I, Ball

01 · The game

Steer Iball to the top of the level

Iball is a ball with an eye, and he is out to rescue four friends from Terry Ball. Each of the sixteen levels is a vertical track of walls. The screen scrolls up as Iball pushes towards the top, and the level ends when he reaches the home straight at the very top of the map. Some walls are electrified and kill him on touch; aliens form out of nothing and home in on him; the timer at the top of the screen counts down from 50.

Some of the aliens are discs. Touching a disc gives Iball an extra: a faster ball, a laser, a shield, a jump to the end of the level. The extras' icons light up in the panels above and below the play area. After every fourth level a friend is rescued, and after the sixteenth the game is over. Andrew Betts wrote the Commodore version, and Rob Hubbard wrote its music.

02 · Music

Music: Rob Hubbard's tune, played by the game's own driver

The title plays the music, and so does play if the player presses 1 on the title screen. Press 2 instead and the game plays its sound effects.

The driver plays three voices, each from a track: a list of pattern numbers, with a transpose before some of them. A pattern is a run of notes with their lengths and instrument changes. There are 35 patterns (pattern_ptrs, $E849) and 18 instruments of 16 bytes (instruments, $E70B), whose flags switch on vibrato, a pulse-width sweep, an arpeggio, a filter sweep, or a drum table that changes the waveform and pitch each frame. C64-Wiki says the tune is a remix of Cabaret Voltaire's songs; Wikipedia names "Whip Blow" and "I Want You".

03 · Sound

Sound: 22 effects, and the events that ask for them

With effects chosen, a second player runs in place of the music. The game asks it for an effect by writing $40 plus the effect's number into one byte per voice.

An effect is a frequency that slides by a step, and the step itself changes each tick. One of eight scripts says when to slide up, when to slide down and when to go back to the start (effect_script_data, $F050). Each effect's 16 bytes (effect_defs_a and effect_defs_b) give its starting note, waveforms, length and script.

04 · Speech

Sound: three spoken words, played through the volume register

The SID has no way to play recorded sound, but changing its master volume makes a click, and changing it thousands of times a second makes a waveform. Each byte of a sample holds two four-bit volumes. CIA 2's timer interrupts the processor every sample_pitch + 1 cycles, and each interrupt writes the next volume into $D418. The three samples fill $A000-$BFFF. The game plays the first on the title screen, the second when Iball dies, and the third at the end of a game.

While a sample plays, the game stands still. irq_panel_end ($4506) checks sample_busy and, if it is set, does not hand on to the next interrupt of the frame, so none of the game's logic runs until the word has been said.

05 · One frame

One frame of play, rebuilt from memory

Four raster interrupts cut the frame in bands, each setting the line of the next. The top and bottom panels use extended colour mode and the text character set, each with its own eight sprites: the extras' icons, the timer's two digits, the score and the lives. The play area between them uses multicolour characters from the playfield's set at $3000 and eight sprites of its own. The panels' sprites are the same eight sprites as the play area's, moved and repainted three times a frame.

06 · The playfield

A level is a list of tiles, built into a map before play

A level's description is 360 bytes: 60 rows of six tile numbers (level_1 at $7600, and fifteen more after it). A tile is four characters by four, so a row of tiles is 24 characters wide, the width of the playfield. Before play, build_level_map ($4B2F) turns the tiles into a map of 240 rows of 24 characters, at $097E. The playfield is that map seen through a window of 25 rows. Each time the window moves by a whole row, draw_playfield copies all 600 characters again.

What a character does depends only on its number. Characters 1 to 5 are the shots and the lasers. Characters $06 and up are walls, and characters $EE and up are electrified walls, which kill. Multicolour register 1 changes colour every frame, so whatever is drawn in that colour flashes.

07 · Iball

Inertia: a push changes Iball's speed by one

Iball moves with two speeds, across and up or down, each with a direction. A push in the direction he is moving adds one to that speed; a push the other way takes one off, and below zero turns him round (push_up to push_right, limit_speeds). The controls are read every second time the top interrupt runs, so holding the stick changes the speed by one every second frame or so. A speed that reaches the limit, 3, goes back to 2, so his top speed is 2 pixels a frame. With the Turbo Boost the vertical limit is 4, and with the Fuel Injection 5. Every 20 times the top interrupt runs, friction takes one off every speed, so Iball drifts to a stop when the stick is let go.

Iball never goes above line $6C of the screen. Pushing past it hands his speed to the scroll, so the map moves instead (move_iball). The sides of the screen turn him round. Walls are found by reading the screen: probe_background copies the 12 characters under him, three rows of four, and bounce_iball reverses his direction when the row or column he moves towards holds a wall. A hit on the top row while the map is in its top two rows is the end of the level.

08 · Controls

Every key in a column does the same job

The manual gives Z, X, O and K for left, right, up and down, P for fire, SPACE for the Smart Missile and M for the Molecule Bomb. The game reads a key by driving rows of the keyboard low and reading one column. Most of its tests drive seven rows at once, so every key in that column on those rows does the job. O is the up key, and so are E, T, U, @, the up-arrow and Q.

The same groups answer on the title screen, where the jobs change. I, the key C64-Wiki gives for the "IBALL!" sample, is in the fire column, so it starts a game. X and the other right keys say "IBALL!", Z "OH NO!", and SPACE "GAME OVER!".

09 · Discs

The discs always give their extras in the same order

The first alien to form in a game is a disc. After that, a disc forms once 21 aliens have been killed (disc_countdown). Touching a disc gives the extra at the next place in the sequence; after the 32nd place the sequence repeats its last four. A death moves the sequence back to the last checkpoint and takes away the extra given at that checkpoint, so the next disc gives it again (disc_rollback, $6648).

10 · Aliens

Each alien's sprite decides how it hunts

Up to seven aliens share sprites 1 to 7. A new one appears at a random place, away from walls and at least 20 pixels from Iball (spawn_alien), and spends 50 passes flickering through colours while it forms (forming_step). A forming alien cannot hurt Iball. Every alien moves with the map as it scrolls (ride_scroll), and on top of that it steers towards Iball, but which way depends on the sprite it got (steer_aliens):

Their speed rises with the level. Across, an alien moves 1 pixel a pass on levels 1 to 4, 2 on levels 5 to 13 and 3 on the last three (alien_speed_x). Up and down, it adds 1, 2 or 3 to the scroll's speed, from levels 1, 9 and 16 (alien_accel_y). Iball's shots are characters on the screen, so an alien dies when a shot character is among the 12 characters around it (alien_shot).

11 · Time

The timer counts the game's logic, not seconds

The work of each frame is split between the interrupts. The third, irq_logic at line $92, runs the aliens, the shots, the scroll and the timer. It can take longer than a frame, and when its interrupt comes round again before it has finished, the new interrupt returns at once. So the more there is on the screen, the more frames the logic misses. The timer, timer_tick, takes a second off every 50 passes of that logic, not every 50 frames.

Counted in the emulator with checkpoints on irq_logic's work and on timer_tick, only while Iball was alive and no sample played. The timer also stands still while Iball materialises or dies, in hyperspace, and while a word is said.