Reputation: 147
When and how does CPU Switch from Kernel mode to User Mode On X86 : What exactly does it do? How does it makes this transition?
Upvotes: 3
Views: 1779
Reputation: 239341
In x86 protected mode, the current privilege level that the CPU is executing in is controlled by the two least significant bits of the CS
register (the RPL
field of the segment selector).
So a switch from kernel mode (CPL=0)
to user mode (CPL=3)
is accomplished by replacing a kernel-mode CS value with a user-mode one. There's many ways to do this, but one typical one is an IRET
instruction which pops the EIP
, CS
and EFLAGS
registers from the stack.
Upvotes: 6
Reputation: 4024
iret
does this for example. See the code here (INTERRUPT_RETURN macro)
Upvotes: 2