It's a 32bit compatibility mode, which has the same x86 command set and registers as 32bit protected mode (http://en.wikipedia.org/wiki/X86-64). Only rax (eax) is changed due to the result of the mode change, other registers are preserved. 

Example:
    
    ; Entry from 64bit mode

    mov   rax , 126
    mov   rbx , 1
    int   0x60

   use32

     ; 32bit code here
     ; ..

    mov   eax , 126
    mov   ebx , 2
    int   0x60

  use64

     ; Use 64bit again



Last Edited By: villemt Jan 21 15 1:11 PM. Edited 2 times.