Karateka

01 · The game

The game

The player is a karateka, a karate fighter, sent to rescue Princess Mariko from the warlord Akuma's fortress. The way in runs along a cliff path, through the palace hall, down into the dungeon and into Akuma's chamber. Guards come one at a time, each in new headgear and with more strength than the last, and Akuma sends his hawk at the hero in the palace and the dungeon.

A fight is fought from a fighting stance: three keys punch high, middle and low, three more kick, and two step forward and back. The orange arrows at the bottom left are the hero's strength and the blue ones at the right his opponent's. Each blow that lands takes an arrow, and the last one kills. A hero who walks or runs into a fight dies of the first blow, and so does one who comes to Mariko in the stance.

The title, and the first guard on the cliff. Apple2TS 3.7.0 as an enhanced Apple IIe, from Brøderbund's original disk (reference/).

02 · One frame, rebuilt from memory

The title and the demonstration fight, run from the listing

With no key pressed, the title shows the credits, the logo and the scrolling prologue, plays out a scene in Akuma's chamber and then a fight on the cliff that plays itself. Here the title's own code from the Source tab does all of that in the browser.

Every frame is drawn on the hi-res page the player is not looking at, and then the two are swapped. The page being drawn is kept in $07 ($20 for page 1, $40 for page 2), and flip ($7609) shows it. On a IIe or IIc the flip first waits for the vertical blank, after a look at the machine's ID byte ($779A); on an older Apple II it flips at once. There is no timer. A frame takes as long as its drawing takes, and a tune or a sound holds the frame up while it plays.

A fighter is drawn from three pictures: his legs, in 42 poses, his torso, in 38, and his head or headgear. Tables of columns and rows for each pose place them ($6000-$62FF), and draw_figure ($6811) draws them. A fighter facing left is the same pictures drawn mirrored, each byte turned round through a table of 256 reversed bytes ($0980). The guards are told apart by their headgear: types 0 and 6 are pictures the engine keeps, and types 1 to 5 are each stage's own, at $BE00. Akuma is drawn by a routine of the last stage's own ($BC00).

03 · The guards' brain

How a guard chooses his next move

Once both fighters are in the stance and 11 columns apart or closer, a guard's move in each frame comes from the distance between them and the next number of the game's random sequence. Choose the opponent and the distance: each square is one of the 256 values the random number can hold as the guard thinks, coloured by what he does.

The random numbers come from random ($6C95), which multiplies the last one by 17 and adds $A1, keeping the low byte. That sequence runs through all 256 values before it repeats, so over a fight the squares above are equally likely. The first number is held against three thresholds for the distance, read from three small tables in the stage: at or above the first, the guard throws no blow; between the first and the second, he tries a kick; between the second and the third, a punch; below the third, a step forward.

A move tried is not yet made. It must pass the guard's chance for it, set by his headgear ($D7 a kick, $D8 a punch, $D9 a step), against the next random number, and a blow's height comes from the number after that: high, middle or low a third of the time each. Akuma never aims low ($6C71). A move that fails its chance falls through to the case of no blow, in which the guard steps in or holds his place, by a fourth table.

For distances 7 to 11 the fourth table is read at $BFF9-$BFFD, and four of those five bytes are not chances at all. They are the stage's entry, JMP $B700 (4C 00 B7), and the stage's number, the first of its check bytes. So at distance 9 a guard who holds back never steps in, because the chance is the low byte of $B700, and at 10 he steps in 183 times in 256, because it is the high byte (Discoveries).

The other cases are simpler. A guard in the stance kicks a standing hero within 8 columns, if his kick chance allows, and steps towards one further off, and towards a hero in the stance 12 columns away or more. A guard standing takes the stance when the hero does or comes within 13 columns, and bows back when the hero bows.

04 · Music

Music: two voices beating on one speaker

The Apple II's speaker clicks each time the program touches $C030. Karateka's tune player, play_tune ($0D00), clicks it for two voices at the same pitch, the second voice's count one shorter, so that the two drift against each other and the note shimmers. These are the sixteen tunes the game stores.

A tune is two header bytes and then notes of three. The first header byte is the tempo, which the player writes into its own code ($0D5E); the second is the page the second voice reads, where $C0 clicks the speaker and any other page is silent. Each note is a length and a pitch, given as a coarse and a fine count, and a coarse count with its top bit set is a rest. Three bytes are commands:

Every tune starts with $FE or $FD, so each one sets the two voices afresh.

