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

Variable: tileData

Altitude and shape data for landscape tiles

Variable: xObject

The x-coordinates in 3D space for the 3D objects

Variable: yObjectHi

The y-coordinates in 3D space for the 3D objects (high byte)

Variable: zObject

The z-coordinates in 3D space for the 3D objects

Variable: objectYawAngle

The yaw angle for each object (i.e. the horizontal direction in which they are facing)

Variable: yObjectLo

The y-coordinates in 3D space for the 3D objects (low byte)

Variable: objectTypes

The object types table for up to 64 objects

Variable: drawViewPitchLo

Storage for the pitch angles of tiles and object points for drawing the current landscape view (low bytes)

Variable: drawViewPitchHi

Storage for the pitch angles of tiles and object points for drawing the current landscape view (high bytes)

Variable: xPolygonPointHi

Pixel x-coordinates for all the points in the polygon that we are currently drawing (high bytes)

Variable: drawViewYawLo

Storage for the yaw angles of tiles and object points for drawing the current landscape view (low bytes)

Workspace: Main variable workspace

The main block of game variables

Subroutine: NMIHandler

The NMI handler at the start of the NMI workspace

Variable: irq1Address

Stores the previous value of IRQ1V before we install our custom IRQ handler

Subroutine: Multiply8x8

Calculate (A T) = T * U

Subroutine: GetAngleFromCoords (Part 1 of 3)

Given the coordinates along two axes, calculate the pitch or yaw angle to those coordinates

Subroutine: GetAngleFromCoords (Part 2 of 3)

Overflow and accuracy calculations

Subroutine: GetAngleFromCoords (Part 3 of 3)

Calculate the arctangent to get the angle

Subroutine: GetRotationMatrix (Part 1 of 5)

Calculate the rotation matrix for rotating the pitch or yaw angle for the sights into the global 3D coordinate system

Subroutine: GetRotationMatrix (Part 2 of 5)

Calculate sin(H G) for smaller angles

Subroutine: GetRotationMatrix (Part 3 of 5)

Calculate sin(H G) for bigger angles

Subroutine: GetRotationMatrix (Part 4 of 5)

Loop back to calculate cos instead of sin

Subroutine: GetRotationMatrix (Part 5 of 5)

Apply the correct signs to the result

Variable: xStoreMatrix

Temporary storage for X so it can be preserved through calls to GetRotationMatrix

Subroutine: CrackerSeed

Obfuscated storage for the high byte of the landscape number as part of the anti-cracker code

Subroutine: GetAngleInRadians

Convert a 16-bit angle into radians, restricted to a quarter circle

Subroutine: Multiply8x16

Multiply an 8-bit and a 16-bit number

Subroutine: ScanKeyboard

Scan the keyboard for a specific key press

Subroutine: GetSineAndCosine

Calculate the sine and the cosine of an angle

Subroutine: Multiply16x16

Multiply a sign-magnitude 16-bit number and a signed 16-bit number

Subroutine: Absolute16Bit

Calculate the absolute value (modulus) of a 16-bit number

Subroutine: Negate16Bit

Negate a 16-bit number

Subroutine: MainTitleLoop

The main title loop: display the title screen, fetch the landscape number/code, preview the landscape and jump to the main game loop

Subroutine: SecretCodeError

Display the "WRONG SECRET CODE" error, wait for a key press and rejoin the main title loop

Variable: secretCode0000

The secret entry code for landscape 0000 (06045387)

Subroutine: ClearScreen

Clear the screen to a specified background

Subroutine: PanLandscapeView

Pan the landscape and update the landscape view

Variable: highestPitchAngle

The pitch angle of the highest point that the player can look at with the sights

Variable: lowestPitchAngle

The pitch angle of the lowest point that the player can look at with the sights

Subroutine: ResetVariables

Reset all the game's main variables

Subroutine: ResetTilesObjects

Reset the tile visibility table and deallocate all object numbers

Subroutine: CheckForKeyPresses

Check for various game key presses and update the key logger and relevant variables (during the interrupt handler)

Subroutine: FocusOnKeyAction

Tell the game to start focusing effort on the action that has been initiated, such as a pan of the landscape, absorb, transfer etc.

Subroutine: CheckForSamePanKey

Check to see whether the same pan key is being held down compared to the last time we checked

Variable: spaceKeyDebounce

A variable to flag whether the SPACE key has been pressed, so we can implement debounce

Subroutine: PlaceObjectBelow

Attempt to place an object on a tile that is below the maximum altitude specified in A

Subroutine: GetNextSeed0To30

Set A to the next number from the landscape's sequence of seed numbers, converted to the range 0 to 30

Subroutine: ProcessGameplay

A gameplay loop that processes all game key presses, returning to the main game loop when the player moves, quits, loses or pans

Subroutine: UpdateEnemyTimers

Update the timers that control the enemy tactics

Subroutine: ResetScreenAddress

Reset the address of the start of screen memory

Subroutine: InitialiseSights

Initialise the variables used to manage the sights, so the sights appear in the middle of the screen

Subroutine: ScanForGameKeys

Scan for game key presses and update the key logger

Variable: gameKeys

Negative inkey values for the game keys

Variable: keyLoggerConfig

The configuration table for storing keys the key logger

Subroutine: DrawTitleView

Draw the main title screen, the secret code screen or the landscape preview

Variable: yTextViewer

The y-coordinate of the viewer for the large 3D text on the title screen

Variable: textViewerPitch

The pitch angle of the viewer for the large 3D text on the title screen

Variable: xTextViewer

The x-coordinate of the viewer for the large 3D text on the title screen

Variable: titleOffset

An offset to apply to the large 3D text for the title screen

Variable: zTextViewer

The z-coordinate of the viewer for the large 3D text on the title screen

Variable: textViewerYaw

The yaw angle of the viewer for the large 3D text on the title screen

Variable: viewType

Storage for the type of title view we are drawing in DrawTitleView (title screen or landscape preview)

