1. Types of Code Segments Conforming Code Segment
There are two types of code segments:
Conforming Code Segment
Nonconforming Code Segment

2. Conforming Code Segment
An executable segment whose descriptor has the conforming bit set
It permits sharing of procedures that may be called from various privilege levels but should
execute at the privilege level of the calling procedure.
Example: math libraries
When control is transferred to a conforming segment, the CPL does not change

3. Nonconforming Code Segment
Most code segments are not conforming
For nonconforming segments, control can be transferred without a gate only to executable segments at the same level of privilege
To transfer control to higher privilege levels(not numerical)-for e.g. application want to use system service- CALL instruction need to be used with call-gate descriptors
JMP instruction never transfer control to a nonconforming segment whose DPL ≠ CPL.

4. Privilege Check for Control Transfer without Gate

5. Gate Descriptors
To provide protection for control transfers among executable segments at different privilege levels, the uses gate descriptors.
There are four kinds of gate descriptors:
Call gates  for far jmp & call
Trap gates  for exceptions
Interrupt gates  for interrupts
Task gates  for task switching

6. Call gates A call gate descriptor may reside in GDT or LDT
A call gate has two primary functions:
1. To define an entry point of a procedure
2. To specify the privilege level of entry point
When h/w recognizes that the destination selector refers to a gate descriptor the operation of the instruction is expanded by the contents of the call gate.

7. Call gates
The selector and offset fields of a gate form a pointer to the entry point of a procedure
Format of Call Gate

8. Privilege Check for Control Transfer with Gate
The far pointer (48-bit logical address of two components: a 16-bit segment selector and a 32-bit offset) operand of the control transfer instruction does not point to the segment and offset of the target instruction; rather the selector part of the pointer selects a gate, and the offset is not used.

9. Privilege Check for Control Transfer with Gate

10. Privilege Check for Control Transfer with Gate
Four different privilege levels are used to check the validity of a control transfer via a call gate:
1. CPL
2. RPL of the selector used to specify the call gate.
3. DPL of the gate descriptor.
4. DPL of the descriptor of the target segment

11. Privilege Check for Control Transfer with Gate
Gates can be used for control transfers to numerically smaller privilege levels or to the same privilege level
For a JMP instruction to a nonconforming segment, the privilege rules that must be satisfied are :
MAX (CPL,RPL) ≤ gate DPL (numerically)
target segment DPL = CPL (numerically)

12. Privilege Check for Control Transfer with Gate
For a CALL instruction, the privilege rules that must be satisfied are :
MAX (CPL,RPL) ≤ gate DPL (numerically)
target segment DPL ≤ CPL (numerically)

13. Privilege Check for Control Transfer with Gate