FLAGS register

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The FLAGS register is the status register in Intel x86 microprocessors that contains the current state of the processor. This register is 16 bits wide. Its successors, the EFLAGS and RFLAGS registers, are 32 bits and 64 bits wide, respectively. The wider registers retain compatibility with their smaller predecessors.

The fixed bits at bit positions 1, 3 and 5, and carry, parity, adjust, zero and sign flags are inherited from an even earlier architecture, 8080 and 8085. The adjust flag used to be called auxiliary carry bit in 8080 and half-carry bit in the Zilog Z80 architecture.

FLAGS[edit]

Intel x86 FLAGS register[1]
Bit # Mask Abbreviation Description Category =1 =0
FLAGS
0 0x0001 CF Carry flag Status CY(Carry) NC(No Carry)
1 0x0002 Reserved, always 1 in EFLAGS [2][3]  
2 0x0004 PF Parity flag Status PE(Parity Even) PO(Parity Odd)
3 0x0008 Reserved[3]  
4 0x0010 AF Adjust flag Status AC(Auxiliary Carry) NA(No Auxiliary Carry)
5 0x0020 Reserved[3]  
6 0x0040 ZF Zero flag Status ZR(Zero) NZ(Not Zero)
7 0x0080 SF Sign flag Status NG(Negative) PL(Positive)
8 0x0100 TF Trap flag (single step) Control
9 0x0200 IF Interrupt enable flag Control EI(Enable Interrupt) DI(Disable Interrupt)
10 0x0400 DF Direction flag Control DN(Down) UP(Up)
11 0x0800 OF Overflow flag Status OV(Overflow) NV(Not Overflow)
12-13 0x3000 IOPL I/O privilege level (286+ only),
always 1[clarification needed] on 8086 and 186
System
14 0x4000 NT Nested task flag (286+ only),
always 1 on 8086 and 186
System
15 0x8000 Reserved,
always 1 on 8086 and 186,
always 0 on later models
 
EFLAGS
16 0x0001 0000 RF Resume flag (386+ only) System
17 0x0002 0000 VM Virtual 8086 mode flag (386+ only) System
18 0x0004 0000 AC Alignment check (486SX+ only) System
19 0x0008 0000 VIF Virtual interrupt flag (Pentium+) System
20 0x0010 0000 VIP Virtual interrupt pending (Pentium+) System
21 0x0020 0000 ID Able to use CPUID instruction (Pentium+) System
22-31 0xFFC0 0000 Reserved System
RFLAGS
32-63 0xFFFF FFFF...
...0000 0000
Reserved  

Note: The mask column in the table is the AND bitmask (as hexadecimal value) to query the flag(s) within FLAGS register value.

Usage[edit]

All FLAGS registers contain the condition codes, flag bits that let the results of one machine-language instruction affect another instruction. Arithmetic and logical instructions set some or all of the flags, and conditional jump instructions take variable action based on the value of certain flags. For example, jz (Jump if Zero), jc (Jump if Carry), and jo (Jump if Overflow) depend on specific flags. Other conditional jumps test combinations of several flags.

FLAGS registers can be moved from or to the stack. This is part of the job of saving and restoring processor context, against a routine such as an interrupt service routine whose changes to registers should not be seen by the calling code. Here are the relevant instructions:

  • The PUSHF and POPF instructions transfer the 16-bit FLAGS register.
  • PUSHFD/POPFD (introduced with the i386 architecture) transfer the 32-bit double register EFLAGS.
  • PUSHFQ/POPFQ (introduced with the x64 architecture) transfer the 64-bit quadword register RFLAGS.

In 64-bit mode, PUSHF/POPF and PUSHFQ/POPFQ are available but PUSHFD/POPFD are not.[4]:4-349,4-432

The ability to push and pop FLAGS registers lets a program manipulate information in the FLAGS in ways for which machine-language instructions do not exist. For example, the cld and std instructions clear and set the direction flag (DF), respectively; but there is no instruction to complement DF. This can be achieved with the following assembly code:

pushf        ; Use the stack to transfer the FLAGS
pop ax       ; ...into the AX register
push ax      ; and copy them back onto the stack for storage
xor ax, 400h ; Toggle (complement) DF only; other bits are unchanged
push ax      ; Use the stack again to move the modified value
popf         ; ...into the FLAGS register
; Insert here the code that required the DF flag to be complemented
popf         ; Restore the original value of the FLAGS

By manipulating the FLAGS register, a program can determine the model of the installed processor. For example, the alignment flag can only be changed on the 486 and above. If the program tries to modify this flag and senses that the modification did not persist, the processor is earlier than the 486.

Starting with the Intel Pentium, the CPUID instruction reports the processor model. However, the above method remains useful to distinguish between earlier models.

See also[edit]

References[edit]

  1. ^ Intel 64 and IA-32 Architectures Software Developer's Manual (PDF). 1. May 2012. pp. 3–21.
  2. ^ Intel 64 and IA-32 Architectures Software Developer’s Manual (PDF). 1. Dec 2016. p. 78.
  3. ^ a b c "Silicon reverse engineering: The 8085's undocumented flags". www.righto.com. Retrieved 2018-10-21.
  4. ^ Intel 64 and IA-32 Architectures Software Developer’s Manual (PDF). 2B. May 2012.