Map of the source code

Workspace: Zero page

Mainly temporary variables that are used a lot

Workspace: Stack variables

Variables that share page 1 with the stack

Workspace: Main variable workspace

The main block of game variables

Subroutine: SetupScreen

Set up the screen mode and load the dashboard image

Variable: loadDashboard

The OS command string for loading the dashboard image

Variable: disableCursor

The VDU command for disabling the cursor

Variable: pleaseWaitText

The "Please wait" message shown when the game loads

Subroutine: DrawCanopy

Move code around, clear the edges of the canopy view, draw the canopy edges and rivets, and jump to the main code

Subroutine: VduPoint

Draw a point on-screen using the standard VDU routines

Subroutine: VduMove

Move the graphics cursor using the standard VDU routines

Subroutine: VduDraw

Draw a line using the standard VDU routines

Subroutine: VduPlot

Perform a plot command using the standard VDU routines

Subroutine: DrawRivet

Draw a square rivet (2 pixels across, 4 pixels high)

Subroutine: ClearRows

Clear the specified number of rows on-screen

Subroutine: ProjectPoint (Part 1 of 3)

Project a point onto the screen (i.e. convert from 3D coordinates to screen coordinates)

Subroutine: ProjectPoint (Part 2 of 3)

Calculate the screen coordinates of the projected point

Subroutine: ProjectPoint (Part 3 of 3)

Move the projected coordinates to the centre of the screen and update the point's status byte

Subroutine: DivideScaled

Divide a 16-bit number by a scaled 16-bit number

Subroutine: ScaleUp

Scale up a 16-bit number until it doesn't fit into 16 bits any more

Subroutine: ScaleDown (Part 1 of 4)

Scale down the results of divisions done using the ScaleUp and DivideScaled routines

Subroutine: ScaleDown (Part 2 of 4)

Balance the scale factors for the x- and y-coordinates

Subroutine: ScaleDown (Part 3 of 4)

Scale the x-coordinate

Subroutine: ScaleDown (Part 4 of 4)

Scale the y-coordinate

Subroutine: DrawClippedLine (Part 1 of 6)

Clip a line to fit on-screen, starting with the line deltas

Subroutine: DrawClippedLine (Part 2 of 6)

Work out whether the line's start point is on-screen

Subroutine: DrawClippedLine (Part 3 of 6)

Work out whether the line's end point is on-screen

Subroutine: DrawClippedLine (Part 4 of 6)

Calculate the starting point and direction for our clipped vector line

Subroutine: DrawClippedLine (Part 5 of 6)

Calculate the deltas for our clipped vector line

Subroutine: DrawClippedLine (Part 6 of 6)

Add the clipped line to the line buffer and draw it

Subroutine: DrawCanopyLine (Part 1 of 9)

Draw a line in the canopy view

Subroutine: DrawCanopyLine (Part 2 of 9)

Modify the line drawing routine for a shallow horizontal slope

Subroutine: DrawCanopyLine (Part 3 of 9)

Draw a line as a shallow horizontal slope

Subroutine: DrawCanopyLine (Part 4 of 9)

Draw a part of the line, working down the screen

Subroutine: DrawCanopyLine (Part 5 of 9)

Draw a part of the line, working up the screen

Subroutine: DrawCanopyLine (Part 6 of 9)

Draw a part of the line, working along the screen

Subroutine: DrawCanopyLine (Part 7 of 9)

Modify the line drawing routine for a steep vertical slope

Subroutine: DrawCanopyLine (Part 8 of 9)

Draw a line as a steep vertical slope

Subroutine: DrawCanopyLine (Part 9 of 9)

If the line was clipped, draw a line from the clipped coordinates to the edge of the screen

Subroutine: ModifyDrawRoutine

Modify the drawing routines to draw in the correct colour for the current colour cycle

Subroutine: ClipBestEndOfLine

Clip a line at the start or end point, depending on which is best

Subroutine: ClipStartOfLine (Part 1 of 5)

Check whether the line is completely off-screen

Subroutine: ClipStartOfLine (Part 2 of 5)

Work out the deltas depending on the direction of slope

Subroutine: ClipStartOfLine (Part 3 of 5)

Work out the deltas depending on the steepness of slope

Subroutine: ClipStartOfLine (Part 4 of 5)