Subroutine: SpawnEnemies

Calculate the number of enemies for this landscape, add them to the landscape and set the palette accordingly

Subroutine: SpawnPlayer

Add the player object to the landscape, ideally placing it below all the enemies and in the bottom half of the landscape

Subroutine: SpawnTrees

Add trees to the landscape, ideally placing them below all the enemies in the landscape

Subroutine: CheckSecretCode (Part 1 of 2)

Generate the secret code for this landscape and optionally check it against the entered code in the keyboard input buffer

Variable: landscapeColour3

Physical colours for colour 3 in the game palette for the different numbers of enemies

Subroutine: CheckSecretCode (Part 2 of 2)

Check the results of the secret code matching process, and if the secret codes match, jump to PlayGame to play the game

Variable: landscapeColour2

Physical colours for colour 2 in the game palette for the different numbers of enemies

Subroutine: AddEnemiesToTiles

Add the required number of enemies to the landscape, starting from the highest altitude and working down, with one enemy per contour

Subroutine: GetTilesAtAltitude

Return a list of tile blocks at a specified altitude

Variable: leadingBitMask

A table for converting the number of leading clear bits in a number into a bit mask with the same number of leading zeroes

Subroutine: GetHighestTiles

Calculate both the highest tiles in each 4x4 block of tiles in the landscape and the altitude of the highest tile in the landscape

Subroutine: UpdateScanner

Update the scanner, if required

Subroutine: UpdateScannerNow

Update the scanner to a new state

Variable: scannerStatic

Storage for a random number that's used to generate static in the scanner

Variable: scannerBlock

A counter for the eight character blocks that make up the scanner

Variable: scannerState

The current state of the scanner (black, static or green)

Variable: scannerPixelByte

Pixel bytes for the three states of the scanner (black, static and green)

Subroutine: ApplyEnemyTactics

Apply tactics to an enemy object, setting things up so the next call applies tactics to the next enemy object

Subroutine: MoveOnToNextEnemy

Update enemyObject so the next time we consider applying enemy tactics, we apply them to the next enemy, looping from 7 to 0

Subroutine: ApplyTactics (Part 1 of 8)

Apply tactics to the Sentinel or a sentry

Subroutine: ApplyTactics (Part 2 of 8)

Process the tactics for a meanie

Subroutine: ApplyTactics (Part 3 of 8)

If the enemy has any residual energy, try expending it onto the landscape in the form of a tree (and end tactics if successful)

Subroutine: ApplyTactics (Part 4 of 8)

If configured, search the landscape for a suitable target for the enemy to drain of energy

Subroutine: ApplyTactics (Part 5 of 8)

Look for a suitable robot to drain of energy, or look for a drainable tree or boulder if there are no suitable robots

Subroutine: ApplyTactics (Part 6 of 8)

Rotate the enemy and make a rotation sound

Subroutine: ApplyTactics (Part 7 of 8)

Drain energy from the enemy's target object, or try scanning for a tree to turn into a meanie if the target's tile is obscured

Subroutine: ApplyTactics (Part 8 of 8)

Redraw the object on the screen, optionally with a dithered effect

Subroutine: CheckEnemyGaze (Part 1 of 2)

Check to see whether the current enemy can see a specific target object of a specific type

Subroutine: AlterCrackerSeed

Create an altered version of the anti-cracker seed-related data, as part of the anti-cracker code

Subroutine: CheckEnemyGaze (Part 2 of 2)

Calculate whether the current enemy can see the specified object

Variable: xStoreEnemyGaze

Temporary storage for X so it can be preserved through calls to CheckEnemyGaze

Subroutine: GetPlayerDrain

Calculate whether the player is being scanned by an enemy and whether the enemy can see the player's tile

Subroutine: ResetMeanieScan

Reset the data stored for any meanie scans that the enemy has tried in the past, so we can start looking with a clean slate

Subroutine: ScanForMeanieTree

Scan through the objects in the landscape to see if any of them are trees that are suitable for turning into a meanie

Subroutine: DrainObjectEnergy

Drain energy from an object into an enemy, transforming it into an object with an energy level of one unit less (if applicable)

Subroutine: ExpendEnemyEnergy

Drain one unit of energy from an enemy and expend it onto the landscape by spawning a tree, if possible

Subroutine: FinishLandscape

Add the player's energy to the landscape number to get the number of the next landscape and display that landscape's secret code

Subroutine: FindObjectToDrain

Find a suitable target object for an enemy to drain

Subroutine: AbortWhenVisible

Abort applying the tactics for this gameplay loop if updating the object on-screen will corrupt a screen pan

Subroutine: FinishEnemyTactics

Stop applying tactics to the current enemy and return to the ProcessGameplay routine to continue with the gameplay loop

Subroutine: CheckObjVisibility

Check whether an object is visible on-screen and should therefore not be changed if a pan operation is about to happen

Subroutine: ProcessActionKeys (Part 1 of 2)

Process an action key press from key logger entry 1 (absorb, transfer, create, hyperspace, U-turn)

Subroutine: SetPlayerIsOnTower

Set up the playerIsOnTower value for checking the game is won, as part of the anti-cracker code

Subroutine: ProcessActionKeys (Part 2 of 2)

