4-10
PROCEDURE CALLS, INTERRUPTS, AND EXCEPTIONS
4.3.6.CALL and RET Operation Between Privilege Levels
When making a call to a more privileged protection level, the processor does the following (refer
to Figure 4-2):
1.Performs an access rights check (privilege check).
2.Temporarily saves (internally) the current contents of the SS, ESP, CS, and EIP registers.
3.Loads the segment selector and stack pointer for the new stack (that is, the stack for the
privilege level being called) from the TSS into the SS and ESP registers and switches to
the new stack.
4.Pushes the temporarily saved SS and ESP values for the calling procedures stack onto the
new stack.
5.Copies the parameters from the calling procedures stack to the new stack. (A value in the
call gate descriptor determines how many parameters to copy to the new stack.)
6.Pushes the temporarily saved CS and EIP values for the calling procedure to the new stack.
7.Loads the segment selector for the new code segment and the new instruction pointer from
the call gate into the CS and EIP registers, respectively.
8.Begins execution of the called procedure at the new privilege level.
When executing a return from the privileged procedure, the processor performs these actions:
1.Performs a privilege check.
2.Restores the CS and EIP registers to their values prior to the call.
3.(If the RET instruction has an optional n argument.) Increments the stack pointer by the
number of bytes specified with the n operand to release parameters from the stack. If the
call gate descriptor specifies that one or more parameters be copied from one stack to the
other, a RET n instruction must be used to release the parameters from both stacks. Here,
the n operand specifies the number of bytes occupied on each stack by the parameters. On
a return, the processor increments ESP by n for each stack to step over (effectively
remove) these parameters from the stacks.
4.Restores the SS and ESP registers to their values prior to the call, which causes a switch
back to the stack of the calling procedure.
5.(If the RET instruction has an optional n argument.) Increments the stack pointer by the
number of bytes specified with the n operand to release parameters from the stack (refer to
the explanation in step 3).
6.Resumes execution of the calling procedure.