Calculate where to clip the line

Subroutine: ClipStartOfLine (Part 5 of 5)

Move the start point to the clipped position and return it

Subroutine: SwapLinePoints

Copy an end point into a start point

Subroutine: EraseCanopyLines

Draw all the lines from a line buffer to erase them

Subroutine: Multiply16x16Mix

Multiply two 16-bit numbers with different sign bits (one in bit 7 of the high byte, the other in bit 0 of the low byte)

Subroutine: Multiply8x8

Multiply two unsigned 8-bit numbers

Subroutine: ProjectAxisAngle

Convert the rotation angles of the plane into coordinates

Subroutine: Sine16Bit

Calculate the sine of a 16-bit number

Subroutine: Multiply4x16

Multiply a 4-bit and a 16-bit number

Subroutine: SetObjPointCoords (Part 1 of 2)

Calculate the coordinate for a point within an object

Subroutine: SetObjPointCoords (Part 2 of 2)

Apply the correct scale factor to the matrix multiplication

Subroutine: AddTempToPoint (Part 1 of 2)

Add the xTemp1 vector to a point, store the result in another point, and set the result to hidden if it overflows

Subroutine: Add16x16Bit0

Add two 16-bit numbers that have their signs in bit 0

Subroutine: SetMatrices

Set up matrices 1 to 4

Subroutine: SetMatrixEntry

Calculate a matrix entry

Subroutine: Multiply16x16

Multiply two unsigned 16-bit numbers

Subroutine: Multiply16x16Bit0

Multiply two 16-bit numbers that have their signs in bit 0

Subroutine: SetPointCoords

Calculate the coordinates for a point

Subroutine: CopyTempToPoint

Set a specified point to (xTemp1, yTemp1, zTemp1)

Subroutine: UpdateIndicator (Part 1 of 15)

Update a single indicator on the dashboard

Subroutine: UpdateIndicator (Part 2 of 15)

Calculations for the compass (indicator 0)

Subroutine: UpdateIndicator (Part 3 of 15)

Calculations for the airspeed indicator (indicator 1)

Subroutine: UpdateIndicator (Part 4 of 15)

Calculations for the altimeter's small "hour" hand (indicator 2)

Subroutine: UpdateIndicator (Part 5 of 15)

Calculations for the altimeter's "minute" hand (indicator 3)

Subroutine: UpdateIndicator (Part 6 of 15)

Logic for checking which indicator to update

Subroutine: UpdateIndicator (Part 7 of 15)

Calculations for the vertical speed indicator (indicator 4)

Subroutine: UpdateIndicator (Part 8 of 15)

Logic for checking which indicator to update

Subroutine: UpdateIndicator (Part 9 of 15)

Calculations for the turn indicator (indicator 5), the bottom part of the slip-and-turn indicator

Subroutine: UpdateIndicator (Part 10 of 15)

Calculations for the slip indicator (indicator 6), the top part of the slip-and-turn indicator

Subroutine: UpdateIndicator (Part 11 of 15)

Calculations for the artificial horizon (indicator 7)

Subroutine: UpdateIndicator (Part 12 of 15)

Logic for checking which indicator to update

Subroutine: UpdateIndicator (Part 13 of 15)

Calculations for the rudder indicator (indicator 9)

Subroutine: UpdateIndicator (Part 14 of 15)

Calculations for the joystick position display (indicator 8 or 10)

Subroutine: UpdateIndicator (Part 15 of 15)

Calculations for the thrust indicator (indicator 11)

Subroutine: DrawIndicatorBar

Draw a vertical bar on indicator 9 (rudder) or 11 (thrust)

Subroutine: DrawIndicatorHand

Apply min and max limits to an indicator value and draw a hand on the indicator

Subroutine: DrawIndicatorLine

Draw a line on indicators 0 to 7, i.e. a dial hand (0-6) or an artificial horizon (7)

Subroutine: DrawJoystickCross

Draw a cross in the joystick position display (indicator 8 or 10)

Subroutine: GetHandVector

Vector line calculation for a hand on indicators 0 to 6

Subroutine: DrawVectorLine (Part 1 of 3)

Draw a line: set up pixel bytes and slope variables

Subroutine: DrawVectorLine (Part 2 of 3)

