CPU Comparision with Z80

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Comparision with 8080

Basically, the gameboy CPU works more like an older 8080 CPU rather than like a more powerful Z80 CPU. It is, however, supporting CB-prefixed instructions. Also, all known gameboy assemblers using the more obvious Z80-style syntax, rather than the chaotic 8080-style syntax.

Comparision with Z80

Any DD-, ED-, and FD-prefixed instructions are missing, that means no IX-, IY-registers, no block commands, and some other missing commands. All exchange instructions have been removed (including total absence of second register set), 16bit memory accesses are mostly missing, and 16bit arithmetic functions are heavily cut-down. The gameboy has no IN/OUT instructions, instead I/O ports are accessed directly by normal LD instructions, or by special LD (FF00+n) opcodes. The sign and parity/overflow flags have been removed. The gameboy operates approximately as fast as a 4MHz Z80 (8MHz in CGB double speed mode), execution time of all instructions has been rounded up to a multiple of 4 cycles though.

Moved, Removed, and Added Opcodes

 Opcode  Z80             GMB
 08      EX   AF,AF      LD   (nn),SP
 10      DJNZ PC+dd      STOP
 22      LD   (nn),HL    LDI  (HL),A
 2A      LD   HL,(nn)    LDI  A,(HL)
 32      LD   (nn),A     LDD  (HL),A
 3A      LD   A,(nn)     LDD  A,(HL)
 D3      OUT  (n),A      -
 D9      EXX             RETI
 DB      IN   A,(n)      -
 DD      <IX>            -
 E0      RET  PO         LD   (FF00+n),A
 E2      JP   PO,nn      LD   (FF00+C),A
 E3      EX   (SP),HL    -
 E4      CALL P0,nn      -
 E8      RET  PE         ADD  SP,dd
 EA      JP   PE,nn      LD   (nn),A
 EB      EX   DE,HL      -
 EC      CALL PE,nn      -
 ED      <pref>          -
 F0      RET  P          LD   A,(FF00+n)
 F2      JP   P,nn       LD   A,(FF00+C)
 F4      CALL P,nn       -
 F8      RET  M          LD   HL,SP+dd
 FA      JP   M,nn       LD   A,(nn)
 FC      CALL M,nn       -
 FD      <IY>            -
 CB3X    SLL  r/(HL)     SWAP r/(HL)


The unused (-) opcodes will lock-up the gameboy CPU when used.