Process an action key press from key logger entry 1 (absorb, transfer, create, hyperspace, U-turn)

Subroutine: GetSightsVector

Calculate the angles of the vector from the player's eyes to the sights

Subroutine: GetVectorForAngles

Convert a vector from pitch and yaw angles into a 3D cartesian vector

Subroutine: MultiplyCoords

Multiply a 16-bit signed number and a 16-bit sign-magnitude value

Subroutine: DivideBy16

Divide a 16-bit sign-magnitude number by 16

Subroutine: AddVectorToCoord

Add a vector to a coordinate

Subroutine: FollowGazeVector (Part 1 of 5)

Follow a gaze vector from a viewing object to determine whether the viewer can see a flat tile or platform (i.e. boulder or tower)

Subroutine: FollowGazeVector (Part 2 of 5)

Calculate the altitudes of the four corners in a non-flat tile

Subroutine: FollowGazeVector (Part 3 of 5)

Calculate whether the viewing object's gaze is obstructed by a tile of shape 4 or 12 (i.e. a tile with one horizontal edge)

Subroutine: FollowGazeVector (Part 4 of 5)

For non-flat tiles with two horizontal edges, work out which tile edge to use when checking for obstruction of the gaze vector

Subroutine: FollowGazeVector (Part 5 of 5)

For non-flat tiles with two horizontal edges, work out whether the tile edge obstructs the gaze vector

Variable: tileEdges

A table to map tile shapes and gaze vector direction to tile edges

Subroutine: GetTileAltitude

Calculate the altitude of a tile, optionally including platform objects and trees in the calculation

Subroutine: CheckForTileCentre

Calculate max(|xCoordLo - 128|, |zCoordLo - 128|)

Subroutine: GetObjectCoords

Get an object's coordinates

Subroutine: DeleteObject

Delete an object, removing it from the landscape and vacating its object number

Subroutine: PlaceObjectOnTile

Place an object on a tile, putting it on top of any existing boulders or towers

Variable: tileNumber

The tile number to which we are adding an object in the PlaceObjectOnTile routine

Subroutine: DrawUpdatedObject

Draw an updated object on-screen, optionally with a dithered effect, and with or without the surrounding landscape

Variable: screenBufferAddr

Storage for the address for the current drawing operation in the screen buffer

Variable: objScreenAddr

The screen address of the object we are updating

Variable: ditherOuterLoop

A counter for the inner loop when dithering objects to the screen in the DitherScreenBuffer routine

Variable: yawAdjustmentHi

The yaw offset of the left edge of the object being updated, relative to the left edge of the screen, in yaw angles

Subroutine: GetObjVisibility

Calculate whether any part of an object is visible on-screen, and if so, which character columns it spans on the screen

Variable: objectHalfWidth

Each object's width in terms of tile widths, for half the object

Subroutine: SpawnObject

Add a new object of the specified type to the objectTypes table

Subroutine: UpdatePlayerEnergy

Update the player's energy levels by adding or subtracting the amount of energy in a specific object

Variable: objectTypeEnergy

The amount of energy required to create each object or the amount energy acquired when absorbing each object

Subroutine: PerformHyperspace

Hyperspace the player to a brand new tile, ideally at the same altitude as the current tile

Subroutine: DrawObjectStack

Draw an entire stack of objects

Subroutine: FillScreen

Fill a rectangular in screen memory or the screen buffer with the specified background

Variable: backgroundEven

Pixel bytes for even pixel rows in the screen background

Variable: backgroundOdd

Pixel bytes for odd pixel rows in the screen background

Variable: pixelBitMask

A table for converting a pixel number in the range 0 to 3 into a screen mode 5 bit mask with that pixel's bits set and others clear

Variable: colourPixels

A table that maps logical colours 0 to 3 to a four-pixel byte in that colour

Variable: pixelsToLeft

Pixel byte with all the pixels to the left of position X set

Variable: pixelsToRight

Pixel byte with all the pixels to the right of position X set

Variable: leftPixels

Pixel byte with all the pixels to the right of position X + 1 set, to give the complementary pixel byte to pixelsToRight

Variable: buffersOffsetLo

An offset to add to the address of the right column screen buffer (low byte)

Variable: buffersOffsetHi

An offset to add to the address of the right column screen buffer (low byte)

Subroutine: DrawPolygonLines (Part 1 of 4)

Draw an analysed polygon into the screen buffer

Subroutine: DrawPolygonLines (Part 2 of 4)

Process the line overflowing the sides of the buffer, or fitting into one pixel byte, and loop on to the next polygon pixel line

Subroutine: DrawPolygonLines (Part 3 of 4)

Draw the left and right edges of the polygon line and fall into part 4 to draw the line in between

Subroutine: DrawPolygonLines (Part 4 of 4)

Draw a horizontal pixel line of a specific length in character columns

Subroutine: GetTileVisibility

For each tile in the landscape, calculate whether the player can see that tile, to speed up the process of drawing the landscape

Variable: visibileBitMask

A table for converting a number in the range 0 to 7 into a bit mask with only that bit set, when counting from the left

Subroutine: GetRowVisibility (Part 1 of 2)

Set up the calculations to determine whether each tile corner in a tile row is obscured from the player by any intervening landscape

Subroutine: CheckSecretStash

Check whether the secret code stash is correctly set up, as part of the anti-cracker code

Subroutine: GetRowVisibility (Part 2 of 2)

Calculate whether each tile corner in a tile row is obscured from the player by any intervening landscape

Subroutine: GetTileAltitudes

Calculate tile corner altitudes and maximum tile corner altitudes for each tile in the landscape