Calculate the coordinates of the next pixel as we step along the line by one pixel

Subroutine: DrawVectorLine (Part 3 of 3)

Plot a pixel at (I, J)

Subroutine: ArtificialHorizon

Vector line calculation for the artificial horizon on indicator 7

Subroutine: DrawOrthoLine

Draw an orthogonal line (i.e. vertical or horizontal)

Subroutine: ScaleSigned

Scale an indicator value by 4 or 16, retaining the sign and adding sensitivity for smaller values

Subroutine: UpdateDashboard

Update two indicators on the dashboard, one from 0-6, one from 7-11, cycling through them with each subsequent call

Subroutine: UpdateFlightModel (Part 1 of 4)

Apply any axis control key presses to the current axis values

Subroutine: UpdateFlightModel (Part 2 of 4)

Apply any throttle key presses to the current thrust value

Subroutine: UpdateFlightModel (Part 3 of 4)

Process the undercarriage, brake, flaps and fire keys

Subroutine: UpdateFlightModel (Part 4 of 4)

Set up matrices, apply the flight model and update the dashboard

Subroutine: ProcessOtherKeys

Apply the undercarriage, brakes, flaps and fire keys

Subroutine: IndicatorU

Update the undercarriage indicator ("U") and related variables

Subroutine: IndicatorF

Update the flaps indicator ("F") and related variables

Subroutine: FireGuns

Create the bullet objects and send them on their way

Subroutine: IndicatorB

Update the brakes indicator ("B")

Subroutine: IndicatorT

Update the Theme indicator ("T")

Subroutine: ScanKeyboard

Scan the keyboard for a specific key

Subroutine: UpdateKeyLogger

Scan the keyboard for keys in the two key tables and update the key logger

Subroutine: AddPointToObject

Add a point vector to an object's coordinates

Subroutine: SetObjectToOrigin

Set an object's coordinates to (0, 0, 0)

Subroutine: ReadADCChannel

Read the joystick position from one of the ADC channels

Subroutine: ResetVariables

Reset most variables to prepare for a new flight

Subroutine: ResetRadar

Reset the radar display

Subroutine: StartGame

Reset the high score, set up the gunfire sound envelope and start a new game

Subroutine: NewGame

Start a new game

Subroutine: MainLoop (Part 1 of 15)

Start the main loop by processing gunfire and bullets

Subroutine: MainLoop (Part 2 of 15)

Reset object statuses and related points

Subroutine: MainLoop (Part 3 of 15)

Make the sound of firing, if appropriate

Subroutine: MainLoop (Part 4 of 15)

Check whether aliens have invaded Acornsville

Subroutine: MainLoop (Part 5 of 15)

Update lines, check flying skills, increment main loop counter, update the radar

Subroutine: MainLoop (Part 6 of 15)

Check whether any aliens have been hit

Subroutine: MainLoop (Part 7 of 15)

Process the terminate key

Subroutine: MainLoop (Part 8 of 15)

If we fire the guns on the runway, enable the Theme

Subroutine: MainLoop (Part 9 of 15)

Fill up the tank if the engine is switched off, and process the volume keys

Subroutine: MainLoop (Part 10 of 15)

Award points for a successful landing

Subroutine: MainLoop (Part 11 of 15)

Process engine start and stop

Subroutine: MainLoop (Part 12 of 15)

Spend at least 9 centiseconds processing lines from the linesToHide list

Subroutine: MainLoop (Part 13 of 15)

Process more lines and update the view out of the canopy

Subroutine: MainLoop (Part 14 of 15)

Handle the score display

Subroutine: MainLoop (Part 15 of 15)

Update the status of any new line points

Subroutine: UpdateLinesToShow

Update the linesToShow list, moving any lines that aren't visible into the linesToHide list

Subroutine: ProcessLinesToShow

Process the linesToShow list, projecting all the lines onto the screen and moving any that aren't visible to the linesToHide list

Subroutine: ProcessLinesToHide

Process an unprocessed line from the linesToHide list

Subroutine: ShowOrHideLine

Process a line, working out its visibility and adding it to the linesToShow or linesToHide list

Subroutine: ProcessLine (Part 1 of 7)

Process a line, rotating and transforming it to the correct coordinates and calculating its visibility

