The cartridge

32,768 bytes, mapped into pages 1 and 2 of the MSX, that is 0x4000 to 0xBFFF. Not a MegaROM: no bank switching anywhere.

sha256  02bdce8ceafe6af05a45b61a689e78807551df663afd152090f7cd9cac827dd4

Two headers, not one

0x4000 carries the standard "AB" header, with INIT at 0x40B1 and STATEMENT, DEVICE and TEXT all zero. That is the one the BIOS reads.

0x4010 carries a second one, "CD" 07 40, and this cartridge never reads it: it is there for the one plugged in next door. It is the header of the Konami Game Master, the house cheat cartridge, and the layout of the bytes that follow is not guesswork - it comes from hand-running the Game Master's own reader. That reader copies 21 bytes from 0x4012, takes the catalogue number and then walks a flags byte at 0x4014: one rra per field, and a bit at zero means the field is present.

With flags 0x08 -only bit 3 set- the fields come out like this, and the last one ends exactly at 0x4025, one byte before code resumes:

offset	field
`0x4015`	`0xE000` and `0x04`: the scene variable, and the scene where cheats apply
`0x4018`	`0xE076` and `0x05`: the stage variable, and how many stages: five
`0x401B`	`0xE070`: the lives
`0x401D`	`0xE06C`: the high score
`0x401F`	`0xE068`: the score
`0x4021`	`0xE002`: the mode bits
`0x4023`	`0x4103`: a routine of this cartridge, which the Game Master calls across slots to set a stage up

The field that is missing -bit 3- is the pointer to the block of game data the Game Master knows how to save to and load from disk. Twin Bee declares none.

The hidden mark

The last nine useful bytes of the cartridge, at 0xBFF7:

ba b7 9a ac 81 91 06 40 aa

Nobody in this cartridge reads them. It is the signature Konami left at the end of its ROMs: the title in katakana, the 0x06 length, the 07 40 of RC-740 and the 0xAA that closes it. This was discovered by Manuel Pazos, and without his work these bytes would be padding.

And its last six bytes are exactly the ones Nemesis (RC-742) goes hunting for through slots 0, 0x80, 0x84, 0x88 and 0x8C: that is how Nemesis knows whether Twin Bee is plugged in next door.

Video

The register table lives at 0x47F1 and is written from register 7 down to 0 -ld c,8, then dec c- so it reads backwards from how it is stored:

register	value	what it means
R0	`0x02`	SCREEN 2
R1	`0xE2`	16 KB, screen and interrupt on, 16x16 sprites
R2	`0x0E`	name table at `0x3800`
R3	`0x7F`	colour at `0x0000`, and the rest is a mask
R4	`0x07`	patterns at `0x2000`
R5	`0x76`	sprite attributes at `0x3B00`
R6	`0x03`	sprite patterns at `0x1800`
R7	`0xE4`	ink 14 on background 4, the blue of the sea

R3 and R4 are the trap of SCREEN 2: they are not addresses, they are a base and a mask. Read as addresses they put the tables in the wrong place, the shapes come out right and the colours come out in stripes. Here they land the other way round from the usual: patterns above, colour below.

Memory map

0x4000 - 0x4025   the two headers
0x4025 - 0x4103   the framework: interrupt hook, dispatchers, INIT
0x4103 - 0x4172   the routine the Game Master calls, and the scene dispatch
0x4182 - 0x4535   the eight scenes
0x4535 - 0x4652   the score, in BCD
0x4652 - 0x47F9   the decompressor and the VRAM routines
0x47F9 - 0x48B4   the joysticks
0x48B4 - 0x4D1E   the intro and the title screen, with their scripts
0x4D1E - 0x518C   the sound player
0x518C - 0x5D20   the tunes
0x5D20 - 0x6CD1   the game: ships, shots, bombs, bells and clouds
0x6CD1 - 0x6F8F   the scenery: scroll, map script and stretches
0x6F8F - 0x8E8F   the 248 row blocks
0x8E8F - 0x994A   sprite patterns
0x994A - 0xA14B   scenery patterns and colour
0xA14B - 0xAD25   collisions and waves
0xAD25 - 0xBFF7   the enemy engine, the trigonometry and the boss
0xBFF7 - 0xC000   the hidden mark

The RAM

The game clears 0xE000 to 0xF0FF on boot and puts the stack at the top of it. Some landmarks:

0xE000  the scene, and 0xE001 the step inside it
0xE003  the frame counter, which half the cartridge looks at
0xE007  what the joystick reads, and 0xE009 what has just been pressed
0xE010  the three sound channels, 0x13 bytes each
0xE076  the stage, and 0xE07D the number you see, in BCD
0xE0A0  the two ships
0xE100  the shots
0xE160  the fourteen enemies, 0x20 bytes each
0xE320  the boss
0xE400  the map, decompressed
0xEB00  the join table, brought back from VRAM
0xEC40  the screen buffer, 768 bytes
0xEF80  the sprite buffer, 32 sprites