Subroutine: DrawLandscapeView (Part 1 of 3)

Set up a number of variables for drawing the landscape view

Subroutine: DrawLandscapeView (Part 2 of 3)

Work through the landscape, drawing one row of tiles/objects at a time, from the back row to the front row

Subroutine: DrawLandscapeView (Part 3 of 3)

Draw a tile row in two parts, one on either side of the viewer

Variable: quadrantOffsets

Offsets into the tile view data tables for the four different viewing directions

Subroutine: GetTileViewEdges

For a given tile row, work out the edges of the visible portion of the row in the current player view, as left to right tile numbers

Subroutine: GetTileEdgeToLeft

Move one tile to the left along the tile row that we are analysing for view edges and tile angles

Subroutine: GetTileEdgeToRight

Move one tile to the right along the tile row that we are analysing for view edges and tile angles

Subroutine: GetTileViewAngles (Part 1 of 4)

Calculate the pitch and yaw angles for a tile corner, relative to a viewer object (e.g. the player), and whether it is on-screen

Subroutine: GetTileViewAngles (Part 2 of 4)

Fetch the tile data for the tile corner we are analysing

Subroutine: GetTileViewAngles (Part 3 of 4)

Calculate the pitch angle for the tile corner

Subroutine: GetTileViewAngles (Part 4 of 4)

Calculate how much of the tile is on-screen

Subroutine: DrawLandscapeRow

Draw a row of tiles between the left visible edge and the right visible, in two parts towards each side of the viewer

Subroutine: FlipBufferType

Flip the buffer type between buffer type 0 (left row buffer) and buffer type 1 (right row buffer)

Subroutine: ConfigureBuffer

Set up the variables required to configure the screen buffer to a specific buffer type

Variable: buffersOrigin

The offset to add to yaw angles for each screen buffer to convert from the origin in the buffer centre to the origin on the left

Variable: xBuffersLeft

The left edge of each screen buffer in pixels

Variable: xBuffersWidth

The width of each screen buffer in pixels

Variable: buffersMinYaw

Minimum allowed yaw angles for points in the screen buffer

Subroutine: ConfigureObjBuffer

Set up the variables required to configure the screen buffer for updating an object

Subroutine: DrawTileAndObjects

Draw a tile and any objects stacked on it

Subroutine: DrawFlatTile

Draw a flat tile in the correct colour for the chess board effect that we use to draw the landscape

Subroutine: SetSecretStash

Alter the secret code stash, as part of the anti-cracker code

Subroutine: DrawOneFaceTile

Draw a tile with one quadrilateral (four-sided) face

Subroutine: DrawSlopingTile

Draw a sloping tile that is not shape 4 or 12

Subroutine: DrawTwoFaceTile

Draw a tile with two triangular (three-sided) faces

Subroutine: DrawPolygon

Draw a polygon

Subroutine: GenerateLandscape

Generate tile data for the landscape

Subroutine: ProcessTileData

Process the tile data for all tiles in the landscape

Subroutine: SmoothTileData

Smooth the entire landscape

Subroutine: GetTileData

Get the tile data and tile data address for a specific tile

Subroutine: SmoothTileCorners (Part 1 of 4)

Smooth a row or column of tile corners (a "strip of tiles")

Subroutine: SmoothTileCorners (Part 2 of 4)

Smooth a strip by moving each outlier tile corner to the altitude of its closest immediate neighbour (in terms of altitude)

Subroutine: SmoothTileCorners (Part 3 of 4)

Smooth a strip by setting the tile corner altitudes to the average of the current tile corner altitude and three following corners

Subroutine: SetCrackerSeed

Set up anti-cracker tile-related data that can be checked in the CheckCrackerSeed routine

Subroutine: SmoothTileCorners (Part 4 of 4)

Copy the smoothed strip data back into the tileData table

Subroutine: GetTileShape

Calculate the shape of the tile anchored at (xTile, zTile)

Variable: trianglePointAdd

The value to add to the second point number to get the third point number when drawing a tile face as two triangles

Variable: tileShapeColour

Tile colours by shape and the orientation of the viewer

Variable: triangleStart

The number of the first point in each two-face shape that is drawn as a pair of triangles

Subroutine: GetPolygonLines (Part 1 of 6)

Calculate the points in a two-face tile polygon when it consists of a pair of triangles

Subroutine: GetPolygonLines (Part 2 of 6)

The main entry point for the routine to calculate the horizontal lines in filled polygon and prepare them for drawing on-screen

Subroutine: GetPolygonLines (Part 3 of 6)

Convert all the polygon point yaw angles into pixel x-coordinates (for larger yaw angles that convert into a 16-bit x-coordinate)

Subroutine: GetPolygonLines (Part 4 of 6)

Convert all the polygon point yaw angles into pixel x-coordinates (for smaller yaw angles that convert into an 8-bit x-coordinate)

Subroutine: GetPolygonLines (Part 5 of 6)

Loop through all the edges in the polygon and call the correct routines to process one-byte, two-byte or horizontal edges

Subroutine: TracePolygonEdge (Part 1 of 8)

Trace a polygon edge, populating xPolygonRight or xPolygonLeft with the x-coordinate of the edge for each y-coordinate

Subroutine: TracePolygonEdge (Part 2 of 8)

Trace a polygon edge with a steep gradient by stepping along the y-axis

Subroutine: TracePolygonEdge (Part 3 of 8)

Trace a polygon edge with a shallow gradient by stepping along the x-axis

Subroutine: TracePolygonEdge (Part 4 of 8)

