Space Rogue

01 · The game

A trader in the Far Arm

The story opens with the player's ship, the Princess Blue, answering a distress call from a derelict and being destroyed by the Manchi. The player is left with the derelict's scout ship, the Sunracer, and earns a living with it by trading and fighting while the plot comes out in conversation.

Flight happens in a cockpit with a filled-polygon view of ships, stations, planets and Malir gates, the wormholes between star systems. Pressing N leaves the cockpit for Navigation Control, a chart of the system the ship is in, where HELM flies to a chosen sector. Docking with a base leaves the ship altogether: the player walks around it in a top-down view, trades, talks to people and, in some cantinas, plays the arcade game HIVE!. The ships, the rules their computer pilots fly by and a fight to set up are on Ships and combat.

02 · The screens

Every screen is one bitmap, rebuilt here from memory

The game never uses a character screen in play. The cockpit, the chart and the bases are each one multicolour bitmap, with a frame of its own unpacked from the disk around the part that changes.

Each picture is a frame recorded in VICE, the memory and the video chip's registers at the start of a frame and every write to them during it, drawn by the site's renderer. All three are a bitmap at $C000 with its screen matrix at $E000. The bitmap runs under the I/O chips, so the engine keeps the processor port at $34, all RAM, and banks the chips in only around the code that needs them (bank_all_ram, $0E85). The engine draws the next picture into a second buffer at $4000 and flips the two (flip_buffers, $8ED3).

03 · Tiles

One chart drawer for space, the bases and HIVE!

Navigation, the bases and the HIVE! cabinet each load a tile set of 128 tiles to $4000 and a map to $5100, and the same four routines in the engine draw a window of 11 by 9 tiles from them.

A tile set's 4 KB of pixels are laid out for the drawing loop rather than for a person. A tile is 16 pixel rows of two bytes, and tile t's row r is the byte at $4080 + $100 × r + t for its left half and $4000 + $100 × r + t for its right, so one index register reaches every tile in a page. The kinds at $5000 say what each tile does, and the colours at $5080 give each tile its two colours in the screen matrix.

The window is built in a buffer of 99 cells at $FA00: fill_chart_window ($3069) copies the map's tiles around the centre, with a fixed tile past the map's edge, draw_map_objects ($2F9C) puts the 32 map entries in, animate_chart ($8638) advances the animated tiles, and draw_chart copies the cells to the bitmap. In NAV.TIL the high four bits of a kind are the sector's kind instead, which QUERY reports: open space, nebula, a shard field, asteroids, an A3 or G2 star, binary stars or a black hole.

04 · A system map

Karonus, with its planets placed for the date

A system map is 32 by 32 sectors. Its first kilobyte is the grid of tiles; after it come 32 entries for the planets, bases, gates, storms and notes, and a few bytes for the system as a whole.

An entry whose motion is an orbit is put on a circle of radius 3 or 6 round the centre of the chart when the map is loaded, and moved on once a day of HELM travel: the small orbit has 16 places and steps every two days, the large one 32 places and steps every four (orbit_small, $2E11; orbit_large, $2E31). After 128 days every planet is back where it started. The day count starts at 01 MAR 12, and the calendar has no leap years (month_days, $20AF).

05 · Sound

Sound: the engine hum restarts its wave every frame

There is no music in the game. The engine plays fourteen effects on the SID, and when the first voice has nothing to play it hums at a pitch set by the ship's speed.

An effect is a voice, a starting frequency, a step added to it every frame, an envelope, a waveform, a length in frames, the frame on which the gate closes and a number of repeats ($11AD-$1238). Most effects use the third voice: the hits, the explosions, the alarm and the hazards, nearly all of them noise. The player's beam, the missiles and the beep use the second, and the door and HIVE!'s recall the first, which silences the hum while they play.

The hum lives in the first voice's idle frames ($114B). Every frame it sets and clears the test bit, which stops the oscillator and starts its wave again from zero, and then turns the gate on or off by turns with a triangle wave at the speed times 8 in the SID's units, so the hum is a triangle under 33 Hz cut into pieces 25 times a second. Going backwards it hums at the speed's size (slot_speed, $0415). In navigation and at a base the hum is off (hum_on, $1E), and while the disk turns every frequency is masked to zero (voice_mask, $1D).

06 · Pictures

Every picture is one run-length stream

The title, the story pictures, the ending and the frames around the cockpit, the chart, the bases and the HIVE! cabinet are all packed the same way, and all thirteen unpack with the same few lines.

A byte n below $80 repeats the byte after it n times; $80 + n copies the n bytes after it; a zero ends the picture, and the byte after it is the border's colour. The output is a bitmap with its screen matrix after it, at +$2000, and for the story pictures a page of colours at +$2400 that the intro and the ending copy into the colour RAM (colours_to_ram, $718B). The engine's frames have no colour page: the engine leaves the colour RAM white, which is why every frame's fourth colour is white.

