A simple program instruction consists of two parts [what-to-do] and [what-to-do-it-on]. The first part is called the Operation and the second the Operand. The program is stored in the Read Only Memory and carried out in the order specified by the instructions. Once the microprocessor is given the address of the first instruction (on start-up) it progresses sequentially from that location (incrementing the program counter value which contains the next instruction address). The operation part of the register issues instructions to the instruction decoder. This sets up the control paths and registers within the microprocessor. It also puts the R/W line in the required state for the chosen operation. The operand will hold the data which is copied into the Arithmetic and Logic Unit where it is added, compared, ANDed etc., with the data in the A Register as required by the operation part of the instruction. The Accumulator always stores the result of the last arithmetic or logic instruction. TEST YOUR UNDERSTANDING: Explain the difference between an Instruction Register and an Instruction Decoder.
