Instructions and Addresses

An important characteristic of the architecture of a computer is the number of addresses contained in its instruction format. Arithmetic operations generally require two input operands and produce one result, so that a three-address instruction format would seem natural. However, there are arguments against this arrangement, and decisions about the actual number of addresses to be contained within one instruction are generally based on the intuitive feelings of the designer(s) in relation to economic considerations, the expected nature of implementation, and the type of operand address and its size. An important distinction exists between register addresses and store addresses, for example; if the instruction for a particular computer contains only register addresses, so that its main store is addressed indirectly through some of these registers, then up to three addresses can be accommodated in one instruction. On the other hand, where full store addresses are used, multiple-address instructions are generally regarded as prohibitively expensive both in terms of machine complexity and in terms of the static and dynamic code requirements. Thus one store address per instruction is usually the limit (in which case arithmetic operations are performed between the content of the store location and the content of an implicit accumulator), although some computers have variable-sized instructions and allow up to two full store addresses in a long instruction format. Included here are examples of computer systems which have three, two, one and zero-address instruction formats, together with a discussion in each case of the relationships which exist between each of these formats and the corresponding hardware organisation.