[Valgrind-developers] segment registers

[Nicholas N. <nj...@ca...>]

Hi,

I just learnt about segment registers properly for the first time.  Now I
see what the fuss is all about.  I have some questions about them, I would
appreciate answers from any kind soul...

- Who uses them?  Some ordinary apps, I guess... does glibc, or other
relevant libraries?

- AFAICT, we virtualise the LDT, but not the GDT(?)  Linux has the
modify_ldt() syscall, does it not use the GDT at all so we don't have to
worry about it, or is there something else happening?

- if segment registers are in use, if I print a pointer, I'm really only
printing the offset within the current segment, rather than the true
virtual address, right?  (Which is why V must convert it, for the skin's
sake.)

- AFAICT, the following is true
    - %cs is implicit operand to every instruction fetch?
    - %ss is implicit operand to every stack instruction?
    - %ds is implicit operand to all non-stack, non-string ops?
    - %es is implicit operand to string ops?
    - segment overrides are required to use %fs, %gs

  When segment overrides are used, the GETSEG/USESEG pair is used to
  convert far pointers to flat addresses.  That's fine.  But what happens
  when there is no segment override?  Eg. if we set %ds, which affects
  most ops, are GETSEG/USESEG pairs introduced then?

Thanks.

N