The BIOS enters at 0x4012 on every retrace. The first thing it decides is what this frame is for, by reading 0xE043: bit 0 means only the demo and the sound, bit 6 only the sound. With no game running, all it does is bump the frame counter.
Then it decides which scene, by reading 0xE040 one bit at a time with rrca:
| bit | |
|---|---|
| 0 | the death |
| 1 | the time bonus (inside the game) |
| 2 | stage cleared |
| 3 | the game is over |
| 4 | build the stage |
With none of them set, the normal game runs: the lanes, the frog, the river creature, the frog to be rescued and the collisions, in that order.
0x57FF is not a dispatcher that jumps. It takes the return address as the base of a table embedded right behind the call, returns the Ath word in BC, walks forward to the 0xFF that closes the table and rets to the byte after it.
Four places use it, and it is the one thing a tracer cannot follow on its own: those four tables have to be declared by hand, or 700 bytes of label lists get read as instructions.
Eight rows move — four of river and four of road — and they do not fit in one frame, so they are spread over three (0x55E5). Each row also has its own clock in a single byte: the low nibble counts down and the high one says how often (0x5682).
Every object is a four-byte record: type and flags, position in quarters of a character, signed speed, and the diving counter.
Types 7 to 12 animate, but only if bit 7 of the fourth byte is set — and that bit is written by hand, at two addresses when the stage is built. The counter runs from 0x80 to 0xC0 and on the way the type goes up one at 0x90 and another at 0xA0, then back down at 0xB0 and 0xC0. Since the type is what picks the drawing, the turtle sinks and comes back up on its own.
Types 9 and 12 have all four collision margins at zero (0x49C7): those are the fully submerged ones, which is why they do not hold the frog up.
The only thing in the river drawn with sprites — four of them. It opens and closes its mouth on a 32-step cycle: 22 steps with drawing 0x40 and 10 with 0x3C. With the mouth open, 0x524F checks the head separately and it kills instead of carrying.
Everything is checked against the frog in one place (0x5185): the river creature, the frog to be rescued, the screen edges and, if the frog is in the water, whichever row of logs it is standing on. Which row that is comes from how far up it has got (0xE28A).
In the water, touching something is what saves you. On the road it is what kills you.
No random generator. The frog is driven by a list of fifteen bytes at 0x584F, one per move, in the same format as the controls. The pointer at 0xE049 is reset to the start whenever a stage is built, and only its low byte is incremented — so the list cannot cross a page boundary.