Top of the stack (so, pushes below this point).

Tracks whether the current screen has been fully initialised. Set to 01 after the HUD header is drawn, and cleared to 00 when a fresh screen setup is needed.

Bitmask representing the current directional input. 1F means no input. Each bit represents a direction when reset:

Bit	State
0	Fire
1	Up
2	Down
3	Right
4	Left

Stores the last directional input for animation and speed purposes.

00=facing right, 01=facing left.

Controls which input device is active:

Byte	State
00	Not yet detected
1F	Kempston joystick
80	Keyboard

Tracks whether Percy is falling:

Byte	State
00	Not falling
FE	Falling (active)
FF	Falling (impact)

Set to 01 when Percy has collided with scenery and the move was rejected.

Set to FF when Percy is dropping an egg, and cleared to 00 when the egg routine is complete.

Set to FF when Percy is carrying a worm, and cleared to 00 when he's not.

A delay so Percy's energy bar doesn't deplete too quickly while he's flying.

Set to FF when a worm has been dropped. Cleared to 00 when there's no worm being processed.

Set to FF when Percy has landed on a platform. Cleared to 00 when no platform is beneath him.

Percy's current movement speed in pixels per frame. Ranges from 01 (minimum) to 04 (maximum). Accelerates while a direction is held, decelerates when input changes or is released.

Animation frame counter for Percy's wing flap cycle. Increments each frame Percy moves horizontally, cycling from 01 to 04.

Controls the animation frame cycle. Counts up from 01 to 04 (first half), then bit 7 is set and it counts back down (second half). Bits 0-2 hold the frame index, bit 7 indicates the second half.

The current level number. E.g. 01 for the first level.

Number of worms Percy has remaining to collect for the current level.

The ASCII character displayed on screen to represent the current lives count.

A rotating bit pattern used to animate the nest chicks. Each bit controls the animation frame for one chick, rotated each frame so they animate independently.

Pointer to the current room's object data, calculated from the room number and the base at Room_Object_Data. Used to track the state of collectible worms and other interactive elements in the current room.

Current pitch value used for sound effects (item collection, falling). Adjusted each frame during the sound.

The two-byte drawing position used by Draw_Room_Tile to track where the next tile should be placed.

The low byte is the column and the high byte is the row.

Set to FF when entering a new room. Checked by hazard handlers to reset their positions and state when the room is repopulated.

00=not paused, FF=paused; set at Game_Initialise.

Copy of LivesDisplayCharacter. Set when lives are initialised, cleared at end of each frame. When non-zero the main game handlers run; when zero the full sprite draw runs.

Holds the pointer to the currently active tile set.

Points to the tile data for the current room.

The ID of the current Room (00-0B).

Workspace for scoring calculations.

Counts down each frame. When it reaches zero, the red bird picks a new random flight direction.

Controls which direction the red bird traverses across rooms. 00=moving left (entering from the right), 01=moving right (entering from the left). Toggled when the red bird wraps past the room boundary.

When non-zero, the red bird is stunned. Counts down each frame until it reaches 01 then the stun ends and a short reappearance delay begins.

Copy of the initial stun timer value, stored when the red bird is first stunned.

The room number the red bird is currently in.

Seeded from the Memory Refresh Register when the red bird is initialised for each room.

The red bird's current movement direction within a room (00-07), indexing into the direction jump table at Direction_Jump_Table. Changes randomly each time the direction change timer at RedBirdDirectionChangeTimer expires.

The red bird's current flight speed, looked up from the per-level speed table at RedBirdSpeedTable based on the current level number.

Counts down to trigger a new flight path setup via HandleRedBird_SetupFlight.

Pointer to the current room's flight path boundary data at Table_RedBirdFlightPath_01. Each boundary entry is 04 bytes: Y-min, Y-max, X-min, X-max.

Counts down each frame while the red bird is waiting to enter the current room. When it reaches zero, the red bird appears.

Flight speed for each level. Indexed by zero-based level number.

Cycles through 00-07 to animate the red bird's wings. Divided by 02 and added to the frame base of 18 to select the sprite frame.

Boundary data for the butterfly's movement, used by the direction jump table at Direction_Jump_Table.

