Enemy/object slots. +$00/$01 X/Y (current), +$02/$03 sprite frame/page (current), +$04/$05 X/Y (target), +$06/$07 sprite frame/page (next), +$08/$09/$0A clip bounds, +$0B flags (bit 2 flip, bit 4 wall-solid, bit 5 draw-active), +$11 AI delay timer, +$12 AI state. See 5BDA for the player's own instance and docs/Object-Record.md for the full field writeup.

Player's object record

Jump table at the head of the code. Every entry is a jump to the real routine, so callers reach the common services through fixed addresses even when the targets move.

This entry point is used by the routines at 8EE3, E1D1, E224, E6FE and EF37.

This entry point is used by the routine at EF37.

This entry point is used by the routine at ED52.

This entry point is used by the routine at DEEF.

This entry point is used by the routines at 83B8, 85AB, 86F5 and EF37.

This entry point is used by the routines at 8158, 8EE3, DE91, E6FE, ECC1 and EF37.

This entry point is used by the routines at 8EE3 and ECC1.

Copies the byte at $643A+A into the active slot $6433+A to start sound A, or clears all seven slots when A=7.

Silences all seven sound channels by clearing their counters.

Scans the seven sound slots at $6433 for an active channel and dispatches to its handler through the address table at $6446, with the decay routine 6495 queued as the return.

Fades the active sound effects, stepping each channel's counter toward silence at its own rate and freeing the slot once it lands there.

Plays a two-stage sound stepped by the counter at $6434: a higher pitch while it is under $0A and a lower one above, reloading the counter to $14 when it expires.

Plays a rasping tone by toggling the speaker while flickering the border, the pitch taken from its sound parameter and each cycle padded with a short delay.

Turns its sound parameter into a pitch and plays a short beep.

Turns its sound parameter into a pitch and plays a short beep.

Turns its sound parameter into a pitch and plays a short beep.

Plays a sound whose pitch band steps with the counter at $6435 (raw below $0A, then based at $12 or $1C for higher values) for a fixed 10-cycle duration via 6575.

Turns the active sound parameter at $6433 into a pitch (base $88, or higher parameters mapped to a shorter period) and plays an 8-cycle beep via 6575.

An unreferenced copy of the beeper loop -- byte-for-byte the same code as 6575, left in the binary but never called.

Sounds a square-wave beep by toggling the speaker for L half-cycle pairs, each half delayed by the pitch value in A so a smaller value gives a higher tone.

This entry point is used by the routine at 6550.

Walks the effect slots, ticking each active one and leaving the free slots alone.

Ticks one object's countdown. Most calls just decrement it; when it reaches zero the object's work record is rebuilt -- position copied, sprite frames picked with a small wobble, the object stamped onto the map -- so it only redraws every few frames.

Reports whether a value is a multiple of four, handing back a default of $12 either way.

Walks the five fixed-size slots looking for one marked free, and if it finds one fills in the caller's address and a starting attribute so the slot becomes active.

Initialises the object work-record at $6661 with its clip bounds and marks all five slots in the $6648 table as free ($80).

Starts the scrolling title text: points the reader at the message, colours the strip it runs along, and arms the interrupt handler that steps it.

Shifts the whole banner left by rotating each screen byte and pulling in the next pixel column from a small font-row buffer. A timer paces it so a fresh character is fed in only every few frames.

Feeds the next character of the scrolling title message into the display buffer $681E: reads it via the pointer at $681B, wrapping at the $1F end marker, and copies its 7-row font glyph from $7D28.

Adds to the score and redraws it, rippling the carry through the digits and printing each one from the font. Crossing a threshold awards an extra life with a sound.

Converts a packed row/column in HL to a screen address, halving the row to a character line via 98D9 and nudging down one pixel line when the row's low bit is set.

Clears B screen pixel-rows starting at HL by pushing zero words through SP, advancing one pixel line (with third-boundary wrap) each iteration and restoring the real SP on exit.

Runs one frame of the screen wipe. It blanks a band of rows either side of a moving boundary and repaints the colour behind it, finishing and clearing the busy flag once the counter reaches the end.

Draws the control-type indicator tile: derives the graphic page from the joystick type in $FDBC (bit 5 selecting a mirrored index) and paints it via 69B4 with count $0A.

Repaints the status bar a column at a time, walking across it and colouring each column in turn.

Works out where one column of the status bar sits in the attribute area and paints it.

Works out where a tile sits in the attribute area from its row and column, then copies the tile's colour bytes into place.

Sets up attract mode: saves the caller's stack, initialises the system, clears the last key and control type, then drops into the main menu.

Main menu: shows the score table, clears the screen and starts the scrolling credits, then waits for a menu key and branches to start the game, pick a joystick or redefine the keys.

This entry point is used by the routines at 6E08 and 6F1D.

This entry point is used by the routines at 6A1E and 7143.

Redefine Keys screen. It clears the existing assignments, then works down the prompt list asking for one key at a time and refusing any already taken, finishing by switching the control type to keyboard.

Checks a key against the six already assigned on the Redefine Keys screen, reporting a clash so the same key cannot be used twice.

Prepares the status bar: fills its area, points the interrupt's scroll handler at the status-bar updater, then copies the panel graphic in row by row.

Waits for the currently held key to be released and then for the next press, so one keystroke cannot be read twice.

Clears the playfield and the status area, blanking both their attributes and their bitmaps.

Blanks a rectangle of the screen bitmap, working down row by row and clearing the given number of bytes across.

Draws the menu box: paints the border in the requested colour with the ink matched to the paper, blanks the interior, then repaints the border over the cleared area.

Scrolls the level in at the start of an Area, running the scroll for a fixed run of frames or until the Area flag says it has arrived.

Outer game loop. It prepares each Area and runs the main loop; when the player dies it restores their grenades, takes a life and replays the Area from its last spawn point, ending the game once the lives run out. A surviving player gets the Area-complete sequence and moves on.

Player is dead

Black screen at the end of an Area

Adds an amount to the grenade count and stores the new total.

Redraws the grenade count in the status bar.

Prints a two-digit number, counting out the tens and then the units as characters.

Game Over. A score that does not make the table drops straight back to the menu; otherwise it draws the letter grid and lets the player steer around it with the joystick, picking out a name one letter at a time before the entry is written into the table.

Enter name for the new Top Score record

Name entry is finished: waits for the key to lift, clears the screen and drops back to the menu.