Subroutine: ProcessLine (Part 2 of 7)

Process runway lines

Subroutine: ProcessLine (Part 3 of 7)

If a line is part of a multi-point object, extract the other points in the line so we can check them all

Subroutine: ProcessLine (Part 4 of 7)

Process bullet lines

Subroutine: ProcessLine (Part 5 of 7)

Calculate the object's coordinates and visibility

Subroutine: ProcessLine (Part 6 of 7)

Check the visibility of all the object's points on the stack

Subroutine: ProcessLine (Part 7 of 7)

Check distance and z-coordinates and return the final result

Subroutine: SetObjectCoords (Part 1 of 11)

Calculate the coordinates for an object's location

Subroutine: SetObjectCoords (Part 2 of 11)

Pre-process the bullets (objects 12, 13, 14 or 15)

Subroutine: SetObjectCoords (Part 3 of 11)

Logic for checking which objects to pre-process

Subroutine: SetObjectCoords (Part 4 of 11)

Pre-process the object groups (objects 6, 7, 8 or 9)

Subroutine: SetObjectCoords (Part 5 of 11)

Logic for checking which objects to pre-process

Subroutine: SetObjectCoords (Part 6 of 11)

Pre-process dormant aliens (object group 30)

Subroutine: SetObjectCoords (Part 7 of 11)

Pre-process feeding and flying aliens (objects 31, 32 and 33)

Subroutine: SetObjectCoords (Part 8 of 11)

Process the object

Subroutine: SetObjectCoords (Part 9 of 11)

Process the next object in the group, if applicable, or return from the subroutine if not

Subroutine: SetObjectCoords (Part 10 of 11)

Process the next dormant alien (object group 30)

Subroutine: SetObjectCoords (Part 11 of 11)

Update the object status and return from the subroutine

Subroutine: CheckObjDistance

Check whether an object is within the visible distance for that object, along just one axis

Subroutine: ResetLineLists

Reset the line lists at linesToShow and linesToHide

Subroutine: FlipColours

Flip the values of colourCycle and colourLogic to cycle to the next colour state

Subroutine: SetColourToWhite

Set a logical colour to white

Subroutine: SetColourToBlack

Set a logical colour to black

Subroutine: SetLogicalColour

Set a logical colour to a physical colour

Subroutine: DrawCanopyView

Draw the main view out of the canopy

Subroutine: ProcessHorizonLine

Calculate coordinates for the horizon's start and end points

Subroutine: UpdateRadarBlip

Update a blip on the radar (runway or alien)

Subroutine: DrawRadarBlip

Draw a blip on the radar (runway or alien)

Subroutine: AlienInAcornsville

Move an alien towards Acornsville and check whether it has reached it yet (and if so, end the game)

Subroutine: GetRadarVector

Calculate the radar line vector for a line (the runway) or a dot (an alien)

Variable: colour1L2R

Pixel bytes for drawing canopy lines left to right in colour 1

Variable: colour1R2L

Pixel bytes for drawing canopy lines right to left in colour 1

Variable: colour2L2R

Pixel bytes for drawing canopy lines left to right in colour 2

Variable: colour2R2L

Pixel bytes for drawing canopy lines right to left in colour 2

Variable: colour1Row

Pixel bytes for erasing canopy lines in colour 1

Variable: colour2Row

Pixel bytes for erasing canopy lines in colour 2

Variable: previousCompass

Stores the value of the compass heading when we draw the runway on the radar, so we can erase the line later

Variable: previousTime

Stores the current time (low byte only), so we can process lines in a timely fashion during the main loop

Subroutine: ClearCanopy

Clear the canopy to black, leaving the canopy edges alone

Subroutine: FillCanopy

Fill the canopy with a specified colour, leaving the canopy edges alone

Subroutine: FillCanopyRows

Fill multiple screen rows with a particular colour, avoiding the canopy edges

Subroutine: Delay

Delay for a specified number of loops

Subroutine: UpdateBullets

Move the bullets through the air

Subroutine: SpawnAlien

If the Theme is enabled and the current wave does not yet have eight aliens in it, spawn a new alien

Subroutine: UpdateAliens (Part 1 of 5)

Update the aliens so they progress through their feeding or attack cycles

Subroutine: UpdateAliens (Part 2 of 5)

