Intel IA-32 - Manual (Page 60)

2-12 Vol. 3A
SYSTEM ARCHITECTURE OVERVIEW
2.3 SYSTEM FLAGS AND FIELDS IN THE EFLAGS REGISTER
The system flags and IOPL field of the EFLAGS register control I/O, maskable hardware inter-
rupts, debugging, task switching, and the virtual-8086 mode (see Figure 2-4). Only privileged
code (typically operating system or executive code) should be allowed to modify these bits.
The system flags and IOPL are:
TF Trap (bit 8) — Set to enable single-step mode for debugging; clear to disable single-
step mode. In single-step mode, the processor generates a debug exception after each
instruction. This allows the execution state of a program to be inspected after each
instruction. If an application program sets the TF flag using a POPF, POPFD, or IRET
instruction, a debug exception is generated after the instruction that follows the POPF,
POPFD, or IRET.
IF Interrupt enable (bit 9) — Controls the response of the processor to maskable hard-
ware interrupt requests (see also: Section 5.3.2, “Maskable Hardware Interrupts”). The
flag is set to respond to maskable hardware interrupts; cleared to inhibit maskable hard-
ware 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.
Figure 2-4. System Flags in the EFLAGS Register
31 22 21 20 19 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 5 4 3 2 1 0
0
C
F
A
F P
F 1
D
F
I
F
T
F S
F Z
F N
T
0 0
V
I
P
V
I
F
O
F
I
O
P
L
VIF — Virtual Interrupt Flag
TF — Trap Flag
Reserved
Reserved (set to 0)