Z-80 OpCode Formation Rules: Introductory note -- Binary form of opcodes Example: LD r,r' the 8-bit binary opcode is 01dddsss ...where "ddd" is a three-bit field specifying the destination, and "sss" is a three-bit field specifying the source. is shown below as r r' 01 xxx xxx Detailed info LD Instruction Format: OPCODE r r' CYCLES ---------------------------------------------------- 01 XXX XXX LD r,r' dst,src 4 Register r,r' -------------- A = 111 B = 000 C = 001 D = 010 E = 011 H = 100 L = 101 read more about this in the z80syntx.htm