Status register

A status register or flag register (also: condition code register, program status word, PSW, etc.) is a collection of flag bits for a processor. An example is the FLAGS register of the x86 architecture.

The status register is a hardware register which contains information about the state of the processor. Individual bits are implicitly or explicitly read and/or written by the machine code instructions executing on the processor. The status register in a traditional processor design includes at least three central flags: Zero, Carry, and Overflow, which are set or cleared automatically as side effects of arithmetic operations. They may then be tested via conditional branch or jump instructions. A status register may often have other fields as well, such as more specialized flags, interrupt enable bits, and similar types of information. During an interrupt, the status of the thread currently executing can be preserved (and later recalled) by storing the current value of the status register along with the program counter and other active registers into the machine stack or a reserved area of memory.

The most common flags

This is a list of the most common CPU status register flags, implemented in almost all modern processors.

Flag	Name	Description
Z	Zero flag	Indicates that the result of an arithmetic or logical operation (or, sometimes, a load) was zero.
C	Carry flag	Enables numbers larger than a single word to be added/subtracted by carrying a binary digit from a less significant word to the least significant bit of a more significant word as needed. It is also used to extend bit shifts and rotates in a similar manner on many processors (sometimes done via a dedicated X flag).
S / N	Sign flag / Negative flag	Indicates that the result of a mathematical operation is negative. In some processors, the N and S flags are distinct with different meanings and usage: One indicates whether the last result was negative whereas the other indicates whether a subtraction or addition has taken place.
V / O / W	Overflow flag	Indicates that the signed result of an operation is too large to fit in the register width using twos complement representation.

Other flags

This is a partial list of other, less common status flags that various CPUs support.

Flag	Name	Description
H	Half-carry flag	Indicates that a bit carry was produced between the nibbles (typically between the 4-bit halves of a byte operand) as a result of the last arithmetic operation. Such a flag is generally useful for implementing BCD arithmetic operations on binary hardware.
P	Parity flag	Indicates whether the number of set bits of the last result is odd or even.
I	Interrupt flag	Indicates that an interrupt is currently active, and that the currently executing code is part of an interrupt handler routine.
S	Supervisor flag	Indicates that CPU is operating in supervisor mode, having enhanced permissions enabled and access to special instructions typically reserved for the operating system itself, as opposed to the normal "user" instruction set.

See also

• Control register
• Flag byte
• Flag (computing)
• FLAGS register (computing)