summary: Calculate the forces for when we are on the ground Deep dive: The flight model On-ground calculations
Context: See this subroutine in context in the source code References: No direct references to this subroutine in this source file

This part does the following: * If we are on the ground, then: * Apply ground steering to yTurn if the rudder is used and forward speed is >= 20. * Stop the plane from rolling by setting the roll rate in dzTurn to 0. * If the undercarriage is up, prevent the plane from pitching forward below the level of the ground. * Calculate the effect of being on the ground on the forces and landing status: * If the plane is stationary, set landingStatus = %01000000 and skip to the side velocity step below. * If the plane is travelling forwards at a speed of 10 or less, set zLinear = -256 and landingStatus = %01000000 and skip to the side velocity step below. * If the plane is on the runway, the undercarriage is down and the brakes are off, set landingStatus = 0 and skip to the side velocity step below. * Otherwise, set landingStatus = 0 and subtract the following from zLinearHi (from the least slowdown to the biggest slowdown): * 7 if the plane is not on the runway and: undercarriage is down and brakes are off * 11 if the plane is on the runway and: undercarriage is up or undercarriage is down and brakes are on * 50 if the plane is not on the runway and: undercarriage is up or undercarriage is down and brakes are on * 248 if the plane is going backwards (zVelocityP < 0) In other words, the following factors slow us down when travelling fowards along the ground: * Having the brakes on * Having the undercarriage up * Having a bad approach * Calculate the effect of side velocity on xLinear: xLinear = -xVelocityPLo * 128
LDA L \ Fetch the value of onGround that we stored in L above BNE fmod4 \ If onGround is non-zero then we are on the ground, so \ jump to fmod4 to keep going JMP fmod17 \ We are not on the ground, so jump to fmod17 to move on \ to the next part .fmod4 \ We start by processing the effect of the rudder \ control on the ground steering (as the rudder control \ doubles up as the ground steering control) LDX #0 \ Set (X Y) = 0, to use as the value of yTurn below LDY #0 LDA zVelocityPHi \ If the forward airspeed is negative, jump to fmod6 to BMI fmod6 \ set (X Y) = 0 BNE fmod5 \ If the high byte of the forward airspeed is non-zero, \ jump to fmod5 to set (X Y) = rudderPosition \ If we get here then the high byte of the forward \ airspeed is zero LDA zVelocityPLo \ If the low byte of the forward airspeed is less than CMP #20 \ 20, jump to fmod6 to set (X Y) = 0 BCC fmod6 .fmod5 LDY rudderPosition \ If the position of the rudder is positive, jump to BPL fmod6 \ fmod6 to skip the following instruction DEX \ Decrement X to &FF, so X contains the correct sign in \ the 16-bit number: \ \ (X Y) = rudderPosition .fmod6 \ By the time we get here, (X Y) contains: \ \ * 0 if the forward airspeed is negative or < 20 \ \ * rudderPosition otherwise \ \ This is the effect of ground steering, which is \ controlled by the rudder control when we are on the \ ground, and only works when the plane is travelling \ forward with a minimum speed of 20 (as it works by \ applying brakes to the wheels, which needs speed to \ work) \ \ Let's call this figure groundSteering STY yTurnHi \ Set (A yTurnHi) = (X Y) TXA \ = groundSteering LDX #1 \ Set X as a shift counter in the following loop, so we \ shift left by 2 places .fmod7 ASL yTurnHi \ Set (A yTurnHi) = (A yTurnHi) << 1 ROL A DEX \ Decrement the shift counter BPL fmod7 \ Loop back until we have shifted left by 2 places, so: \ \ (A yTurnHi) = (A yTurnHi) * 4 \ = groundSteering * 4 STA yTurnTop \ Set yTurn = (A yTurnHi) \ \ So we have now set yTurn to groundSteering * 4, so the \ plane steers on the ground when we apply the rudder LDX #&82 \ Set (dzTurnTop dzTurnHi) = 0 JSR ResetVariable \ \ This also sets A = 0 STA dzTurnLo \ Set dzTurnLo = 0, so by now we have: \ \ (dzTurnTop dzTurnHi dzTurnLo) = 0 \ We now work out the effect of the various landing \ configurations to get the amount that the plane is \ being slowed down by being on the ground LDY ucStatus \ If ucStatus is non-zero then the undercarriage is BNE fmod9 \ down, so jump to fmod9 to move on to the brake checks LDA xRotationHi \ If the plane's rotation about the x-axis is positive AND dxTurnTop \ or the calculated rate of change of rotation around BPL fmod8 \ x-axis is positive, jump to fmod8 skip the following \ If we get here then both the following are negative: \ \ * The plane's current rotation about the x-axis \ \ * The calculated rate of change of rotation around \ the x-axis \ \ In other words, the nose has dipped below the forward \ horizontal and is heading down further, which can't \ happen when the undercarriage is up and we are on the \ ground, so we now set dxTurn to 0 to stop the plane \ from pitching any further into the ground LDX #&80 \ Set (dxTurnTop dxTurnHi) = 0 JSR ResetVariable \ \ This also sets A = 0 STA dxTurnLo \ Set dxTurnLo = 0, so by now we have: \ \ (dxTurnTop dxTurnHi dxTurnLo) = 0 .fmod8 JSR CheckPlaneOnRunway \ Check whether the plane is over the runway BCC fmod10 \ If the plane is on the runway, then jump to fmod10 to \ set A = 11 LDA #50 \ Set A = 50 and jump to fmodll (this BNE is effectively BNE fmod11 \ a JMP as A is never zero) .fmod9 LDX brakesStatus \ If brakesStatus is non-zero then the brakes are on, so BNE fmod8 \ jump to fmod8 \ If we get here then the brakes are off and the \ undercarriage is down JSR CheckPlaneOnRunway \ Check whether the plane is over the runway BCC fmod15 \ If the plane is on the runway, then jump to fmod15 to \ setlandingStatus = 0 LDA #7 \ Set A = 7 and jump to fmod11 (this BNE is effectively BNE fmod11 \ a JMP as A is never zero) .fmod10 LDA #11 \ Set A = 11 .fmod11 LDX zVelocityPHi \ If the plane is going backwards, jump to fmod12 to set BMI fmod12 \ A = 248 BNE fmod13 \ If the plane is moving forwards, jump to fmod13 to \ subtract A from zLinearHi \ If we get here then zVelocityPHi = 0 LDX zVelocityPLo \ If the plane is stationary (i.e. both zVelocityPLo and BEQ fmod14 \ zVelocityPHi = 0), jump to fmod14 to leave zLinear \ untouched and set landingStatus = %01000000 CPX #11 \ If the plane is moving forwards at a speed of 11 or BCS fmod13 \ more, jump to fmod13 to subtract A from zLinearHi LDA #0 \ Set zLinear = -256 STA zLinearLo LDA #&FF STA zLinearHi BNE fmod14 \ Jump to fmod14 to set landingStatus = %01000000 (this \ BNE is effectively a JMP as A is never zero) .fmod12 LDA #248 \ Set A = 248 .fmod13 \ We do not reach this point if any of the following are \ true: \ \ * The plane is stationary, in which case we already \ moved on with landingStatus = %01000000 \ \ * The plane is travelling forwards at a speed of 10 \ or less, in which case we already moved on with \ zLinear = -256 and landingStatus = %01000000 \ \ * The plane is on the runway, the undercarriage is \ down, the brakes are off, in which case we already \ moved on with landingStatus = 0 \ \ Otherwise, we get here and A is one of the following: \ \ * 248 if the plane is going backwards \ \ * 11 if the plane is on the runway and: \ either undercarriage is up \ or undercarriage is down, brakes are on \ \ * 50 if the plane is not on the runway and: \ either undercarriage is up \ or undercarriage is down, brakes are on \ \ * 7 if the plane is not on the runway and: \ undercarriage is down, brakes are off \ \ We now subtract the value of A from zLinearHi STA P \ Set zLinearHi = zLinearHi - A SEC LDA zLinearHi SBC P STA zLinearHi JMP fmod15 \ Jump to fmod15 to set landingStatus = 0 .fmod14 LDA #%01000000 \ Set A = %01000000 to use as the value of landingStatus BNE fmod16 \ Jump to fmod16 to set the value of landingStatus (this \ BNE is effectively a JMP as A is never zero) .fmod15 LDA #0 \ Set A = 0 to use as the value of landingStatus .fmod16 STA landingStatus \ Set landingStatus to the value in A LDA xVelocityPLo \ Set P = xVelocityPLo / 2 LSR A \ STA P \ and shift bit 0 of xVelocityPLo into the C flag LDX #0 \ Set X = 0 STX slipRate \ Set slipRate = 0 TXA \ Set A = 0 ROR A \ Shift the C flag into bit 7 of V, so bit 7 of V now STA V \ contains the bit 0 of xVelocityPLo that we shifted \ into the C flag above LDA xVelocityPHi \ Set Q = P with the sign bit from xVelocityPHi AND #%10000000 \ = xVelocityPLo / 2 with the correct sign ORA P STA Q \ If xVelocityPLo is %vvvvvvvv and the sign bit of \ xVelocityPHi is %s, then this sets \ \ (Q V) = %svvvvvvv v0000000 \ \ which is xVelocityPLo << 7, or xVelocityPLo * 128 TXA \ Set A = 0 SEC \ Set xLinear = (0 0) - (Q V) SBC V \ = -xVelocityPLo * 128 STA xLinearLo \ \ starting with the low bytes TXA \ And then the high bytes SBC Q STA xLinearHi