3-11
BASIC EXECUTION ENVIRONMENT
As the IA has evolved, flags have been added to the EFLAGS register, but the function and
placement of existing flags have remained the same from one family of the IA processors to the
next. As a result, code that accesses or modifies these flags for one family of IA processors
works as expected when run on later families of processors.
Figure 3-7. EFLAGS Register
3129
3028272625242322212019181716
0
R
F
I
D
A
C
V
M
X Virtual-8086 Mode (VM)
X Resume Flag (RF)
X Nested Task (NT)
X I/O Privilege Level (IOPL)
S Overflow Flag (OF)
C Direction Flag (DF)
X Interrupt Enable Flag (IF)
X Alignment Check (AC)
X ID Flag (ID)
X Virtual Interrupt Pending (VIP)
1513
141211109876543210
0
C
F
A
F
P
F1
D
F
I
F
T
F
S
F
Z
F
N
T
0
0
000000000
V
I
P
V
I
F
O
F
I
O
P
L
X Virtual Interrupt Flag (VIF)
X Trap Flag (TF)
S Sign Flag (SF)
S Zero Flag (ZF)
S Auxiliary Carry Flag (AF)
S Parity Flag (PF)
S Carry Flag (CF)
S Indicates a Status Flag
C Indicates a Control Flag
X Indicates a System Flag
Reserved bit positions. DO NOT USE.
Always set to values previously read.
