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.