Flag (computing)
Encyclopedia
In computer programming
, flag can refer to one or more bit
s that are used to store a binary
value or code
that has an assigned meaning, but can refer to uses of other data types. Flags are typically found as members of a defined data structure
, such as a database record
, and the meaning of the value contained in a flag will generally be defined in relation to the data structure it is part of. In many cases, the binary value of a flag will be understood to represent one of several possible states or statuses. In other cases, the binary values may represent one or more attributes in a bit field
, often related to abilities or permissions, such as "can be written to" or "can be deleted". However, there are many other possible meanings that can be assigned to flag values. One common use of flags is to mark or designate data structures for future processing.
Within microprocessor
s and other logic devices, flags are commonly used to control or indicate the intermediate or final state or outcome of different operations. Microprocessors typically have, for example, a status register
that is composed of such flags, and the flags are used to indicate various post-operation conditions, such as when there has been an arithmetic overflow
. The flags can be utilized in subsequent operations, such as in processing conditional jump instruction
s. For example a je (Jump if Equal) instruction in the X86 assembly language will result in a jump if the Z (zero) flag was set by some previous operation.
A command line switch is also referred to as a flag. Command line
programs often start with an option parser
that translates command line switches into flags in the sense of this article.
Computer programming
Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
, flag can refer to one or more bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s that are used to store a binary
Binary numeral system
The binary numeral system, or base-2 number system, represents numeric values using two symbols, 0 and 1. More specifically, the usual base-2 system is a positional notation with a radix of 2...
value or code
Code
A code is a rule for converting a piece of information into another form or representation , not necessarily of the same type....
that has an assigned meaning, but can refer to uses of other data types. Flags are typically found as members of a defined data structure
Data structure
In computer science, a data structure is a particular way of storing and organizing data in a computer so that it can be used efficiently.Different kinds of data structures are suited to different kinds of applications, and some are highly specialized to specific tasks...
, such as a database record
Row (database)
In the context of a relational database, a row—also called a record or tuple—represents a single, implicitly structured data item in a table. In simple terms, a database table can be thought of as consisting of rows and columns or fields...
, and the meaning of the value contained in a flag will generally be defined in relation to the data structure it is part of. In many cases, the binary value of a flag will be understood to represent one of several possible states or statuses. In other cases, the binary values may represent one or more attributes in a bit field
Bit field
A bit field is a common idiom used in computer programming to compactly store multiple logical values as a short series of bits where each of the single bits can be addressed separately. A bit field is most commonly used to represent integral types of known, fixed bit-width. A well-known usage of...
, often related to abilities or permissions, such as "can be written to" or "can be deleted". However, there are many other possible meanings that can be assigned to flag values. One common use of flags is to mark or designate data structures for future processing.
Within microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
s and other logic devices, flags are commonly used to control or indicate the intermediate or final state or outcome of different operations. Microprocessors typically have, for example, a status register
Status register
A status register or flag register is a collection of flag bits for a processor. An example is the FLAGS register of the x86 architecture....
that is composed of such flags, and the flags are used to indicate various post-operation conditions, such as when there has been an arithmetic overflow
Arithmetic overflow
The term arithmetic overflow or simply overflow has the following meanings.# In a computer, the condition that occurs when a calculation produces a result that is greater in magnitude than that which a given register or storage location can store or represent.# In a computer, the amount by which a...
. The flags can be utilized in subsequent operations, such as in processing conditional jump instruction
Branch (computer science)
A branch is sequence of code in a computer program which is conditionally executed depending on whether the flow of control is altered or not . The term can be used when referring to programs in high level languages as well as program written in machine code or assembly language...
s. For example a je (Jump if Equal) instruction in the X86 assembly language will result in a jump if the Z (zero) flag was set by some previous operation.
A command line switch is also referred to as a flag. Command line
Command-line interface
A command-line interface is a mechanism for interacting with a computer operating system or software by typing commands to perform specific tasks...
programs often start with an option parser
Parsing
In computer science and linguistics, parsing, or, more formally, syntactic analysis, is the process of analyzing a text, made of a sequence of tokens , to determine its grammatical structure with respect to a given formal grammar...
that translates command line switches into flags in the sense of this article.
See also
- Status registerStatus registerA status register or flag register is a collection of flag bits for a processor. An example is the FLAGS register of the x86 architecture....
- Flag byte
- FLAGS register (computing)FLAGS 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...
- Program status wordProgram status wordThe Program status word is an IBM System/360 architecture and successors control register which performs the function of a Status register in other architectures, and more....
- Control registerControl registerA control register is a processor register which changes or controls the general behavior of a CPU or other digital device. Common tasks performed by control registers include interrupt control, switching the addressing mode, paging control, and coprocessor control.-CR0:The CR0 register is 32 bits...