In computer

processors A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and ...

the parity flag indicates if the numbers 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 10 (1010 in binary) then the parity flag would be 1.

x86 processors

x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was introd ...

processors, the parity flag reflects the parity only of the ''

least significant byte In computing, bit numbering is the convention used to identify the bit positions in a binary number. Bit significance and indexing In computing, the least significant bit (LSB) is the bit position in a binary integer representing the binary ...

'' of the result, and is set if the number of set bits of ones is even (put another way, the parity bit is set if the sum of the bits is even). According to 80386

Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 seri ...

manual, the parity flag is changed in the x86 processor family by the following instructions: * All arithmetic instructions; * In conditional jumps, parity flag is used, 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 is to check an unrelated x87-FPU flag. The

FPU FPU may stand for: Universities * Florida Polytechnic University, in Lakeland, Florida, United States * Franklin Pierce University, in New Hampshire, United States * Fresno Pacific University, in California, United States * Fukui Prefectural Un ...

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 In computer processors the carry flag (usually indicated as the C flag) is a single bit in a system status register/flag register used to indicate when an arithmetic carry or borrow has been generated out of the most significant arithmetic logic ...

, C2 in the parity flag and C3 in the

zero flag The zero flag is a single bit flag that is a central feature on most conventional CPU architectures (including x86, ARM, PDP-11, 68000, 6502, and numerous others). It is often stored in a dedicated register, typically called status register or ...

.{{cite web , url=http://www.intel.com/products/processor/manuals/index.htm , title=Intel 64 and IA-32 Architectures Software Developer's Manual Volume 1: Basic Architecture , date=January 2011 , pages=97–98 The C2 flag is set when e.g. incomparable

floating point In computing, floating-point arithmetic (FP) is arithmetic that represents real numbers approximately, using an integer with a fixed precision, called the significand, scaled by an integer exponent of a fixed base. For example, 12.345 can be r ...

values (

NaN Nan or NAN may refer to: Places China * Nan County, Yiyang, Hunan, China * Nan Commandery, historical commandery in Hubei, China Thailand * Nan Province ** Nan, Thailand, the administrative capital of Nan Province * Nan River People Given nam ...

or unsupported format) are compared with the FUCOM instructions.

x86 processors

References

See also