Parity flag
Encyclopedia
In computer
processors the parity flag indicates if the number of set bits is odd or even in the binary representation of the result of the last operation. It is normally a single bit in a processor status register.
For example, assume a machine where a set parity flag indicates even parity. If the result of the last operation were 26 (11010 in binary), the parity flag would be 0 since the number of set bits is odd. Similarly, if the result were 102 (1100110 in binary) then the parity flag would be 1.
The parity flag is usually used in conditional jumps, where e.g. the JP instruction jumps to the given target when the parity flag is set and the JNP instruction jumps if it is not set. The flag may be also read directly with instructions such as PUSHF, which pushes the flags register on the stack.
One common reason to test the parity flag actually has nothing to do with parity. The FPU
has four condition flags (C0 to C3), but they can not be tested directly, and must instead be first copied to the flags register. When this happens, C0 is placed in the carry flag
, C2 in the parity flag and C3 in the zero flag
. The C2 flag is set when e.g. incomparable floating point
values (NaN
or unsupported format) are compared with the FUCOM instructions.
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
processors the parity flag indicates if the number of set bits is odd or even in the binary representation of the result of the last operation. It is normally a single bit in a processor status register.
For example, assume a machine where a set parity flag indicates even parity. If the result of the last operation were 26 (11010 in binary), the parity flag would be 0 since the number of set bits is odd. Similarly, if the result were 102 (1100110 in binary) then the parity flag would be 1.
x86 Processors
In x86 processors, the parity flag reflects the parity only of the least significant byte of the result, and is set if the number of set bits of ones is even. According to 80386 Intel manual, the parity flag is changed in the x86 processor family by the following instructions:- All arithmetic instructions;
- Compare instruction (equivalent to a subtract instruction without storing the result);
- Logical instructions - XOR, AND, OR;
- the TESTTEST (x86 instruction)In the x86 assembly language, the TEST instruction performs a bitwise AND on two operands. The flags SF, ZF, PF, CF, OF and AF are modified while the result of the AND is discarded. There are 9 different opcodes for the TEST instruction depending on the type and size of the operands. It can compare...
instruction (equivalent to the AND instruction without storing the result). - the POPF instruction
The parity flag is usually used in conditional jumps, where e.g. the JP instruction jumps to the given target when the parity flag is set and the JNP instruction jumps if it is not set. The flag may be also read directly with instructions such as PUSHF, which pushes the flags register on the stack.
One common reason to test the parity flag actually has nothing to do with parity. The FPU
Floating point unit
A floating-point unit is a part of a computer system specially designed to carry out operations on floating point numbers. Typical operations are addition, subtraction, multiplication, division, and square root...
has four condition flags (C0 to C3), but they can not be tested directly, and must instead be first copied to the flags register. When this happens, C0 is placed in the carry flag
Carry flag
In computer processors the carry flag is a single bit in a system status register used to indicate when an arithmetic carry or borrow has been generated out of the most significant ALU bit position...
, C2 in the parity flag and C3 in the zero flag
Zero flag
The Zero Flag is a single bit flag that is a central feature on most conventional CPU architectures . It is often stored in a dedicated register, typically called status register or flag register, along with other flags...
. The C2 flag is set when e.g. incomparable floating point
Floating point
In computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...
values (NaN
NaN
In computing, NaN is a value of the numeric data type representing an undefined or unrepresentable value, especially in floating-point calculations...
or unsupported format) are compared with the FUCOM instructions.
See also
- x86 architectureX86 architectureThe term x86 refers to a family of instruction set architectures based on the Intel 8086 CPU. The 8086 was launched in 1978 as a fully 16-bit extension of Intel's 8-bit based 8080 microprocessor and also introduced segmentation to overcome the 16-bit addressing barrier of such designs...
- x86 assembly languageX86 assembly languagex86 assembly language is a family of backward-compatible assembly languages, which provide some level of compatibility all the way back to the Intel 8008. x86 assembly languages are used to produce object code for the x86 class of processors, which includes Intel's Core series and AMD's Phenom and...
- x86 Flags RegisterFLAGS register (computing)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...