Manage alien slots 31 and 32, and if there's a vacancy, wake up a dormant alien, move it into the slot and start its feeding cycle

Subroutine: UpdateAliens (Part 3 of 5)

If alien slot 33 is free and there's an alien waiting to take off, promote it into slot 33

Subroutine: UpdateAliens (Part 4 of 5)

When an alien reaches the next feeding stage, double its size

Subroutine: UpdateAliens (Part 5 of 5)

Check whether the whole wave has been destroyed, and award points accordingly

Subroutine: ResizeFeedingAlien

Change the size of an alien so it grows bigger as it feeds

Subroutine: CheckIfAlienIsHit (Part 1 of 2)

Extract the alien's feeding stage, ready for the hit calculations in part 2

Subroutine: CheckIfAlienIsHit (Part 2 of 2)

Check to see whether the alien has been hit, and if so, initiate the explosion

Subroutine: GetAlienWeakSpot

Calculate the coordinates of an alien's weak spot

Subroutine: CheckAlienWeakSpot

Check whether an object is close enough to an alien's weak spot to be hitting it

Subroutine: GetTrailVectorStep

Calculate 1/32 of the vector for a bullet trail

Subroutine: ShowUpsideDownBar

Show or hide the upside down bar in the artificial horizon indicator

Subroutine: ProcessRunwayLine (Part 1 of 5)

Calculate coordinates and visibility for a runway line

Subroutine: ProcessRunwayLine (Part 2 of 5)

Calculate coordinates and visibility for the runway outline

Subroutine: ProcessRunwayLine (Part 3 of 5)

Calculate visibility for the runway dashes

Subroutine: ProcessRunwayLine (Part 4 of 5)

Construct the dashes down the middle of the runway

Subroutine: ProcessRunwayLine (Part 5 of 5)

Clip any portion of the runway outline that's behind us

Subroutine: SetPointVisibility

Set the status byte for multiple points to indicate that their coordinates and visibility have been calculated

Subroutine: DrawHalfHorizon

Draw half of the horizon line

Subroutine: FillUpFuelTank

Fill up the fuel tank by 1/65th of a full tank every four iterations of the main loop

Subroutine: UpdateFuelGauge

Update the fuel gauge every 16 iterations of the main loop

Subroutine: DrawFuelPixel

Draw or erase a pixel on the fuel gauge on the dashboard

Subroutine: ProcessVolumeKeys

Adjust the volume when the volume keys are pressed

Subroutine: RetractFlapsIfFast

Retract the flaps if we are going faster than 150 mph

Variable: xObjectPoint

Scaled x-coordinates of the points that make up objects, relative to the object's anchor point

Variable: yObjectPoint

Scaled y-coordinates of the points that make up objects, relative to the object's anchor point

Variable: zObjectPoint

Scaled z-coordinates of the points that make up objects, relative to the object's anchor point

Variable: xRadarBuffer

The x-coordinates of the runway and alien on the radar

Variable: yRadarBuffer

The y-coordinates of the runway and alien on the radar

Variable: distanceFromHit

The distance from the alien we just hit, so we can work out whether we get hit by turbulence

Variable: feedingStage

The feeding stage of the alien we are currently processing

Variable: hitObjectId

The object ID of the alien we just hit (30 to 33)

Variable: hitTimer

The time since we hit an alien, so we can time its explosion

Variable: fuelUsedLo

The low byte of the current batch of fuel used

Variable: fuelUsedHi

The high byte of the current batch of fuel used

Variable: fuelLevel

The current fuel level

Variable: explodeTo

The end point ID for exploding each of the four alien slots

Variable: explodeFrom

The starting point ID for exploding each of the four alien slots

Variable: scoreLo

Score (low byte)

Variable: scoreHi

Score (high byte)

Variable: highScoreLo

High score (high byte)

Variable: highScoreHi

High score (low byte)

Subroutine: SetEngine

Set the engine status

Variable: skillZoneLo

Low byte of the skill zone coordinates for testing flying skills

Variable: skillZoneHi

High byte of the skill zone coordinates for testing flying skills

Variable: skillZoneSize

Sizes of the skill zones for testing flying skills

Variable: shift4Right

Lookup table for shifting a byte four places to the right, to extract the high nibble

