The code

629 named routines, 5,301 instructions, 2,357 line comments: 44.5 % of the listing commented, and no routine below 10 %.

Where everything starts

0x4061  INIT             hooks the interrupt and clears the RAM
0x4010  interrupt        ALL the game runs in here
0x411e  bucle_principal  the Konami logo rising up the screen

The interrupt guards itself with two locks, keeps the frame counter at 0xE000, and at the end reads the VDP status again: if another interrupt is pending it re-executes itself instead of returning.

How a frame is split

0x7bb0 splits the work into two halves that alternate frame by frame, on bit 0 of the frame counter:

Even frames	Odd frames
the batted ball (0x620a)	the fielding (0x7373)
the pitch (0x4ccc)	the batter (0x7080)
the runners (0x5e80)	the pick-up (0x7692)
	the machine (0x7d04)

The records

Almost everything that moves is described by a record pointed at with IX. The slots the movement engine at 0x4b53 uses:

+0x00  flags: bit 0 running, bit 6 "moved this frame"
+0x01  sub-pixel accumulator, 16 bits
+0x03  the step added every frame, 16 bits
+0x07  X in pixels           +0x08  Y in pixels
+0x0c  min X  +0x0d  max X  +0x0e  min Y  +0x0f  max Y
+0x10  direction flags: bit 7 the dominant axis, bits 6 and 5 the signs
+0x11  how much is left to travel in X
+0x12  how much is left to travel in Y

The sub-pixel trick is King's Valley's: the fine position lives in 16 bits, the step is added every frame, and the accumulator's high byte is how many whole pixels to move; it is spent and zeroed, and the remainder is kept for the next frame.

Where the state lives

Address	What
0xE000	the frame counter, raised by the interrupt
0xE001	the slow counter, lowered every 32 frames
0xE002	the demo autopilot is in charge
0xE00C..0xE011	the two pads: read, previous read and edges
0xE100	the pitcher's state
0xE140	the batter's
0xE180..0xE1FF	the four bases, 32 bytes each
0xE220..0xE33F	the nine fielders
0xE340	the ball thrown between bases
0xE360..0xE36E	the pitch meter and its bar
0xE380	the running word
0xE388	the fielding word
0xE400	the state of the play
0xE408..0xE40E	strikes, balls, outs, inning, runs
0xE440..0xE446	league, teams and the two random seeds
0xE660..0xE686	the sound engine, three eleven-byte channels

The calculator

The whole flight of a batted ball is worked out in four bytes of RAM, 0xE060 to 0xE063, with two routines that are a multiply and a divide done by hand:

• 0x6539, multiply: sixteen rounds of shifting 24 bits and adding.
• 0x6519, divide: eight rounds of shifting and subtracting.

And there is a smaller divide besides, the one at 0x4cb0, which works on 0xE700 and computes the slope of a trajectory: it is called twice in a row (0x4ca1) to get two decimals.

How a player is drawn

A player is three stacked 16x16 sprites, one per colour, and the cartridge does not keep all the poses in VRAM: every time the pose changes it uploads all three patterns - 32 bytes each - from the ROM, decompressing them with the same interpreter that draws the screens.

The table at 0x5048 holds one pointer per pose, and each points at a four-word record: the three scripts - one layer each - and a pointer to six offset bytes, one signed (row, column) pair per layer.

Fielders are cheaper: a single layer and one sprite, and there the table at 0x5048 is used differently - what it returns is the script directly, not a four-word record.

The sound engine

Three channels, eleven bytes of state each, starting at 0xE661. The way in is 0x6c0e, which takes the sound number in A and refuses to play anything if 0xE002 says the demo is in charge.

The dispatcher at 0x6c20 decides how many channels a sound fits in and only takes it if its number is higher than the one already playing: the low six bits of the number act as a priority.

A sound's script, as 0x6ced reads it:

0xFF  end: the channel goes quiet
0xFE  back to the start, counting the laps
0x2n  change the note duration to n
0x1n  noise: n goes to PSG register 6
rest  a note, with its duration in the high nibble

And there is a second path, behind bit 7 of the mode, with up to three prefixes in front of the note: 0xDn the duration unit, 0xFn the volume decay and 0xEn the octave, which is done by doubling the frequency that many times.

The tools

tools/z80trace.py    the flow trace, with the !skip directive
tools/mkasm.py       the listing, from the trace and the notes
tools/guiones.py     the script interpreter, redone in Python
tools/pantallas.py   the screens and the poses, drawn from the ROM
tools/densidad.py    how much of the listing is commented, routine by routine
tools/presupuesto.py that not one byte is left unassigned