Flag byte
Encyclopedia
A flag word is a generic term for a word (value) used to indicate conditions within a binary
computer. In particular, the byte can be used to show up to 8 discrete conditions. Essentially, each bit
represents one flag
, capable of showing two states.
A flag word can be seen as a bit array with a short, constant length.
processor's status register
, the following information was held within 8 bits:
As you can see, a lot of information about the state of the processor can be packed into 8 bits.
exit status
as a flag word. In this case, the exit code is by the programmer to pass status information to another process. An imaginary program which returns the status of 8 burglar alarm switches connected to the printer port could set each of the bits in the exit code in turn, depending on whether the switches are closed or open.
the status byte with a mask byte
. The mask byte should have only the bit corresponding to the flag you want to read set, as in the example below.
Suppose that status byte 103 (decimal) is returned, and that we want to check flag bit 5.
The flag we want to read is number 5 (counting from zero) - so the mask byte will be
. ANDing 32 with 103 gives 32, which means the flag bit is set. If the flag bit was not set, the result would have been 0.
In modern computing, the shift
operator (<<) can be used to quickly perform the power-of-two. In general, a mask with the Nth bit set can be computed as
(1 << n)
Thus to check the Nth bit from a variable v, we can perform the operation
bool nth_is_set = (v & (1 << n)) != 0
To set a bit, OR the status byte with a mask byte. Any bits set in the mask byte or the status byte will be set in the result.
int setBit(int val, int bit_position)
{
return val | (1 << bit_position);
}
To clear a bit, perform a NOT
operation on the mask byte, then AND it with the status byte. The result will have the appropriate flag cleared (set to 0).
int clearBit(int val, int bit_position)
{
return val & ~(1 << bit_position);
}
To toggle a bit, XOR
the status byte and the mask byte. This will set a bit if it is cleared or clear a bit if it is set.
int toggleBit(int val, int bit_position)
{
return val ^ (1 << bit_position);
}
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...
computer. In particular, the byte can be used to show up to 8 discrete conditions. Essentially, each 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...
represents one flag
Flag (computing)
In computer programming, flag can refer to one or more bits that are used to store a binary value or code that has an assigned meaning, but can refer to uses of other data types...
, capable of showing two states.
A flag word can be seen as a bit array with a short, constant length.
Processor status register
Taking the example of a 6502MOS Technology 6502
The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured microprocessor on the market by a considerable margin, costing less than one-sixth the price of...
processor's 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....
, the following information was held within 8 bits:
- Bit 7. Negative flag
- Bit 6. Overflow flag
- Bit 5. Unused
- Bit 4. Break flag
- Bit 3. Decimal flag
- Bit 2. Interrupt-disable flag
- Bit 1. Carry flag
- Bit 0. Zero flag
As you can see, a lot of information about the state of the processor can be packed into 8 bits.
Unix process exit code
A more general example would be the use of a UnixUnix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
exit status
Exit status
The exit status or return code of a process in computer programming is a small number passed from a child process to a parent process when it has finished executing a specific procedure or delegated task...
as a flag word. In this case, the exit code is by the programmer to pass status information to another process. An imaginary program which returns the status of 8 burglar alarm switches connected to the printer port could set each of the bits in the exit code in turn, depending on whether the switches are closed or open.
Extracting bits from flag words
To read a status byte, assuming your programming language does not offer this facility by default, is quite easy. You simply need to ANDLogical conjunction
In logic and mathematics, a two-place logical operator and, also known as logical conjunction, results in true if both of its operands are true, otherwise the value of false....
the status byte with a mask byte
Mask (computing)
In computer science, a mask is data that is used for bitwise operations.Using a mask, multiple bits in a byte, nibble, word can be set either on, off or inverted from on to off in a single bitwise operation.-Masking bits to 1:...
. The mask byte should have only the bit corresponding to the flag you want to read set, as in the example below.
Suppose that status byte 103 (decimal) is returned, and that we want to check flag bit 5.
The flag we want to read is number 5 (counting from zero) - so the mask byte will be
. ANDing 32 with 103 gives 32, which means the flag bit is set. If the flag bit was not set, the result would have been 0.In modern computing, the shift
Logical shift
In computer science, a logical shift is a bitwise operation that shifts all the bits of its operand. Unlike an arithmetic shift, a logical shift does not preserve a number's sign bit or distinguish a number's exponent from its mantissa; every bit in the operand is simply moved a given number of bit...
operator (<<) can be used to quickly perform the power-of-two. In general, a mask with the Nth bit set can be computed as
(1 << n)
Thus to check the Nth bit from a variable v, we can perform the operation
bool nth_is_set = (v & (1 << n)) != 0
Changing bits in flag words
Writing, reading or toggling bits in flags can be done only using the OR, AND and NOT operations - operations which can be performed quickly in the processor.To set a bit, OR the status byte with a mask byte. Any bits set in the mask byte or the status byte will be set in the result.
int setBit(int val, int bit_position)
{
return val | (1 << bit_position);
}
To clear a bit, perform a NOT
Negation
In logic and mathematics, negation, also called logical complement, is an operation on propositions, truth values, or semantic values more generally. Intuitively, the negation of a proposition is true when that proposition is false, and vice versa. In classical logic negation is normally identified...
operation on the mask byte, then AND it with the status byte. The result will have the appropriate flag cleared (set to 0).
int clearBit(int val, int bit_position)
{
return val & ~(1 << bit_position);
}
To toggle a bit, XOR
Exclusive disjunction
The logical operation exclusive disjunction, also called exclusive or , is a type of logical disjunction on two operands that results in a value of true if exactly one of the operands has a value of true...
the status byte and the mask byte. This will set a bit if it is cleared or clear a bit if it is set.
int toggleBit(int val, int bit_position)
{
return val ^ (1 << bit_position);
}
See also
- Bit fieldBit fieldA 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...
- Bit array
- Flag (computing)Flag (computing)In computer programming, flag can refer to one or more bits that are used to store a binary value or code that has an assigned meaning, but can refer to uses of other data types...
- 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....
- 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...