Speedball

01 · The game

Five a side in steel armour

Two teams of five, a keeper and four outfield players each, fight over a steel ball in a walled arena. There are no fouls.

You steer whichever of your outfield players is nearest the ball, and the rest of the team runs itself. Fire throws the ball, or, without it, jumps or slides into a tackle. The pitch scrolls up and down; the ball goes into a goal only along the ground, and a warp tunnel in each side wall sends it out of the other wall. Tokens and power-up tiles appear on the floor, and after each match the tokens buy bribes and better figures in a shop.

The menu offers a knockout cup of ten rounds, a league of eleven teams, and a league for two players. You choose one of three teams, Verna, Lacata or Draco, each with its own stamina, power and skill.

02 · The music

Music: a goal plays the scoring side's own tune

The menu's credits name Dave Whittaker for the music and the sound effects. Of his seven tunes, two are short jingles for a goal, one for each side, and green's plays again at full time.

The driver is the disk's file FILE0, which loads into memory at $0810. The SID has three voices, each playing one note at a time. Each tune has seven bytes in tune_table ($131E): its tempo, then where each voice finds its list of patterns, the short runs of notes the tune is built from. Once a frame the driver adds the tempo to a running total (music_frame_voices, $094E). Each time the total goes past 255 and starts again, every voice moves on a step in its pattern, so a larger tempo plays faster. Tunes 5 and 6, the goal jingles, end on their own; the others loop.

03 · Sound effects

Sound: over half the driver's effects are never played

The driver holds 26 sound effects and the launcher's hum. The game asks for twelve of the effects and never for the other fourteen.

An effect is a record of 20 bytes: its length in frames, and for voices 1 and 3 a starting pitch, an envelope (how the volume rises and falls), a waveform (the shape of the tone) and a slide (how the pitch moves). Starting one takes those voices from the music (effect_start, $0E7A). The launcher's hum is voice 2 alone (hum_start, $1155), held while the launcher spins and stopped when it shoots the ball out (launcher_sequence, $3739).

04 · One frame

One frame, rebuilt from memory

The match screen is a scrolling pitch built of characters, with the panel split off below it. The players are sprites, small pictures the video chip draws over the screen, and each sprite is used again lower down.

The video chip can interrupt the processor when it reaches a chosen line of the screen as it draws it (a raster line). The processor then stops what it was doing and runs a routine. The game has one such routine (irq_handler, $86D3), and it works through a chain of slots, one each time it runs. A slot holds the line for the next interrupt, the video chip's settings (its registers) for that line, and a routine to run (irq_slot_run, $8704).

In a match the slots place the sprites. The game moves everything once a pass, every second frame. Each pass the ten players, the ball and the four parts of the ball launcher are sorted by height (sprites_sort, $8ADD) and handed out to the chip's eight sprites from the top of the screen down, so one sprite draws several of them. The pitch is a map of 40 columns by 74 rows at $F300, built before each match (pitch_map_build, $917D). The game keeps two screens, shows one and copies the next view into the other when the camera moves a row (scroll_update, $9292).

05 · The pitch

The bounce domes move after every match

Four domes stand on the pitch, and their places change after every match, through ten layouts in turn.

The layout number $1ED0 moves on after every match, from 0 to 9 and round again (match_over, $3320), and domes_place ($9241) turns its four entries into places in the map. A dome turns the ball only while it rolls along the ground (dome_bounce), and the players run straight through it. The ball launcher stands in the middle, where no tile is ever placed (tile_place, $A94C).

06 · Tiles

Tokens come up more often than all the power-ups together

A new tile appears on the floor 50 passes, two seconds, after the last one was taken, and the player you control takes it by standing on it. Eleven times in twenty it is a token, which the shop takes as money; the rest are power-ups.

tile_choose draws a random number from 0 to 19, or to 20 in a two player game, and takes ten from it, counting anything below 0 as 0. The result picks an entry in the table at $BB21, 00 01 02 03 04 05 06 08 08 08 0A. Entry 0 is a token, so eleven numbers in twenty give a token, and W, the ball that tackles whoever it hits, three times as often as any other power-up. J, which reverses the other player's stick, is the twenty-first number and so comes only in a two player game. The manual's electron bolt E (type 7) and eight-way burst M (type 9) are not in the table; their entries in the effect table at $BBBF point at W's routine (tile_ball_tackles, $9A9F).

