Word (computer Architecture) - Uses of Words

Uses of Words

Depending on how a computer is organized, units of the word size may be used for:

• Integer numbers: Holders for integer numerical values may be available in one or in several different sizes, but one of the sizes available will almost always be the word. The other sizes, if any, are likely to be multiples or fractions of the word size. The smaller sizes are normally used only for efficient use of memory; when loaded into the processor, their values usually go into a larger, word sized holder.

• Floating point numbers: Holders for floating point numerical values are typically either a word or a multiple of a word.

• Addresses: Holders for memory addresses must be of a size capable of expressing the needed range of values but not be excessively large, so often the size used is the word though it can also be a multiple or fraction of the word size.

• Registers: Processor registers are designed with a size appropriate for the type of data they hold, e.g. integers, floating point numbers or addresses. Many computer architectures use "general purpose" registers that can hold any of several types of data, these registers must be sized to hold the largest of the types, historically this is the word size of the architecture though increasingly special purpose, larger, registers have been added to deal with newer types.

• Memory-processor transfer: When the processor reads from the memory subsystem into a register or writes a register's value to memory, the amount of data transferred is often a word. In simple memory subsystems, the word is transferred over the memory data bus, which typically has a width of a word or half-word. In memory subsystems that use caches, the word-sized transfer is the one between the processor and the first level of cache; at lower levels of the memory hierarchy larger transfers (which are a multiple of the word size) are normally used.

• Unit of address resolution: In a given architecture, successive address values designate successive units of memory; this unit is the unit of address resolution. In most computers, the unit is either a character (e.g. a byte) or a word. (A few computers have used bit resolution.) If the unit is a word, then a larger amount of memory can be accessed using an address of a given size. On the other hand, if the unit is a byte, then individual characters can be addressed (i.e. selected during the memory operation).

• Instructions: Machine instructions are normally fractions or multiples of the architecture's word size. This is a natural choice since instructions and data usually share the same memory subsystem. In Harvard architectures the word sizes of instructions and data need not be related.