Trace a steep edge that starts off-screen, without storing the coordinates, until we reach the screen and return to part 2

Subroutine: TracePolygonEdge (Part 5 of 8)

Trace a shallow edge that starts off-screen, without storing the coordinates, until we reach the screen and return to part 3

Subroutine: GetPolygonLines (Part 6 of 6)

Split polygon edges whose coordinates are stored in two-byte numbers into smaller sections for processing

Subroutine: TracePolygonEdge (Part 6 of 8)

Trace a polygon edge where the start or end point x-coordinates are two-byte numbers

Subroutine: TracePolygonEdge (Part 7 of 8)

Trace a polygon edge with a steep gradient by stepping along the y-axis (for two-byte x-coordinates)

Subroutine: TracePolygonEdge (Part 8 of 8)

Trace a polygon edge with a shallow gradient by stepping along the x-axis (for two-byte x-coordinates)

Subroutine: CorruptSecretCode

Corrupt the generation process for the landscape's secret code by fetching one more seed number than necessary

Subroutine: ModifyStoringCode

Modify the code in TracePolygonEdge that stores the coordinates of the polygon edge that is being traced

Subroutine: GetNextSeedNumber

Set A to a seed number

Variable: yStoreNextSeed

Temporary storage for Y so it can be preserved through calls to GetNextSeedNumber

Subroutine: PrintNumber

Print a number as a single digit, printing zero as a capital "O"

Subroutine: PrintDigit

Print a numerical digit, printing zero as a capital "O"

Subroutine: SpawnCharacter3D (Part 1 of 2)

Spawn a character on the landscape in large 3D blocks for drawing on the main title screen or secret code screen

Subroutine: CheckCrackerSeed

Check whether the anti-cracker seed-related data is correctly set up, as part of the anti-cracker code

Subroutine: SpawnCharacter3D (Part 2 of 2)

Spawn large 3D blocks for the extracted character definition

Variable: objBlockNumber

A lookup table to convert bit pairs into object numbers for spawning 3D text blocks on the landscape

Subroutine: DrawTitleScreen

Draw the title screen or the screen showing the secret code

Variable: screenType

A variable that determines whether we are drawing the title screen or the secret code screen in the DrawTitleScreen routine

Variable: titleText

The text to draw on the title screen

Subroutine: ReadNumber

Read a number from the keyboard into the input buffer

Subroutine: PrintInputBuffer

Print the contents of the keyboard input buffer

Subroutine: StringToNumber

Convert a string of ASCII digits in the input buffer in-place into a multi-byte BCD number

Subroutine: DigitToNumber

Convert a digit from the input buffer into a number

Subroutine: Print2DigitBCD

Print a binary coded decimal (BCD) number using two digits

Subroutine: GetNextSeedAsBCD

Set A to the next number from the landscape's sequence of seed numbers, converted to a binary coded decimal (BCD) number

Variable: lowNibbleBCD

Storage for the low nibble when constructing a BCD seed number in the GetNextSeedAsBCD routine

Subroutine: SpawnSecretCode3D

Draw the landscape's secret code by spawning a set of large 3D text block objects

Subroutine: PrintLandscapeNum

Print the four-digit landscape number (0000 to 9999)

Subroutine: InitialiseSeeds

Initialise the seed number generator so it generates the sequence of seed numbers for a specific landscape number

Subroutine: ProcessCharacter

Process and print a character from a text token, which can encode another text token or be a one-byte character or VDU command

Subroutine: GetEnemyCount

Calculate the number of enemies for the current landscape

Subroutine: GetNextSeed0To22

Set A to the next number from the landscape's sequence of seed numbers, converted to the range 0 to 22

Subroutine: GetPlayerEnergyBCD

Fetch the player's energy in binary coded decimal (BCD)

Subroutine: MakeSound

Make a sound

Subroutine: DefineEnvelope

Define a sound envelope

Subroutine: MakeSoundEnvelope

Either make a sound or set up an envelope

Variable: aStoreEnvelope

Temporary storage for use in the DefineEnvelope routine

Variable: soundNumberData

A table to map a sound number (0 to 6) to the sound data block(s) that are used to make the sound

Subroutine: ProcessVolumeKeys

Adjust the volume of the sound envelopes when the volume keys are pressed

Variable: volumeLevel

The volume level, which can be changed by pressing "7" and "8"

Variable: envelopeVolumes

A table of offsets into the envelope data for bytes that control the volume of each envelope, so we can change their volume levels

Subroutine: ProcessPauseKeys

Pause or unpause the game when COPY or DELETE are pressed

Subroutine: ProcessMusic

Play the configured music in the background

Subroutine: FlushSoundBuffers

Flush all four sound channel buffers

Subroutine: FlushSoundBuffer0

Flush the sound channel 0 buffer

Subroutine: FlushBuffer

Flush the specified buffer

Subroutine: ProcessSound

Process any sound effects that have been configured so they play in the background (this is called regularly throughout gameplay)

Subroutine: PlayGame

Start playing the generated landscape

Subroutine: MainGameLoop

The main game loop for playing a landscape

Subroutine: ClearIconsScanner

Clear the energy icon and scanner row at the top of the screen

Subroutine: PrintTextToken

Print a recursive text token

Variable: clearPixelMask

A table for converting a pixel number in the range 0 to 3 into a screen mode 5 bit mask with that pixel's bits clear and others set

Subroutine: UpdateIconsScanner

Update the icons in the top-left corner of the screen to show the player's current energy level and redraw the scanner box