Variable: shift4Left

Lookup table for shifting a byte four places to the left, to extract the low nibble

Variable: divisionHi

Division lookup table

Variable: timesTable

Lookup table for multiplication times tables

Variable: maxLineDistance

The furthest distance at which each line is visible

Variable: maxObjDistance

The furthest distance at which each object is visible

Variable: lineBufferR

Line buffer storage for the start x-coordinate (R)

Variable: lineBufferW

Line buffer storage for the max/min x-coordinate (W)

Variable: lineBufferS

Line buffer storage for the start y-coordinate (S)

Variable: lineBufferG

Line buffer storage for the max/min y-coordinate (G)

Variable: lineBufferT

Line buffer storage for the line's |x-delta| (T)

Variable: scoreText

The high score text

Variable: lineEndPointId

The point ID for a line's end point

Variable: objectGroup

The current group number for object IDs 6, 7, 8 and 9

Variable: groupStart

The starting value for each object's group number

Variable: xGroupObjectHi

High byte of the x-coordinate for objects in a group (6 to 9)

Variable: zGroupObjectHi

High byte of the z-coordinate for objects in a group (6 to 9)

Subroutine: CheckTimePassed

Flag whether 9 centiseconds have passed since the last call

Variable: lineStartPointId

The point ID for a line's start point

Variable: alienScore

The scores for killing aliens in the various feeding stages

Variable: dormantAlienScore

The score for killing a dormant alien

Variable: sinLo

Low byte of the sine lookup table

Variable: sinHi

High byte of the sine lookup table

Variable: alienSlot

Slots for up to three aliens that are ready to start moving towards the town

Variable: alienToMove

The number of the alien to move towards Acornsville in this iteration of the main loop

Variable: mainLoopCounter

The main loop counter

Variable: numberOfLines

The total number of lines in Aviator's 3D world

Variable: alienObjectId

Object IDs for each of the eight aliens

Variable: alienState

The state of each of the eight aliens

Variable: alienStatus

Storage for the object status bytes for the four alien objects

Variable: matrix1Lo

The low bytes of matrix 1

Variable: matrix2Lo

The low bytes of matrix 2

Variable: matrix3Lo

The low bytes of matrix 3

Variable: matrix4Lo

The low bytes of matrix 4

Subroutine: CheckFlyingSkills (Part 1 of 2)

Check whether we are performing one of the tests of flying skill

Variable: matrix1Hi

The high bytes of matrix 1

Variable: matrix2Hi

The high bytes of matrix 2

Variable: matrix3Hi

The high bytes of matrix 3

Variable: matrix4Hi

The high bytes of matrix 4

Subroutine: CheckFlyingSkills (Part 2 of 2)

Perform finer checks to see if we are flying under the bridge or along the main street of the town

Variable: scaleFactor

Scale factors for the flight forces (in signed powers of 2)

Variable: lowNibble

Lookup table for the low nibble of a value

Variable: xObjectLo

Low byte of the x-coordinate for an object

Variable: yObjectLo

Low byte of the y-coordinate for an object

Variable: zObjectLo

Low byte of the z-coordinate for an object

Variable: xObjectHi

High byte of the x-coordinate for an object

Variable: yObjectHi

High byte of the y-coordinate for an object

Variable: zObjectHi

High byte of the z-coordinate for an object

Variable: randomNumbers

A list for keeping a list of random numbers

Variable: highNibble

Lookup table for the high nibble of a value

Variable: objectPoints

Sequences of related points that together make up objects

Subroutine: ReadJoystick

Read the joystick axes and fire button and update the aileron, elevator and fire key values accordingly

Variable: divisionLo

Division lookup table

Variable: yLookupLo

Lookup table for converting pixel y-coordinate to low byte of screen address

Variable: yLookupHi

Lookup table for converting pixel y-coordinate to high byte of screen address

Subroutine: DrawCanopyCorners

Draw the diagonal corners at the top of the canopy

Subroutine: RemoveScore

Remove the score display from the screen

Variable: lineBufferV

Line buffer storage for the line direction (V)

Variable: zPointHi

High byte of the z-coordinate for a point

Variable: xPointHi

High byte of the x-coordinate for a point

Variable: xLinearHi

High byte of point 252 (x-coordinate)

