Processor Programming (Continued)
3.3.1.4 EFLAGS Register
The EFLAGS Register contains status information and
controls certain operations on the GXm processor. The
lower 16 bits of this register are referred to as the
EFLAGS register that is used when executing 8086 or
80286 code. Table 3-4 gives the bit formats for the
EFLAGS Register
Table 3-4. EFLAGS Register
Bit
Name
Flag Type
Description
Reserved: Set to 0.
31:22
21
RSVD
ID
--
System
Identification Bit: The ability to set and clear this bit indicates that the CPUID instruction is sup-
ported. The ID can be modified only if the CPUID bit in CCR4 (Index E8h[7]) is set.
20:19
18
RSVD
AC
--
Reserved: Set to 0.
System
Alignment Check Enable: In conjunction with the AM flag (bit 18) in CR0, the AC flag deter-
mines whether or not misaligned accesses to memory cause a fault. If AC is set, alignment faults
are enabled.
17
16
VM
RF
System
Debug
Virtual 8086 Mode: If set while in protected mode, the processor switches to virtual 8086 opera-
tion handling segment loads as the 8086 does, but generating exception 13 faults on privileged
opcodes. The VM bit can be set by the IRET instruction (if current privilege level
is 0) or by task switches at any privilege level.
Resume Flag: Used in conjunction with debug register breakpoints. RF is checked at instruction
boundaries before breakpoint exception processing. If set, any debug fault is ignored on the next
instruction.
15
14
RSVD
NT
--
Reserved: Set to 0.
System
Nested Task: While executing in protected mode, NT indicates that the execution of the current
task is nested within another task.
13:12
IOPL
System
Arithmetic
Control
I/O Privilege Level: While executing in protected mode, IOPL indicates the maximum current
privilege level (CPL) permitted to execute I/O instructions without generating an exception 13
fault or consulting the I/O permission bit map. IOPL also indicates the maximum CPL allowing
alteration of the IF bit when new values are popped into the EFLAGS register.
11
OF
Overflow Flag: Set if the operation resulted in a carry or borrow into the sign bit of the result but
did not result in a carry or borrow out of the high-order bit. Also set if the operation resulted in a
carry or borrow out of the high-order bit but did not result in a carry or borrow into the sign bit of
the result.
10
DF
Direction Flag: When cleared, DF causes string instructions to auto-increment (default) the
appropriate index registers (ESI and/or EDI). Setting DF causes auto-decrement of the index
registers to occur.
9
8
IF
System
Debug
Interrupt Enable Flag: When set, maskable interrupts (INTR input pin) are acknowledged and
serviced by the CPU.
TF
Trap Enable Flag: Once set, a single-step interrupt occurs after the next instruction completes
execution. TF is cleared by the single-step interrupt.
7
6
5
4
SF
ZF
Arithmetic
Arithmetic
--
Sign Flag: Set equal to high-order bit of result (0 indicates positive, 1 indicates negative).
Zero Flag: Set if result is zero; cleared otherwise.
Reserved: Set to 0.
RSVD
AF
Arithmetic
Auxiliary Carry Flag: Set when a carry out of (addition) or borrow into (subtraction) bit position 3
of the result occurs; cleared otherwise.
3
2
RSVD
PF
--
Reserved: Set to 0.
Arithmetic
Parity Flag: Set when the low-order 8 bits of the result contain an even number of ones; other-
wise PF is cleared.
1
0
RSVD
CF
Reserved: Set to 1.
Arithmetic
Carry Flag: Set when a carry out of (addition) or borrow into (subtraction) the most significant bit
of the result occurs; cleared otherwise.
Revision 3.1
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