Open questions

What is not known. None of this is dressed up as a finding anywhere else on this site.

Twelve bytes nobody reads

0x49AB–0x49B6, right behind the eight VDP registers:

49AB  00 70 38 70  00 A0 38 70  00 F0 38 F0

The loop at 0x421E reads eight registers and stops, and no other instruction points here. They have the shape of three sprite records — Y, X, drawing, colour — all three with Y = 0 and drawing 0x38, which is the crocodile's head. What they were for is not known.

Five loose bytes

0x49FF–0x4A03: 01 06 11 16 21. They sit between the collision margins and the first label list, and nothing reaches them. The gaps go 5, 11, 5, 11.

Bit 6 of the type

Layouts 4 and 5 carry 0xCD where 0x8D would do: bit 7 marks the last object of the row and the low nibble is the type, but no instruction looks at bit 6. It may be left over from something.

Why the same character is uploaded B times

0x47DA reads a count and eight bytes of drawing, and pushes that same drawing to video memory as many times as the count says, then again reversed (push hl / pop hl restore the pointer each turn). It has been read, but why it is worth repeating the same character has not been checked in the emulator.

Nothing measured in the emulator

There is not one openMSX figure here: not the cost of the interrupt, not the VDP writes per frame, not how long a lane takes to cross. Everything on this site comes from reading the binary.