CPU Comparison with Z80

Comparison with 8080

The Game Boy CPU has a bit more in common with an older Intel 8080 CPUthan the more powerful Zilog Z80 CPU. It is missing a handful of 8080instructions but does support JR and almost all CB-prefixedinstructions. Also, all known Game Boy assemblers use the more obviousZ80-style syntax, rather than the chaotic 8080-style syntax.

Unlike the 8080 and Z80, the Game Boy has no dedicated I/O bus and noIN/OUT opcodes. Instead, I/O ports are accessed directly by normal LDinstructions, or by new LD (FF00+n) opcodes.

The sign and parity/overflow flags have been removed, as have the 12RET, CALL, and JP instructions conditioned on them. So have EX (SP),HL(XTHL) and EX DE,HL (XCHG).

Comparison with Z80

In addition to the removed 8080 instructions, the other exchangeinstructions have been removed (including total absence of secondregister set).

All DD- and FD-prefixed instructions are missing. That means no IX- orIY-registers.

All ED-prefixed instructions are missing. That means 16-bit memoryaccesses are mostly missing, 16-bit arithmetic functions are heavilycut-down, and some other missing instructions. IN/OUT (C) are replaced withnew LD ($FF00+C) opcodes. Block instructions are gone, but autoincrementingHL accesses are added.

The Game Boy operates approximately as fast as a 4 MHz Z80 (8 MHz in CGBdouble speed mode), with execution time of all instructions having beenrounded up to a multiple of 4 cycles.

Moved, Removed, and Added Opcodes

Note: The unused (-) opcodes will lock up the Game Boy CPU when used.