Variable: xGravityHi

High byte of point 253 (x-coordinate)

Variable: xTempPoint1Hi

High byte of point 254 (x-coordinate)

Variable: xTempPoint2Hi

High byte of point 255 (x-coordinate)

Subroutine: CopyWorkToPoint

Copy a point from the variable workspace to the point tables

Subroutine: CopyPointToWork

Copy a point from the point tables to the variable workspace

Subroutine: SetPointToOrigin

Set a point's coordinates to the origin at (0, 0, 0)

Subroutine: SetPoint

Set a point's coordinates to the value (a, a, a)

Subroutine: CheckLineDistance

Check whether a point on a line is within the visible distance for the line

Subroutine: AddTempToPoint (Part 2 of 2)

Check whether the vector addition overflowed and set the point's visibility accordingly

Subroutine: CheckBridgeAndTown

Check whether we are safely flying under the bridge or down the main street in Acornsville

Subroutine: ScorePoints

Increase the score and make a beep

Subroutine: UpdateScore

Increase the score by a specified number of points

Subroutine: UpdateHighScore

If this is a high score, update the high score

Subroutine: DisplayScore

Print the scores on-screen

Subroutine: PrintScore

Print a score

Subroutine: PrintScoreText

Print text when showing the scores on-screen

Subroutine: Crash

Make a crashing sound, flash the canopy and start a new game

Subroutine: NextObjectGroup

Cycle to the next object group

Subroutine: ExplodeAlien

Explode an alien, making the alien split apart

Subroutine: ScoreHitPoints

Award points for destroying an alien, and remove the alien from its slot and the radar, if required

Subroutine: SetRandomNumber

Set the next item in the randomNumbers list to a new random number and update the pointer to point to it

Subroutine: NextRandomNumber

Point to the next item in the randomNumbers list

Subroutine: MakeSound

Make a sound

Subroutine: TerminateGame

Terminate the current game

Subroutine: DefineEnvelope

Define a sound envelope

Subroutine: MakeSoundEnvelope

Either make a sound or set up an envelope

Subroutine: ToggleEngineSound

Turn the engine sound on or off

Subroutine: ResetEngineSound

Reset the pitch of the engine sound

Subroutine: MakeEngineSound

Make the engine sound, with the choppiness and pitch affected by thrust and airspeed

Subroutine: DrawGunSights

Draw the canopy corners and the gun sights, if shown

Subroutine: ToggleJoystick

Toggle the joystick setting

Variable: envelopeData

Data for two sound envelopes

Variable: soundData

OSWORD blocks for making the various game sounds

Subroutine: PrintTooLate

Print the "TOO LATE!" message at column 6, row 7 on-screen

Variable: tooLateText

The "TOO LATE!" message shown when the aliens land in Acornsville

Variable: xLookupLo

Lookup table for converting pixel x-coordinate to low byte of screen address

Variable: xLookupHi

Lookup table for converting pixel x-coordinate to high byte of screen address

Variable: keyTable1

Internal key numbers of high priority keys that are scanned first

Variable: keyTable2

Internal key numbers of lower priority keys that are scanned second

Variable: axisChangeRate

Stores the amount by which the three axes of movement change when the aileron, elevator or rudder are moved

Variable: keyTable2Lo

Key logger value (low byte) for key presses in keyTable2

Variable: keyTable2Hi

Key logger value (high byte) for key presses in keyTable2

Variable: keyTable1Lo

Key logger value (low byte) for key presses in keyTable1

Variable: keyTable1Hi

Key logger value (high byte) for key presses in keyTable1

Variable: soundData26

The sound of us making contact with the ground while landing

Variable: forceFactor

The factors by which the flight forces are multiplied as part of the scaling process

Variable: dialQuadrant

The value range of a quadrant in each indicator

Variable: xDeltaMax

The maximum x-delta for the hand line in each indicator

Variable: yDeltaMax

The maximum y-delta for the hand line in each indicator

Variable: indicatorLineI

Line buffer storage for the start x-coordinate for each indicator line (I)

Variable: indicatorLineJ

Line buffer storage for the start y-coordinate for each indicator line (J)

