segment registers
overview
Segment registers are specifically used for referencing memory locations. Unlike normal registers, segment registers are 16-bits in size, and always contain a pointer to the start of the memory-specific segment.
segment registers
There are six segment registers which are as follows:
| Segment Register | Purpose | Mnemonic |
|---|---|---|
| CS | Code segment register stores the base location of the code section (.text section), which is used for data access. | "CS" for code segment. |
| DS | The data segment register stores the default location for variables (.data section), which is used for data access. | "DS" for data segment. |
| ES | Extra segment register is used during string operations | "ES" for extra segment. |
| SS | Stack segment register stores the base location of the stack segment and is used when implicitly using the stack pointer or when explicitly using the base pointer. | "SS" for stack segment. |
| FS | This is an extra segment register. | "FS" for free segment. |
| GS | This is an extra segment register. | "GS" for good space. |
import to remember
Each segment register is 16-bits and contains the pointer to the start of the memory-specific segment. The CS register contains the pointer to the code segment in memory. The code segment is where the instruction codes are stored in memory. The processor retrieves instruction codes from memory based on the CS register value and an offset value contained in the instruction pointer (EIP) register. Keep in mind no program can explicitly load or change the CS register. The processor assigns its values as the program is assigned a memory space.
The DS, ES, FS and GS segment registers are all used to point to data segments. Each of the four separate data segments help the program separate data elements to ensure that they do no overlap. The program loads the data segment registers with the appropriate pointer value for the segments and then reference individual memory locations using an offset value.
The stack segment register (SS) is used to point to the stack segment. The stack contains data values passed to functions and procedures within the program.
Segment registers are considered part of the operating system and can neither read nor be changed directly in almost all cases. When working in the protected mode flat model (x86 architecture which is 32-bit), your program runs and receives a 4GB address space to which any 32-bit register can potentially address any of the four billion memory locations except for those protected areas defined by the operating system. Physical memory may be larger than 4GB however a 32-bit register can only express 4,294,967,296 different locations. If you have more than 4GB of memory in your computer, the OS must arrange a 4GB region within memory and your programs are limited to that new region. This task is completed by the segment registers and the OS keeps close control of this.