Subroutine: DrawIcon

Draw a single icon in the top-left corner of the screen (via the icon screen buffer at iconBuffer) and move along to the right

Subroutine: IRQHandler

The main interrupt handler, which gets run 50 times a second to update the game state and check for game key presses

Subroutine: ScrollPlayerView

Scroll the screen and copy data from the screen buffer into screen memory to implement the player's scrolling landscape view

Subroutine: ShowScreenBuffer

Update the player's scrolling landscape view by copying the relevant parts of the screen buffer into screen memory

Subroutine: ShowBufferColumn

Update the player's scrolling landscape view by copying a 2-pixel wide column from the screen buffer into screen memory

Subroutine: ShowBufferRow

Update the player's scrolling landscape view by copying an 8-pixel high character row from the screen buffer into screen memory

Subroutine: ShowBufferBlock

Update the player's scrolling landscape view by copying an 8-byte character block from the screen buffer into screen memory

Variable: updateOffsetLo

The offset within screen memory for the player's landscape view of the area to update following a scroll (low byte)

Variable: updateOffsetHi

The offset within screen memory for the player's landscape view of the area to update following a scroll (high byte)

Variable: scrollScreenLo

The amount to change the start of screen memory in order to scroll the player's landscape view through each direction (low byte)

Variable: scrollScreenHi

The amount to change the start of screen memory in order to scroll the player's landscape view through each direction (high byte)

Variable: screenBufferHi

The value to add to scrollScreenHi for each direction to get the high byte of the screen buffer address of the content to scroll in

Variable: screenBufferLo

The value to add to scrollScreenLo for each direction to get the low byte of the screen buffer address of the content to scroll in

Variable: panAngleToUpdate

Pitch and yaw angles for panning the landscape view, so the output of DrawLandscapeView will be the bit we add when updating the view

Subroutine: StartScrollingView

Start a scroll process in the background by setting the number of scroll steps and the address to start copying new content from

Subroutine: SetBufferAddress

Set screenBufferAddr(1 0) to the address from which the interrupt handler should fetch new content to scroll onto the screen

Subroutine: UseRowBuffer

Configure the row buffer for use

Variable: bufferMaxPitch

Maximum allowed pitch angles for points in the screen buffer

Variable: bufferMinPitch

Minimum allowed pitch angles for points in the screen buffer

Subroutine: UseColumnBuffer

Configure the column buffer for use

Subroutine: SetColumnBufferMax

Set the maximum and minimum pitch angles for the column buffer

Subroutine: MoveSights

Check for up/down/left/right key presses and move the sights accordingly, panning the screen if they go past the screen edges

Subroutine: MoveSightsSideways

Check for the left/right keys and move the sights accordingly, panning to the left or right if they go past the screen edges

Subroutine: MoveSightsUpDown

Check for the up/down keys and move the sights accordingly, panning up or down if they go past the screen edges

Subroutine: SetSightsAddress

Update the address variables for the sights when they move into a new character block or row

Subroutine: DrawSights

Draw the sights on the screen, saving the existing screen contents in the sights pixel byte stash

Variable: pixelByteColour2

A table for converting a pixel number in the range 0 to 3 into a screen mode 5 pixel byte with that pixel set to colour 2 (%10)

Variable: xSightsStep

Steps to take along the x-axis when drawing the sights

Variable: ySightsStep

Steps to take along the y-axis when drawing the sights

Subroutine: RemoveSights

Remove the sights from the screen

Variable: sightsMoveAddrLo

The change to apply to the screen address of the sights when they move into a new character block or row (low byte)

Variable: sightsMoveAddrHi

The change to apply to the screen address of the sights when they move into a new character block or row (high byte)

Subroutine: GetIconRowAddress

Calculate the address in screen memory of the icon and scanner row at the top of the screen

Subroutine: ShowIconBuffer

Display the redrawn icon and scanner row by copying the contents of the icon screen buffer into screen memory

Variable: arctanLo

Table for arctan values when calculating yaw angles (low byte)

Variable: arctanHi

Table for arctan values when calculating yaw angles (high byte)

Variable: tanHalfAngle

Table for hypotenuse lengths given the tangent of an angle

Variable: screenRowAddrLo

Address lookup table for character rows in screen memory (low byte)

Variable: bufferRowAddrLo

Address lookup table for character rows in the screen buffer (low byte)

Variable: screenRowAddrHi

Address lookup table for character rows in screen memory (high byte)

Variable: bufferRowAddrHi

Address lookup table for character rows in the screen buffer (high byte)

Variable: sightsByte

The sights pixel byte stash, which contains the screen pixel bytes behind the sights, so they can be restored to remove the sights

Variable: sightsByteAddrHi

The screen addresses of the bytes in the sights pixel byte stash, to which they can be restored to remove the sights (high byte)

Variable: edgePixelsLeft

Table to convert a polygon colours byte and a pixel offset (0-3) into a pixel byte for the left edge of the polygon pixel line

Variable: edgePixelsRight

Table to convert a polygon colours byte and a pixel offset (0-3) into a pixel byte for the right edge of the polygon pixel line

Variable: tileVisibility

A table for storing the visibility of each tile from the player's point of view, with one bit per tile (1 = visible, 0 = hidden)

Subroutine: secretCodeStash

A stash for calculated values for each iteration in the CheckSecretCode routine

Subroutine: screenBufferRow0

The screen buffer for character row 0

Subroutine: screenBufferRow16

The screen buffer for character row 16 (as part of a column buffer)

Subroutine: screenBufferRow1

The screen buffer for character row 1

Subroutine: screenBufferRow17

The screen buffer for character row 17 (as part of a column buffer)

