Intel 64 and IA-32 Architectures Software Developers Manual Volume 1, Basic Architecture

Vol. 1 3-23

BASIC EXECUTION ENVIRONMENT

3.4.3.3 System Flags and IOPL Field

The system flags and IOPL field in the EFLAGS register control operating-system or

executive operations. They should not be modified by application programs.

The functions of the system flags are as follows:

TF (bit 8) Trap flag — Set to enable single-step mode for debugging;

clear to disable single-step mode.

IF (bit 9) Interrupt enable flag — Controls the response of the

processor to maskable interrupt requests. Set to respond to

maskable interrupts; cleared to inhibit maskable interrupts.

IOPL (bits 12 and 13)

I/O privilege level field — Indicates the I/O privilege level of

the currently running program or task. The current privilege

level (CPL) of the currently running program or task must be

less than or equal to the I/O privilege level 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.

NT (bit 14) Nested task flag — Controls the chaining of interrupted and

called tasks. Set when the current task is linked to the previ-

ously executed task; cleared when the current task is not linked

to another task.

RF (bit 16) Resume flag — Controls the processor’s response to debug

exceptions.

VM (bit 17) Virtual-8086 mode flag — Set to enable virtual-8086 mode;

clear to return to protected mode without virtual-8086 mode

semantics.

AC (bit 18) Alignment check flag — Set this flag and the AM bit in the CR0

clear the AC flag and/or the AM bit to disable alignment

checking.

VIF (bit 19) Virtual interrupt flag — Virtual image of the IF flag. Used in

conjunction with the VIP flag. (To use this flag and the VIP flag

the virtual mode extensions are enabled by setting the VME flag

in control register CR4.)

VIP (bit 20) Virtual interrupt pending flag — Set to indicate that an inter-

rupt is pending; clear when no interrupt is pending. (Software

sets and clears this flag; the processor only reads it.) Used in

conjunction with the VIF flag.

ID (bit 21) Identification flag — The ability of a program to set or clear

this flag indicates support for the CPUID instruction.

For a detailed description of these flags: see Chapter 3, “Protected-Mode Memory

Management,” in the Intel® 64 and IA-32 Architectures Software Developer’s

Manual, Volume 3A.