Publisher Summary This chapter discusses the instruction sets. The operations inside a microprocessor central processing unit (CPU) are governed by instructions read from the program memory. These normally consist of an opcode that defines the type of operation to be carried out and for many instructions there is also an operand that generally indicates the address to be accessed in the main memory. The operand is normally held in one or two memory locations immediately following the opcode. Operations between or on registers inside the CPU are generally defined within the opcode itself so there is no operand and the instruction takes up only one word in memory. During the execution of an instruction the CPU will first read the opcode from the memory and this is then decoded inside the CPU. After decoding the CPU may then make further data transfers from memory to obtain the operand and finally, it will execute the instruction itself that may involve a further data transfer to or from memory. A commonly used instruction in microprocessor systems is the data transfer operation. The reverse operation where data is transferred from a register to memory is generally called a STORE instruction and is often abbreviated to the form STA, STB, and STX.