A power-up lasts 120 passes, ten more for each bribe of the timer, with a countdown of ten stages in the panel (tile_countdown, $9AB3). The computer's team picks up tiles too, but only a token does anything for it (tile_check, $99FA).

07 · Teams

The computer's ten teams grow stronger in order

Your three teams have figures of their own; the ten others are built from templates that grow stronger in order.

A new league or knockout copies the templates into eleven team records (team_record_init, $3F32), and every match loads the computer's side from one of them (opponent_load, $3F81). Lower delays make the computer aim, throw and save sooner, so template 10, Persus, is the strongest. The league keeps the records in template order. The knockout shuffles them, and then loads the wrong one: see Discoveries.

A league awards 100 points for a win, 20 for a draw and 5 for every goal (league_record, $994A). The other four matches of each week are settled with random numbers, weighted by the two teams' tackle figures (league_week_end, $8519). The knockout gives 2 points for a won leg and 1 for a draw, and 3 points take you through to the next round (knockout_round_end, $8195).

08 · Tackles

How a tackle is decided

Two players who touch are measured against each other by the way they face, unless one of them is sliding.

Every pass the game measures the distance between each of the 45 pairs of players (pair_distances, $B038) and then looks for pairs that touch (tackles_check, $8DD6). For two opponents that touch, tackle_resolve ($8EF5) decides. A sliding player brings the other down when a random number from 0 to 255 is at least the victim's tackle figure, unless the victim's side has power-up P. Otherwise a table at $B996 says who wins from the directions the two players face, and a player you control wins only while you hold fire. The loser falls and loses stamina by the winner's power (stamina_drain, $908A); the winner takes the ball if the loser had it.

Below 12 stamina a player runs a step slower (player_dir_step, $9F98), and below 6 a player's throws are a step weaker (player_move, $9E45). Both bugs in this part of the game, a table looked up thirteen bytes early and a skill figure that works backwards, are on the Discoveries tab.

09 · The shop

What the tokens buy

After every match the shop sells eleven items for the tokens you collected.

In a one player game the four items that weaken the computer are kept until the next match loads its team, and then cleared (attributes_handicap, $AEF5): each match, every leg of a knockout round and extra time included, loads the computer's figures afresh, so they last one match. The computer collects tokens as you do and never spends them.

10 · Copy protection

Two timers that must keep in step

The game checks that two timers in the computer's second interface chip are running, and running together, in every frame of a match.

At start-up the game sets timers A and B of CIA 2 (the C64's second interface chip) to the same period, $231F (8,991) cycles of the processor's clock, and starts them four cycles apart (game_start, $201A). It never uses them for timing. Three checks read them instead:

The interrupt handler reads timer B's control register on every interrupt, with an instruction the processor's makers never documented, and when the timer has stopped it sets the match clock's target to 255, so the match never ends (irq_handler, $86D3). The keyboard reader makes the same test and sends the interrupt chain to a slot that does not exist (keys_read, $8891). At the top of the match screen, irq_slot_play_top ($8745) reads timer A and then timer B four cycles later. Started four cycles apart, the two read the same. The game combines them with EOR, which gives 0 when two numbers are equal, and adds the result to $2DD0, which so stays 0. The game merges that byte into what it reads from the joystick, uses it as the flag that waits for the next frame, and uses it to pick the league's save file. On a copy whose timers drift apart, the stick jams and the game hangs.

The timers' registers answer at several addresses, called mirrors, and these reads use a different one each frame (irq_slot_shop_row, $881D). That hides them from a search for the registers' usual addresses.

Nothing in the game says what the check is for, so what follows is our reading of it. A copy of the game's files that starts it the usual way runs game_start, sets the timers going and passes. What fails is a copy made with a freezer cartridge, such as the Action Replay, which stops a running game, saves the whole of memory to an ordinary disk and later starts it again from where it stopped. Copies made that way were a common answer to disks that could not be copied directly. When the game is restarted from such a copy, game_start does not run again, so the cartridge has to restart the timers itself. The chip lets a program read where a timer's count has reached, but not the period it was set to, so the cartridge cannot know to set both to $231F, four cycles apart. The timers then stay stopped, or run out of step, and the game breaks partway through a match in a way that looks like a bug rather than a copy check.