Gameboy Development Forum

Discussion about software development for the old-school Gameboys, ranging from the "Gray brick" to Gameboy Color
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#1 2019-01-22 08:26:17

PinoBatch
Member
Registered: 2018-03-22
Posts: 41

Game Boy CPU isn't a Z80. What is it?

The IC containing the NES CPU is a Ricoh 2A03. It consists of a DMA controller and programmable sound generator wrapped around a barely modified MOS 6502 core. (Before the introduction of NOAC famiclones, it wasn't quite a system on chip because the PPU lay elsewhere.) Programmers are comfortable saying the CPU is a 6502. The Game Boy CPU, on the other hand, uses a special snowflake architecture that's clearly descended from the Intel 8080 but not exactly the same as any other well-known part.

"GBZ80": Fan name for the Game Boy instruction set based on the overall similarity of its unprefixed and $CB-prefixed instructions to those of Zilog's Z80. Invites confusion because the lack of IX/IY register and exchange instructions makes it handle quite unlike a Game Gear's authentic Z80 in practice, particularly when accessing fields of structs.

"LR35902": This originated from a package marking on Game Boy systems using the earliest revision of the processor. Because the die marking was the only part number that looked anything like Sharp's other part numbers, this was popularized by the schematic by Jeff Frohwein. It is believed to refer to the entire system on chip (CPU + memory controller + APU + PPU), just as "2A03" refers to the NES CPU + DMA controller + APU.

"SM83": Someone on the GBDev Discord server claims to have seen datasheets for other Sharp microcontrollers or systems on chip that have the same instruction set as the Game Boy CPU, and they call the core "SM83". I'd like to get the identity of this datasheet into a form that I can cite, as things said only in chat tend to disappear down a memory hole more quickly than things said in a newsgroup post, mailing list post, or other forum post with a URL.

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