Subroutine: screenBufferRow2

The screen buffer for character row 2

Subroutine: screenBufferRow18

The screen buffer for character row 18 (as part of a column buffer)

Subroutine: screenBufferRow3

The screen buffer for character row 3

Subroutine: screenBufferRow19

The screen buffer for character row 19 (as part of a column buffer)

Subroutine: screenBufferRow4

The screen buffer for character row 4

Subroutine: screenBufferRow20

The screen buffer for character row 20 (as part of a column buffer)

Subroutine: screenBufferRow5

The screen buffer for character row 5

Subroutine: screenBufferRow21

The screen buffer for character row 21 (as part of a column buffer)

Subroutine: screenBufferRow6

The screen buffer for character row 6

Subroutine: screenBufferRow22

The screen buffer for character row 22 (as part of a column buffer)

Subroutine: screenBufferRow7

The screen buffer for character row 7

Subroutine: screenBufferRow23

The screen buffer for character row 23 (as part of a column buffer)

Subroutine: screenBufferRow8

The screen buffer for character row 8

Variable: objPointRange

The first and last point numbers for each object

Variable: objPolygonRange

The first and last polygon numbers for each object

Variable: objPolygonPhases

The phase configuration for each object

Variable: sightsByteAddrLo

The screen addresses of the bytes in the sights pixel byte stash, to which they can be restored to remove the sights (low byte)

Variable: objMeanie

The list of polygons and points for the meanie object (polygons 77 to 101, using points 76 to 93)

Variable: enemyYawStep

The yaw angle through which each enemy rotates on each scheduled rotation

Subroutine: screenBufferRow9

The screen buffer for character row 9

Variable: objPointYaw

Polar yaw angles for each of the points in each of the objects

Subroutine: screenBufferRow10

The screen buffer for character row 10

Variable: objPointHeight

The height of each of the points in each of the objects, relative to the object's origin

Subroutine: screenBufferRow11

The screen buffer for character row 11

Variable: objPointDistance

Polar distances for each of the points in each of the objects

Subroutine: screenBufferRow12

The screen buffer for character row 12

Variable: objPolygonData

Various data for object polygons (colour, drawing phase)

Subroutine: screenBufferRow13

The screen buffer for character row 13

Variable: objPolygonAddrLo

Addresses for the list of polygons and points for each object (low byte)

Subroutine: screenBufferRow14

The screen buffer for character row 14

Variable: objPolygonAddrHi

Addresses for the list of polygons and points for each object (high byte)

Subroutine: screenBufferRow15

The screen buffer for character row 15

Variable: objSentinel

The list of polygons and points for the Sentinel object (polygons 102 to 136, using points 94 to 123)

Variable: objTree

The list of polygons and points for the tree object (polygons 52 to 66, using points 51 to 67)

Variable: objBoulder

The list of polygons and points for the boulder object (polygons 67 to 76, using points 68 to 75)

Variable: objTower

The list of polygons and points for the Sentinel's tower object (polygons 137 to 147, using points 124 to 135)

Variable: objTextBlock

The list of polygons and points for the 3D text block object (polygons 148 to 151, using points 136 to 143)

Variable: objRobot

The list of polygons and points for the robot object (polygons 0 to 26, using points 0 to 28)

Variable: objSentry

The list of polygons and points for the sentry object (polygons 27 to 51, using points 29 to 50)

Variable: xPolygonPointLo

Pixel x-coordinates for all the points in the polygon that we are currently drawing (low bytes)

Variable: drawViewYawHi

Storage for the yaw angles of tiles and object points for drawing the current landscape view (high bytes)

Subroutine: GetHypotenuseAngle

Calculate the angle of the hypotenuse in a right-angle triangle given the two non-hypotenuse sides (i.e. two orthogonal axes)

Subroutine: GetPitchAngleDelta

Calculate the pitch angle of a vector relative to an object's pitch angle

Subroutine: GetHypotenuse

Calculate the hypotenuse from an angle and two triangle sides with one lookup and one multiplication (so without a square root)

Variable: yStoreHypotenuse

Temporary storage for Y so it can be preserved through calls to GetHypotenuse

Subroutine: GetRandomNumber

Set A to a random number

Variable: randomGenerator

A 24-bit random number generator that works independently from the landscape seeds and with a much simpler generation algorithm

Variable: shiftGenerator

Storage for randomGenerator(2 1 0) while we shift it left by two places

Subroutine: DrawStars

Draw 80 randomly positioned stars on the screen in colour 2 (white, yellow, cyan or red)

Subroutine: DrawBlackDots

Draw 80 randomly positioned dots on the screen in colour 1 (black)

Subroutine: DrawRandomDots

Draw 80 randomly positioned dots on the screen

Variable: dotCounter

A counter for the number of dots drawn in the DrawRandomDots routine

Variable: dotColour

A flag that determines the colour of the dots drawn by the DrawRandomDots routine

Variable: pixelByteColour1

A table for converting a pixel number in the range 0 to 3 into a screen mode 5 pixel byte with that pixel set to colour 1 (%01)

Subroutine: SetScannerAndPause

Set the scanner update status and delay for 40 empty loops of 256 iterations each (i.e. 10,240 loops)

Subroutine: PrintCharacter

Print a single-byte VDU command or character from a text token, optionally printing a drop shadow if the character is alphanumeric

Subroutine: PrintVduCharacter

Print a one-byte character from a text token or a multi-byte VDU 25 command

Variable: vduCounter

The number of bytes in a VDU 25, n, x; y; command

Variable: tokenOffset

Address offsets for the text tokens (each offset in the table is the offset of the token from tokenBase)

