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Vol. 3 2-13
SYSTEM ARCHITECTURE OVERVIEW
IF Interrupt enable (bit 9) — Controls the response of the processor to
maskable hardware interrupt requests (see also: Section 6.3.2, “Maskable
Hardware Interrupts”). The flag is set to respond to maskable hardware
interrupts; cleared to inhibit maskable hardware interrupts. The IF flag does
not affect the generation of exceptions or nonmaskable interrupts (NMI
interrupts). The CPL, IOPL, and the state of the VME flag in control register
CR4 determine whether the IF flag can be modified by the CLI, STI, POPF,
POPFD, and IRET.
IOPL I/O privilege level field (bits 12 and 13) — Indicates the I/O privilege
level (IOPL) of the currently running program or task. The CPL of the
currently running program or task must be less than or equal to the IOPL to
access the I/O address space. This field can only be modified by the POPF
and IRET instructions when operating at a CPL of 0.
The IOPL is also one of the mechanisms that controls the modification of the
IF flag and the handling of interrupts in virtual-8086 mode when virtual
mode extensions are in effect (when CR4.VME = 1). See also:
Chapter 13,
“Input/Output,” in the Intel® 64 and IA-32 Architectures Software Devel-
oper’s Manual, Volume 1.
NT Nested task (bit 14) — Controls the chaining of interrupted and called
tasks. The processor sets this flag on calls to a task initiated with a CALL
instruction, an interrupt, or an exception. It examines and modifies this flag
on returns from a task initiated with the IRET instruction. The flag can be
explicitly set or cleared with the POPF/POPFD instructions; however,
Figure 2-4. System Flags in the EFLAGS Register
31
22
21 2019
18
17 16
R
F
I
D
A
C
V
M
VM — Virtual-8086 Mode
RF — Resume Flag
NT — Nested Task Flag
IOPL— I/O Privilege Level
IF — Interrupt Enable Flag
AC — Alignment Check
ID — Identification Flag
VIP — Virtual Interrupt Pending
15
13
14 12
11 10
9
8
7
6
543
2
1
0
0
C
F
A
F
P
F
1
D
F
I
F
T
F
S
F
Z
F
N
T
00
V
I
P
V
I
F
O
F
I
O
P
L
VIF — Virtual Interrupt Flag
TF — Trap Flag
Reserved
Reserved (set to 0)