Boot ROM ends with different registers than the official ROM

[max-m]

Hi, in #gbdev at EFnet we discovered that SameBoy’s boot ROM ends with different register values than the official one does. It checks out for the DMG boot ROM, but the CGB ROM differs in both modes of operation. This has thrown bgb’s CGB mode detection off but is fixed now :sweat_smile:

The following values have been optained in bgb using the official Nintendo boot ROMs and freshly built ROMs from the current master.

Nintendo dmg_boot.bin	SameBoy dmg_boot.bin
DMG game	DMG game
AF: $01B0	AF: $01B0
BC: $0013	BC: $0013
DE: $00D8	DE: $00D8
HL: $014D	HL: $014D
SP: $FFFE	SP: $FFFE
PC: $0100	PC: $0100

Nintendo cgb_boot.bin	SameBoy cgb_boot.bin
DMG game	DMG game
AF: $1180	AF: $1180
BC: $3C00	BC: $0000
DE: $0008	DE: $0008
HL: $007C	HL: $007C
SP: $FFFE	SP: $FFFE
PC: $0100	PC: $0100

Nintendo cgb_boot.bin	SameBoy cgb_boot.bin
CGB game	CGB game
AF: $1180	AF: $1180
BC: $0000	BC: $0000
DE: $FF56	DE: $0008
HL: $000D	HL: $007C
SP: $FFFE	SP: $FFFE
PC: $0100	PC: $0100

[LIJI32]

The DMG mode register initial values appear to be dependent on the ROM's header, so I'll need to research a bit more into it. What ROMs were used to test this?

[LIJI32]

Checked my disassembly of the CGB boot ROM. Before booting a DMG game, B is set to the checksum of the title, but ONLY IF it's a Nintendo title with a built in palette. This means the game you were testing is Dr. Mario. :)

[max-m]

f1006d6cf77469092f3b9f266d0a65da9c91ac42 Dr. Mario (World).gb

// Edit: Sniped :)

[LIJI32]

Also, I'm unsure what happens in the AGB-CGB boot ROM, as it hasn't been dumped and it has B set to a different value after booting.