Lode Runner

01 · Room data

The rooms on disk

Each disk board packs its 28 × 16 cells into 224 bytes. The disk reader skips raw sector byte zero and loads those cells from raw offsets 1–224. Each byte holds two cells, with the low nibble read first. Values above nine become blank when the game builds the room.

The disk game keeps its engine in memory and reads a new sector for each board. The Source tab has the resident engine and each of the 150 room loads. The 150 boards occupy tracks 3–12. Those room sectors are identical in the three supplied disks. The Editions tab compares the disk variants with the separate cartridge engine.

Surplus guard and exit markers disappear when a room is built. The disk engine keeps at most five guards and 45 hidden exit cells, even when the stored board contains more. The scan runs from the bottom-right cell toward the top-left, so the first markers encountered get the available slots. $6FA6, $728F, $709C.

Trapdoors and direction

A trapdoor looks like a brick and lets the runner fall through from above. The cell blocks sideways and upward entry. Its picture uses brick tile 1, while the maps keep trapdoor tile 5. $733E, $73C7, $74DE, $7550, $75C8.

02 · Graphics

Bitmap scenery and sprites

Loading the shared C64 renderer.

VICE reference picture

The runner and guards use up to six hardware sprites, in slots 0, 2, 3, 4, 6, and 7. Those sprites move over multicolour bitmap scenery.

The work bitmap at $4000 preserves the scenery. When a shape is erased from the display bitmap at $2000, that background restores the picture beneath the shape.

03 · Digging

When a hole closes

Drilling opens a brick diagonally below the runner. The cell beside the runner must be clear. Concrete blocks resist drilling. $76E6, $77A5.

A completed hole stays open for 180 main-loop passes. Partial refill pictures appear with 20 and 10 passes remaining. At zero, the brick returns. $7B12, $84F1.

Trapping a guard earns 75 points. Burying the trapped guard when the brick returns earns another 75.

A buried guard moves to a respawn position. After the respawn delay, the guard’s sprite returns when that cell is clear in the actor map. $7BD2, $862F.

At the disk version’s default pace, an undelayed hole lasts about six seconds. The measured PAL setup runs about 30 main-loop passes per second. Scoring, drawing, and disk operations can delay those passes, so the timer follows the game’s progress rather than elapsed seconds.

Digging cadence and hole count

The disk’s normal digging cadence limits how many timed holes can overlap. A successful dig takes 13 runner updates, and each continuing main-loop pass also ages the holes. With legal positions and the normal loop and room-reset flow, a 180-pass lifetime allows at most 14 active timed holes: 180 / 13, rounded up. Walking and interrupted digs lengthen the gap between completions. The allocator has room for 30. $611C, $76E6, $77A5, $7B12.

The final drill check

The neighboring cell is checked on every dig update. A nonblank cell, including a guard marker, cancels the dig even on its final update. The brick remains, and no hole timer is allocated. $7712, $77D3.

04 · Guard movement

Guard route costs

Guards first try a supported horizontal chase when they share the runner’s row. Otherwise they look for ladders and drop points within their walking range.

The guard assigns a cost to each candidate exit. A candidate on the runner’s row costs the lateral distance from guard to candidate. An exit above the runner costs its vertical gap plus 100; an exit below costs its gap plus 200.

Equal route costs preserve the guard’s search order. The guard tries its current column before searching left and then right. Each side scan starts at its outer walking limit and works inward. A candidate replaces the previous choice only when its cost is strictly lower. $7FD2, $809C, $8106, $816F.

05 · Points

Gold and points

The status bar hides the score’s highest digit. The counter stores eight decimal digits in four packed-BCD bytes, while the status bar prints the lowest seven. The highest digit remains in the counter and high-score record. Overflow beyond eight digits discards the carry.

Gold is collected only when both movement phases are centered. Collecting the last gold queues a motif and enables hidden exits. The Music tab plays those motifs and explains their changing pitch. $7A4A.

An enabled exit can become a ladder beneath an actor while preserving that actor’s collision marker. With the gold collected, reaching row zero at the centered vertical phase completes the room and awards 1,500 points plus a life. $8D96, $611C, $6162.

06 · Controls and disks

Controls and the editor

Commands labeled Ctrl also accept other modifier combinations with the same letter. The IRQ sets the key byte’s high bit whenever a modifier is active, matching the command codes at $7A10.

The editor reads and writes the same packed sectors used by the game. Its commands play, clear, edit, move, initialize, and clear scores. $65D6.

The editor protects the master disk’s room data. Clearing the master score file is permitted. $6A88.

High scores rank displayed level before points. Before writing an edited record, the game rereads the score sector. $6CAD.

The Black Label disk engine is covered here. Gray uses a different disk-probe path, and Yellow has additional startup differences.

The supplied 16 KB CRT boots a separate engine outside the disk Source tab. The cartridge’s complete controls and editor workflow are outside this disk analysis.