Variable: vduShadowFront

VDU commands for printing the front character of a drop shadow

Variable: vduShadowRear

VDU commands for printing the rear character of a drop shadow

Variable: token0

Background colour blue, print "PRESS ANY KEY" at (64, 100), set text background to black

Variable: token1

Print 13 spaces at (64, 100), print "LANDSCAPE NUMBER?" at (64, 768), switch to text cursor, move text cursor to (5, 27)

Variable: token2

Background colour blue, print "SECRET ENTRY CODE?" at (64, 768), switch to text cursor, move text cursor to (2, 27)

Variable: token3

Background colour blue, print "WRONG SECRET CODE" at (64, 768), print "PRESS ANY KEY" at (64, 100), set text background to black

Variable: token4

Background colour black, print "PRESS ANY KEY" at (192, 64), print "LANDSCAPE" two chars right of (64, 768), move cursor right

Variable: token5

Text token 5: Print "SECRET ENTRY CODE" at (64, 768), "LANDSCAPE" at (192, 704), move cursor right

Variable: token6

Text token 6: Print "PRESS ANY KEY" at (64, 100)

Variable: token7

Text token 7: Move the graphics cursor to (64, 768)

Variable: token8

Text token 8: Move the graphics cursor to (192, 704)

Variable: token9

Text token 9: Move the graphics cursor to (192, 64)

Variable: token10

Text token 10: Configure text to be printed at the graphics cursor and set the background colour to colour 0 (blue)

Variable: token11

Text token 11: Configure text to be printed at the graphics cursor and set the background colour to colour 1 (black)

Variable: token12

Text token 12: Move graphics cursor to (64, 100)

Variable: token13

Text token 13: Print "LANDSCAPE"

Variable: token14

Text token 14: Print "SECRET ENTRY CODE"

Variable: token15

Text token 15: Print five spaces

Variable: token16

Text token 16: Print three spaces

Variable: token17

Text token 17: Print "PRESS ANY KEY"

Variable: musicData

Data for the game's music

Variable: iconData

Screen mode 5 bitmap data for the ten icons that make up the energy icon and scanner row at the top of the screen

Variable: soundData

OSWORD blocks for making the various game sounds

Variable: envelopeData

Data for the six sound envelopes

Variable: sin

Table for sine values

Variable: iconBuffer

The icon screen buffer, which is used to buffer the energy icon and scanner row before writing to screen memory

Variable: xPolygonLeft

The pixel x-coordinate of the left edge of each pixel line in the polygon being drawn

Variable: xPolygonRight

The pixel x-coordinate of the right edge of each pixel line in the polygon being drawn

Variable: stripData

Storage for tile data when smoothing strips of tiles during the landscape generation process

Variable: tilesAtAltitude

Storage for tile blocks at specific altitudes for placing enemies on the landscape

Variable: maxAltitude

The maximum tile altitude for each 4x4 block of tiles

Variable: xTileMaxAltitude

The tile x-coordinate of the highest tile within each 4x4 block of tiles

Variable: zTileMaxAltitude

The tile z-coordinate of the highest tile within each 4x4 block of tiles

Subroutine: GetObjectAngles

Calculate the angles and distances of the vector from the viewer to a specific object

Subroutine: GetObjPointAngles

Calculate the view-relative pitch and yaw angles of all the points in an object

Subroutine: DrawObject

Draw a 3D object

Subroutine: GetHorizontalDelta

Calculate the difference in the x-axis and z-axis between two objects, as both signed and absolute deltas

Subroutine: GetVerticalDelta

Calculate the difference in the y-axis between two objects as a signed delta

Subroutine: ReadKeyboard

Enable the keyboard and read a character from it

Subroutine: ReadCharacter

Read a character from the currently selected input stream

Subroutine: EnableKeyboard

Select the keyboard as the input stream and flush the keyboard buffer

Subroutine: SetColourPalette

Set the logical colours for each of the four physical colours in screen mode 5

Variable: colourPalettes

The logical colours for two mode 5 palettes

Subroutine: DitherScreenBuffer

Dither the contents of the screen buffer onto the screen

Subroutine: ShowGameOverScreen

Display the game over screen

Subroutine: DecayScreenToBlack

Smother the screen with randomly placed black dots to decay the screen to black

Subroutine: JumpToPreview

An intentionally confusing jump point for controlling the main title loop flow when returning from the GenerateLandscape routine

Subroutine: SpawnTitleObject

Spawn the title object (robot or the Sentinel) as object #1

Variable: zTitleObject

The z-coordinate of an object on the title screen, as a delta from the z-coordinate of the viewing object

Subroutine: PreviewLandscape

Draw an aerial preview of the landscape

Variable: titleViewerPitch

The pitch angle of the viewer for objects on the title screen

Variable: yTitleObject

The y-coordinate of an object on the title screen, as a delta from the y-coordinate of the viewing object

Variable: titleObjectYaw

The yaw angle of an object on the title screen

Variable: titleViewerYaw

The yaw angle of the viewer for objects on the title screen

Subroutine: DrawTitleObjects

Draw an object on top of a tower on the title screen

Subroutine: DrawTitleObject

Draw an object on the title screen

Subroutine: PlayMusic

Play a piece of music

Subroutine: ConfigureMachine

Configure the custom screen mode, set the break handler to clear memory, move code, reset timers and set the interrupt handler

Subroutine: ClearMemory

Clear game memory, so that the BREAK key can remove all trace of the game code in early versions of the operating system

Subroutine: Entry

The main entry point for the game