Segment register is 16-bit register, for use in combination with the microprocessor generates a memory address in the other registers.
For the same microprocessor, the functional segment register in real mode and protected mode is different.
Code register CS (Code Segment)
CS microprocessor used for code, CS defines the starting address of the code segment.
In real mode, the CS defines the starting point of a 64K byte memory segment; protected mode code segment which selects a starting address, length, and other required attribute information (read, write, etc. may be performed) is described.
Data segment register DS (Data Segment)
DS is a storage area that contains most of the data used by the program is. DS define the start address of the data segment.
For the protected mode of the 80386 microprocessor and the higher model, the data segment length is limited to 4GB
Additional segment register ES (Extra Segment)
ES is a data segment and an additional store for the destination operand of the operation instruction for certain strings. ES is used to define the start address of the additional segment.
The stack segment register SS (Stack Segment)
A stack is a special memory area of the memory, number of data and address information for storing programs run.
SS define the first address of the stack segment. Determining the memory address in the stack segment register SS from the stack segment and stack pointer register SP / ESP. Further BP / EBP may also be used for addressing data in the stack segment.
FS, and GS segment registers
80386 and higher microprocessors only valid for programs to access two additional memory segments.