Each stage keeps its own tunes behind JMPs at $1000, at the same place in every stage, so that the engine and the stage can ask for the tune for a guard beaten ($1006) or the hero's death ($1003) whichever stage is loaded; the engine itself keeps two of the last stage's ($0FB0, $0FE4). The closing tune has a tempo of $40, against 4 to 8 for every other, which stretches 64 short notes over 19 seconds.

Before each note the player reads the keyboard. A special key such as CONTROL-S is obeyed at once, and any other key is thrown away, so a key pressed during a tune never reaches the fight.

05 · Fighting

A blow on a standing hero kills him outright

Arrows count the blows a fighter can take, but only in the stance. A hero caught standing, running or bowing dies of the first blow, whatever his arrows ($712A); a guard caught the same way only reels.

Whether a blow lands is decided by hit_test ($7366) from the distance between the fighters and the class of the target's pose (pose_class, $70C6, 26 classes for the 42 poses of the legs). A kick always lands within the target's reach plus one column, and the reach is 6 or 7 columns ($7300). Punches land by tables, at the reach ($7348, $7352) and at the reach plus two ($7334, $733E, $735C). A quick punch, thrown while stepping, lands only at exactly its own reach ($745B). Closer than the reach the two collide, and a hero who is moving is pushed back two columns.

Each blow that lands takes an arrow ($0BC1, $0BD2), and arrows come back one at a time while a fighter is not hit ($0BE3). The stages share out the strength. The hero's most arrows and each opponent's always add up to 28, and each guard beaten moves one more from the hero's side to the next guard's, until the hero is down to a floor that each stage sets.

The dungeon's hawk is its last opponent, with 5 arrows that never come back, and the hero has 23 for that fight: 28 again.

06 · Controls

Controls: the low kick is on X

The manual gives Z for the low kick. The game reads X, and Z is the low punch.

A blow pressed while another is under way waits, and is sent again each frame until the fighter can take it ($46, $47). With the joystick, chosen with J (K goes back to the keys), holding the stick up keeps the hero standing, and letting it go puts him in the stance, where the buttons kick and punch at the height the stick points to (player_input, $6E00). Up stands him up again only when no guard or Akuma is within 15 columns and the hawk is not close.

The special keys work at any time, even in the middle of a tune (special_keys, $7697). ESC freezes the game until a key is pressed. CONTROL-S turns the music off, then the sound, then both back on. CONTROL-B swaps the joystick's buttons, CONTROL-E its axes, CONTROL-X reverses left and right and CONTROL-Y up and down, and CONTROL-N puts all four back. CONTROL-R goes back to the title. Each answers with a beep.

07 · Sound

Sound: one recording, played at seven speeds

Seven short effects are clicks spaced by the Monitor ROM's delay routine. The eighth sound is a recording of 2,048 one-bit samples ($0E00), played back slower or faster for a guard falling dead, a doorway kicked open, the hero falling to Akuma, Mariko's kick, Akuma's cry and the hawk.

play_sample ($0DC0) waits $BA turns of a loop between samples and clicks the speaker only where the sample changes, so the speed sets the pitch and the length together: the recording lasts 0.35 seconds at speed 30 and 0.45 at 40.

All the sounds are made at the end of the frame that asked for them (end_frame, $7628), and CONTROL-S silences them. The last stage turns the music and the sound back on whatever the player chose, so the ending is always heard ($B700).

08 · The stages and the disk

Five stages take turns in the same 12 KB

The engine stays in memory from the boot to the end. The title and the four stages each load over the one before, into $1000-$1FFF and $A000-$BFFF.

The disk has no DOS and no files. After the boot sector, everything is read by the game's own loader (load, $0338), twelve 256-byte sectors to a track, with four marks of its own on each track (track_marks, $5610). Stage n starts on track 4n + 7, the castle picture shown before the cliff on track 4. The loader's later parts live in the strips of hi-res page 2 that the screen never shows, and are read from the disk again before every load.

A stage ends with three check bytes: its number, the number EOR $3B and the number EOR $49. The engine reads the stage again until they check, then enters it by the JMP just before them, at $BFFA (load_stage, $020C).

What passes from one stage to the next is only what is in the engine's memory: zero page, with the control chosen, the sound switches and the hero's record.

Pressing RESET does not get out of the game. The game keeps a copy of the ROM in the language card with its reset vector pointed at the game's own handler, which clears memory and boots the disk again (reset_handler, $0309). Every stage, with its rooms, is drawn by its own code on the Maps / levels tab.