The packing is plain run-length, so a picture's size on disk follows its detail. A story picture is a smaller scene on a black screen, and the text fills the rest. SPACE is the title's star field alone and ROGUE the logo on black; the title draws its 3D ship over them.

07 · Controls

Flying the Sunracer from the keyboard, or with two joysticks

The engine reads the keyboard itself, the C128's extra keys included, and looks each key up in a list of 37 commands. Click the table and press a key to find it.

The keys that steer add a step to a turn, pitch or roll rate, and the rate keeps turning the ship until S stops it. Two joysticks fly the ship together: the one in port 2 steers and fires, and pushing the one in port 1 forward or back changes the speed (joystick_speed, $242C; Discoveries). F1 gives the code ESC, which pauses (key_escape, $1438), and F7 gives the code the list puts beside CTRL-E, so F7 switches between cruise and Newtonian flight too.

08 · Bases

The people of the Far Arm stand where the map files put them

Each of the twelve bases is a map file with the same layout as a system's: a floor plan of tiles and 32 entries, which here are the people, the counters, the doors, the notices and the things lying about.

The type of an entry says what it is (map_type, $5590): below $20 a person, by the number of their talk script; $20 to $24 the cargo merchant, a guard, the outfitter, the repair shop and the bartender; $80 and up an item; $A0 a locked door and $A1 a safe. Each person's 64 lines of conversation are disk texts numbered from $C000 plus 64 times the script's number (talk_script, $E569).

Seven people have no base of their own. Each time the ship docks at a base whose map names a visitor's square, one of them, chosen at random, stands there (base_setup, at $E4BC).

At starbases and the carrier the cargo is checked one visit in two, one in eight with forged papers, and the contraband taken ($A62E).

The prices come from the base's map: a base price for each class of cargo, a random part of up to an eighth more, and a multiplier from 2 to 5 for each kind within its class, the same all day (cargo_price, $B027). Maps draws every base.

09 · HIVE!

HIVE! pays only from the fifth drop

The cabinet in the cantinas is a game of its own, loaded over the conversations: a trooper drops onto an island, destroys a number of Manchi mounds and runs back to the drop ship.

Drop n has 3 × n mounds to destroy, and there is no last drop: the game ends only when two Manchi stand next to the trooper at once (hive_caught, $5947). Caught on drop 5 or later, the trooper is paid (n − 4) × 8 credits; caught earlier, the trooper is torn in half and paid nothing. A credit buys a game. The island is loaded again before every game (hive_menu, $572F), and the mounds rise on it during play, up to five at once.

The Manchi chase the trooper with the routine the bases' guards use (station move_chase, $A785), and the screen is the chart drawer's window round the trooper. A reversed branch keeps the wait between mounds at its shortest from the first drop (Discoveries). Maps draws the island.

10 · Text

The game's 4,105 texts are Huffman-coded on the disk

The conversations, the QUERY notes and most of the engine's messages are not in the programs. They are read from side 2 a block at a time and decoded one character at a time with a Huffman tree of 136 bytes.

The tree is a list of pairs, a 0 bit taking the left of a pair and a 1 bit the right. An entry below $80 is the distance on to the next pair, and an entry of $80 and up is a character. Space takes 3 bits, e takes 4, and capitals mostly 9 or more, which suits text that is mostly lower case.

A text is named by a number from $C000, where an address would be. resolve_text ($9BB7) takes a number from $C000 to $CFFF as a disk text, subtracts the counts in SPB's table at $BD00 to find the block of 512 bytes that holds it, reads the block from disk block $01AD on, skips the texts before it and decodes it into $BF00.

11 · Copy protection

Only a saved game has to answer the manual

The manual question comes only when the player loads an old game. A new game starts without it.

The question is one of 32 records, each a page, paragraph, sentence and word of the Sunracer Owner's Guide and the word in capitals, picked by the frame counter AND 31. The check compares only the first four letters. A wrong answer shows ACCESS DENIED over the star field, which the typed answer has altered (Discoveries), and loads the title again (access_denied_key, $7965).

12 · Memory

How the eight parts take turns in memory

The game is a title, an intro and an ending that are programs of their own, and an engine that stays in memory while the flight code, the bases and HIVE! are loaded in and out over each other.

Docking loads SEG1 over SEG0 and SEG1A over the models (load_seg1, $1507); leaving loads SEG0 back, and END over the bytes the base used, which is INIT's 512 bytes under another name. Every file loads through the KERNAL's LOAD, with no fast loader and no protection on the disk.

The odd things the code does, from a raster interrupt that can stop for good to a hidden ninth system, are on Discoveries.