What is the purpose of segment registers in x86 protected mode?

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Some historical background

The 8086 always used a fixed 64KiB Window per segment whose starting address was calculated by (segment register * 16). Since the 80286 there are some special tables in memory (GDT and LDT). These tables contain the starting address, the length and the access rights of a segment. The segment registers (CS, DS, ES, SS – and since 80386: FS, GS) contain indexes into these tables.

So theoretically an operating system may set the offset and the length of a segment in a way it wants to do that: On 8086 DS=0x0123 means: Segment is 64KiB starting from address 0x01230. In 32-bit mode DS=0x0123 may mean: Segment start at address 0xABCD, length is 0xEF bytes – this depends on the content of the GDT and LDT tables created by the operating system. Trying to access a segment outside this range (DS:0x1000 if the length is < 0x1000) will cause an exception (interrupt).

Current situation

However most modern 32-bit operating systems do not really use segment registers any more. Their values are set depending on the mode (kernel or user) because of access rights issues. The starting address is typically 0 and the length is 4GiB.

The real memory protection is done using the MMU so that some areas of memory cannot be accessed in user mode. In modern operating systems the MMU is absolutely essiential. It maps an “absolute” virtual address to a real physical address checking for access right violations.

There is one exception: Some operating systems (Windows and Linux for example) use the FS and/or GS segments to really point to a different memory area.

For this reason in 64-bit mode the x86 processors use the CS register only for access rights issues and FS and GS can be used to add an offset to each address. As far as I know DS, ES and SS are not used while the content of the registers FS and GS does not matter but there are special registers that explicitly give the offset to be added to an operation that uses FS or GS.

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