The binary is 100 % explained: there is not a byte left unassigned nor a routine left unnamed. What is left are why questions, and those the binary does not answer on its own.
The discard on which the machine declares riichi comes from a random draw plus 3, 5 or 7 depending on the difficulty, and that same number is what opens the door for it to call at all. What it does is read, and that the three keys give three different amounts is measured. Which way it pulls is not clear: declaring riichi later means playing more turns without showing your hand, but it also means letting go of hands it could have closed earlier. Settling that means playing many games and counting them, not reading more code.
Before every hand the machine picks a plan and keeps it in one byte. Which gates each bit opens is read —go for a suit, allow the pon, allow the chi, do not defend— and who decides it is read too, with its score and its honba. What is not there is what each plan means in terms of difficulty: whether the one that comes up when the machine is losing is more aggressive or just different.
The first row of both payment tables is 0000, and the 30 fu minimum that in four-player riichi keeps you off it does not exist here. On paper, an open hand made of runs, with a ryanmen wait, won by ron and with a single yaku, collects zero plus the honba. It is read and checked by hand, but not measured: it has not been seen happening in the emulator.
The cartridge ends at 0x9FE4 and behind it there are 8,219 bytes of 0xFF. That is compatible with a 32 KB memory whose last bank was never programmed and with a 24 KB one the dumper read eight too many from, and from the dump the two are identical. Only looking at a real cartridge settles it.
In the south round, a hand with no winner does not move the deal even when the dealer is not tenpai, which is the opposite of what it does in the east round. And if the player wins with fewer than ten discards, a randomly drawn tile overwrites the second-to-last slot of the machine's hand before it is turned over. Both are read from the code; whether they are intent or oversight, it does not say.
There is a third case like it: with four identical tiles in the sorted hand the ron-tile flag is cleared, which changes the fu of that set. The what is clear, the why is not.