User's Manual

Vol. 3 5-23

PROTECTION

The privilege checking rules are different depending on whether the control transfer

was initiated with a CALL or a JMP instruction, as shown in

Table 5-1.

The DPL field of the call-gate descriptor specifies the numerically highest privilege

level from which a calling procedure can access the call gate; that is, to access a call

gate, the CPL of a calling procedure must be equal to or less than the DPL of the call

gate. For example, in

Figure 5-15, call gate A has a DPL of 3. So calling procedures at

all CPLs (0 through 3) can access this call gate, which includes calling procedures in

code segments A, B, and C. Call gate B has a DPL of 2, so only calling procedures at

a CPL or 0, 1, or 2 can access call gate B, which includes calling procedures in code

Figure 5-11. Privilege Check for Control Transfer with Call Gate

Table 5-1. Privilege Check Rules for Call Gates

Instruction Privilege Check Rules

CALL CPL ≤ call gate DPL; RPL ≤ call gate DPL

Destination conforming code segment DPL ≤ CPL

Destination nonconforming code segment DPL ≤ CPL

JMP CPL ≤ call gate DPL; RPL ≤ call gate DPL

Destination conforming code segment DPL ≤ CPL

Destination nonconforming code segment DPL = CPL

CPL

RPL

DPL

Privilege

Check

Call Gate (Descriptor)

Destination Code-

CS Register

Call-Gate Selector

Segment Descriptor