An index register is a fast memory area in a CPU used to iterate through memory addresses or track operations. It can act as a pointer to data arrays and hold loop counters, and is usually accessed through assembly language. Some processors have dedicated index registers, while others use any generic register.
In computer programming and architecture, an index register is an area of memory usually built into the central processing unit (CPU) for use as a very fast counter to iterate through memory addresses or to track operations such as looping . Depending on the type of system architecture, an index register can be a defined and dedicated register among other processor registers, or it can be any generic register. Most commonly, one index register holds the current offset of a memory location, with another register holding the base address, so the combination of the two registers creates a complete memory address. One of the special functions of an index register, when one is specially designated by a CPU, is that it can be used to easily iterate through memory addresses by incrementing or decrementing as needed so that data structures such as arrays and stacks can be traversed.
The index registers on a computer’s CPU are incredibly low-level memory areas that are usually only directly accessible by a programmer through the use of assembly language or a similar low-level programming language. In some of the more common types of processors, two separate registers are referred to as index registers, namely the source index (SI) and destination index (DI) registers. Other processors specifically don’t have index registers or helper operators that require them, meaning that any generic register of the appropriate size can be used.
One of the most frequent uses of an index register is to act as a pointer to a memory location that contains a stream of data that needs to be accessed sequentially. An example can be seen when using a data array where all elements are arranged consecutively in memory. If the index register is used to access an array, it can hold the corresponding value of the offset of the element currently being accessed when added to another register that contains a base address, such as the data segment register . This can make it much easier to complete procedures such as copying the contents of a null-terminated string from a source location to a destination string.
Another use of an index register can be to hold information about loops and other counters. Some system architectures prefer to use the index register to hold the number of iterations that have occurred in a cycle, although other times any register can be used. Also, some assembly instructions rely specifically on the source and destination index registers to perform certain operations, such as reading or writing block memory, as might be done to send information to a screen.
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