Index registers are used to provide more flexibility in addressing modes, allowing the programmer to create a memory address by combining the contents of an address register with the contents of an index register (with displacements, increments, decrements, and other options). In some processors, there are specific index registers (or just one index register) that can only be used only for that purpose. In some processors, any data register, address register, or general register (or some combination of the three) can be used as an index register.
* IBM 360/370: any of the 16 general purpose registers may be used as an index register
* Intel 80x86: 7 of the 8 general purpose registers may be used as an index register (the ESP is the exception)
* MIX: five index registers; named I-registers I1, I2, I3, I4, and I5; five bytes plus sign
* Motorola 680x0, 68300: any of the 8 data registers or the 8 address registers may be used as an index register
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