Table of room/level pairs where butterflies appear. Each entry is two bytes: the room number followed by the level number. Bit 7 of the level byte is set when the butterfly has been collected. The table is terminated by FF.

Toggled each frame to alternate the butterfly's wing animation between two frames.

Countdown timer for the butterfly's current movement direction. When it reaches zero, a new random direction and delay are generated.

Current movement direction for the butterfly, in the range 00-07.

Jump height lookup table. Each entry is the Y offset from ground level for one step of the frog's jump arc. The values rise to a peak of 22 at the midpoint then fall back to 00 forming a smooth parabolic arc over 18 steps.

Crash animation timer. When non-zero, the car cycles through crash frames. Counts down to 01 then clears.

Single byte used as movement boundary data; read at Move_Helicopter.

State variables for the helicopter hazard. Two five-byte state blocks: Helicopter_State_Data_1 (first) and Helicopter_State_Data_2 (second).

Referenced by room handlers at Handler_Room01 and Handler_Room06.

Animation counter, direction flag and step size for the cat.

Animation counter, direction flag and step size for the dog.

State variables for the UFO hazard.

Countdown for the bomb explosion animation. When non-zero, the explosion frames are displayed on Percy.

State variables for the plane hazard.

The horizontal speed of the plane in pixels per frame.

Direction flag for the balloon. 00 = floating right, non-zero = floating left.

Counter for the balloon popping and respawn sequence. Values 01 to 03 show deflating frames, 04 to C7 are the respawn delay, and C8 triggers the respawn.

Animation counter, direction flag and boundary data for the walking paratrooper.

Walking speed for the paratrooper, indexed by room number minus one. Only rooms 05, 06 and 08 have active paratrooper entries.

Direction flag for the spider. 00 = ascending, FF = descending.

State variables for the parachute hazard.

State variables for the cat hazard. Only rooms 02, 03, 05, 06, 07 and 08 have active entries.

State variables for the dog hazard. Only rooms 03 and 07 have active entries.

Lookup table of starting X positions for the parachute, indexed by room number minus one. Only rooms 02, 08 and 09 have active paratrooper entries.

State variables for the walking paratrooper hazard. Only rooms 05, 06 and 08 have active entries.

Used by the Red Bird.

Used by Frog 1, Car Type 1.

Used by Frog 2.

Used by Car Type 3, Cat.

Used by Car Type 2, Dog, Plane.

Used by the Helicopter, Parachute.

Used by Cat, UFO, Plane.

Sprite data for the butterfly collectible.

Backup of Percy_X_Position. First byte is Percy's X from previous frame (used for wing flap animation); written each frame by Draw_Sprites.

Backup of Sprite01_3Wide_X_Position; written each frame by Draw_Sprites.

Backup of Sprite02_3Wide_X_Position; written each frame by Draw_Sprites.

Backup of Sprite03_3Wide_X_Position; written each frame by Draw_Sprites.

Backup of Sprite04_3Wide_X_Position; written each frame by Draw_Sprites.

Backup of Sprite05_3Wide_X_Position; written each frame by Draw_Sprites.

Backup of Sprite06_3Wide_X_Position; written each frame by Draw_Sprites.

Backup of Sprite07_3Wide_X_Position; written each frame by Draw_Sprites.

Backup of Egg_X_Position; written each frame by Draw_Sprites.

Backup of Sprite01_2Wide_X_Position; written each frame by Draw_Sprites.

Backup of Sprite02_2Wide_X_Position; written each frame by Draw_Sprites.

Backup of Sprite03_2Wide_X_Position; written each frame by Draw_Sprites.

Backup of Sprite04_2Wide_X_Position; written each frame by Draw_Sprites.

Backup of Sprite05_2Wide_X_Position; written each frame by Draw_Sprites.

Backup of Sprite06_2Wide_X_Position; written each frame by Draw_Sprites.

Buffer holding the state of interactive objects (worms, nest slots) for all rooms. Each byte can be:

Value	Meaning
00	Empty / available
1E	Worm collected by Percy
1F	Worm delivered to nest

The buffer is 0160 bytes long, pointed to per-room by Room_Object_Data_Pointer.