Segmentation Registers: when are they used?

Implicit uses

    1. Instruction references use the %cs segment register.

    2. General memory references use the %ds segment register.

    3. Stack references, that is, memory references via %ebp or %esp, use
       the %ss segment register.

    4. String references, that is, memory references via string operations
      (e.g. indexing using %esi and %edi, or primitive string operations
      %movs, %stos, or repeat prefix %rep), use the %es segment register

Explicit uses

    1. Overriding implicit rules.  Only rule 2 above can be overridden.
       can do movl %eax, %ss:[%ebx].

    2. can directly modify segment registers via mov or far jmp and call.

    3. can use lds lss, lfs, ... instructions to load a memory reference
       via the selected segment registers.


References

[1] http://www.scs.stanford.edu/05au-cs240c/lab/i386/s02_05.htm

[2] http://www.ic.unicamp.br/~celio/mc404s2-03/addr_modes/intel_addr.html