Base registers or segment registers are used to segment memory. Effective addresses are computed by adding the contents of the base or segment register to the rest of the effective address computation. In some processors, any register can serve as a base register. In some processors, there are specific base or segment registers (one or more) that can only be used for that purpose. In some processors with multiple base or segment registers, each base or segment register is used for different kinds of memory accesses (such as a segment register for data accesses and a different segment register for program accesses).
* IBM 360/370: any of the 16 general purpose registers may be used as a base register
* Intel 80x86: 6 dedicated segment registers: CS (code segment), SS (stack segment), DS (data segment), ES (extra segment, a second data segment register), FS (third data segment register), and GS (fourth data segment register)
* Motorola 680x0, 68300: any of the 8 address registers may be used as a base register
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