Most 8-bit operations can only be performed on the 8-bit accumulator (the A register). For 8-bit operations with two operands, the other operand can be either an immediate value, another 8-bit register, or a memory byte addressed by the 16-bit register pair HL. Direct copying is supported between any two 8-bit registers and between any 8-bit register and an HL-addressed memory byte. Due to the regular encoding of the instruction (using a quarter of available opcode space), there are redundant codes to copy a register into itself (, for instance), which are of little use, except for delays. However, what would have been a copy from the HL-addressed cell into itself (i.e., ) is instead used to encode the halt (HLT (x86 instruction)) instruction, halting execution until an external reset or interrupt occurs.