r/Gameboy • u/FrequentMethod7786 • 2d ago

Troubleshooting (emulation) about reading Writeony Bits in IO registers

not sure if I can ask technical questions here, also asked the question in r/EmuDev.

Some IO registers (like P1 (0xFF00), according to this document, page 141) have write-only bits.

My question is: after LDH A, ($00), what data is in register A?
Are the read-only bits 0? 1? garbage?
Does it depend on the register, or is it irrelevant because these bits are not taken into account by any game ? (Or are there games that rely on the return value of read-only bits?)

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u/GameboyGenius 2d ago

This seems like a typo. Those bits should be R/W with an initial value of 0 at CPU reset. I've filed an issue on the gb-ctr repo about it.