Variable: indicatorBase

The base value for each indicator

Variable: indicatorMin

The minimum value shown on each indicator

Variable: indicatorLineT

Line buffer storage for the indicator line's |x-delta| (T)

Variable: indicatorLineU

Line buffer storage for the indicator line's |y-delta| (U)

Variable: indicatorLineV

Line buffer storage for the indicator line's direction (V)

Variable: yJoyCoord

Temporary storage

Variable: xJoyCoord

Temporary storage

Variable: indicator0To6

The first indicator counter

Variable: indicator7To11

The second indicator counter

Variable: joyCoord

Temporary storage

Variable: altitudeMinutes

The value of the altimeter's large "minute" hand

Subroutine: ApplyFlightModel (Part 1 of 7)

Apply the flight model to the plane, starting by calculating the effect of gravity and the undercarriage springs

Subroutine: ApplyFlightModel (Part 2 of 7)

Convert velocity to the plane's perspective and calculate various aerodynamic forces

Subroutine: ApplyFlightModel (Part 3 of 7)

If we are near to an exploding alien, apply turbulence

Subroutine: ApplyFlightModel (Part 4 of 7)

Calculate the dxTurn and xLinear vectors, and the slipRate

Subroutine: ApplyFlightModel (Part 5 of 7)

Calculate the forces for when we are on the ground

Subroutine: ApplyFlightModel (Part 6 of 7)

Adjust the plane's velocity and turn rate, rotate the plane according to the forces on the aircraft, process landing

Subroutine: ApplyFlightModel (Part 7 of 7)

Calculate fuel usage

Subroutine: AdjustTurn

Adjust the plane's turn rate

Subroutine: AdjustVelocity

Adjust the plane's velocity vector

Subroutine: AdjustRotation

Move the plane and adjust its rotation

Subroutine: ApplyAerodynamics (Part 1 of 3)

Set up various variables to use in the aerodynamics calculations

Subroutine: ApplyAerodynamics (Part 2 of 3)

Check whether the plane is stalling, and if it is, simulate one wing stalling before the other, and make the stalling sound

Subroutine: ApplyAerodynamics (Part 3 of 3)

Calculate various aerodynamic figures

Subroutine: ScaleFlightForces

Scale the flight forces by the relevant scale factors

Subroutine: Multiply8x16

Multiply an 8-bit and a 16-bit number

Subroutine: Negate16Bit

Negate a 16-bit number

Subroutine: GetMoments

Calculate the pitching, rolling and yawing moments due to the current pitch, roll and yaw rates

Subroutine: ApplyFlightControl

Calculate the effects of the primary flight controls (elevator, rudder and ailerons), and implement the "instant centre" feature

Subroutine: ApplyTurnAndThrust (Part 1 of 2)

Calculate the (dxTurn, dyTurn, dzTurn) vector

Subroutine: ApplyTurnAndThrust (Part 2 of 2)

Calculate the (xLinear, yLinear, zLinear) vector

Subroutine: CheckPlaneOnRunway

Check whether the plane is over the runway

Subroutine: ProcessLanding (Part 1 of 7)

If this is an emergency landing, make the landing a bumpy one

Subroutine: ProcessLanding (Part 2 of 7)

If we are too high to be touching the ground then we can't be landing, so stop the landing checks

Subroutine: ProcessLanding (Part 3 of 7)

If we are taxiing, restrict the plane's vertical movement and roll

Subroutine: ProcessLanding (Part 4 of 7)

If we are taxiing with the undercarriage up, restrict pitching

Subroutine: ProcessLanding (Part 5 of 7)

If we are taxiing with the undercarriage down, restrict pitching and tilt the plane backwards

Subroutine: ProcessLanding (Part 6 of 7)

If we are not taxiing, process the landing

Subroutine: ProcessLanding (Part 7 of 7)

We have successfully touched down without crashing, so process the effects of landing on the plane

Subroutine: ApplyBumpyRide

Apply a random amount of roll or bumpiness to the plane

Subroutine: AddScaled

Add or subtract a scaled amount to a variable

Subroutine: ScaleByAltitude

Multiply the high byte of the plane's altitude by a 16-bit number

Subroutine: ResetVariable

Set a 16-bit in the variable workspace to 0

Subroutine: Entry

The main entry point for the game: move code into lower memory and call it