In the x86 processor architecture, memory addresses are specified in two parts called the segment and the offset. One usually thinks of the segment as specifying the beginning of a block of memory allocated by the system and the offset as an index into it. Segment values are stored in the segment registers. There are four or more segment registers: CS contains the segment of the current instruction (IP is the offset), SS contains the stack segment (SP is the offset), DS is the segment used by default for most data operations, ES (and, in more recent processors, FS and GS) is an extra segment register. Most memory operations accept a segment override prefix that allows use of a segment register other than the default one.