The cartridge shares its frame with the other Konami titles of the period: 561 bytes in common with Knightmare (RC-739) and 562 with The Goonies (RC-734), measured with tools/comun_normalizado.py. The dispatcher even falls at the same address as in Knightmare, 0x406C.
0x403D is the whole program. Every frame:
avanza_el_reloj (0x40DB), which counts down the three timers and dispatches the scene;(0xE000) says which one we are in and (0xE001) the step within it. The table at 0x411C hands out, and nearly all of them start with chained djnz: the step arrives in B and each djnz eats one.
Scene 2 is the match. Its per-frame loop is 0x4338, and in thirteen calls it does the whole game: the bell, the sprites, the two boxers, the referee, the punches that land, the damage, the bars and the round.
Each one has an eleven-byte block, and the trick is that there is only one work area: 0x46F9 copies whichever one is due to 0xE221, does everything there and puts it back. That way the code carries no player index.
| block | who | starting column |
|---|---|---|
| 0xE232 | the right-hand one | 22 |
| 0xE255 | the left-hand one | 2 |
(0xE22F) alternates between 0 and 1 on every call and is what says which is which. And each one comes in knowing the other's action, which is the only thing it knows about its opponent.
(0xE228) says what he is doing. The table at 0x4C72 turns the five control bits into one of them:
| pressed | action | |
|---|---|---|
| up | 3 | stand up |
| down | 1 | duck |
| left / right | 9 / 0x0A | walk |
| punch | 8 | |
| punch + up | 5 | |
| punch + down | 6 | |
| punch + left / right | 4 / 7 |
The other 23 combinations give zero. When a punch starts, 0x48D6 adds four to it: actions 9 to 0x0C mean "throwing punch 5 to 8". And 0x0D to 0x10 are wait, take it, back off and go down.
mira_si_entra_el_golpe (0x4C92) settles it once per boxer, and the order is decided by the parity of (0xE23E)+(0xE261). The two index registers are the split: IY to the block of the one throwing and IX to the action of the one receiving. Hence the punch lands with a ld (ix+000h),00eh, which is the action of taking it, and when the threshold is passed (ix+0) becomes 0x10, which is going down.
Damage is two bytes: the level, 0 to 8, and what has piled up inside the level. Each level's threshold comes from 0x5445, and it falls: the more battered you are, the sooner you go down.
pinta_una_figura (0x4F66) redraws a boxer's body by redefining tiles the screen already has in place. Four pointers per figure: two to the patterns and two to the colour, which sits at the same offset with bit 13 cleared (the res 5,a at 0x4F81).
Each boxer has two sets of tiles and they take turns frame by frame: 0x30 and 0x80 for the player, 0x58 and 0xA8 for the opponent. While one is on screen, the other is being built.
And the head and the gloves are not tiles: they are 16x16 sprites that monta_los_sprites_de_una_figura (0x5178) writes in one go, four bytes each, with the spare ones parked on row 0xCF.
Three voices with a fourteen-byte block each from 0xE311. The sound number acts as state and priority at the same time: a new one only gets in if its number is not lower than the one already playing. And how many voices it takes depends on the number: below 7 only the third, from 7 to 0x12 two, and from 0x13 up all three.