The status register, also known as the flag register, program status word, and condition code register, is defined as a set of flag bits within a processor. A register is a processor circuit and is much like a memory location, meaning data can be written to and read from it. Unlike a memory location, the status register does not usually have an address because the microprocessor uses it internally. In an 8-bit central processing unit (CPU), a status register bit, equal to the number 1, or clear, equal to the number 0, can be set for various results of processor operation. The processor sometimes sets or clears the bits itself, but other times a specific program instruction sets or clears the bits.
Woman doing handstand with a computer
Status register bits are also called flags or flag bits and are used by the programmer for certain programming purposes. Each flag in a status register has a unique purpose. The carry flag is set if a previous operation overflowed the seventh bit, or underflowed, or overflowed the carry flag. It is set during logical, comparison, and arithmetic changes. The zero flag is set if the result of the most recent operation was 0.
A flag called “interrupt disable” operates by enabling or disabling the operation of interrupts, which are instructions that temporarily interrupt certain operations so that other operations can be performed. When this particular flag is set, interrupts are not allowed to operate, but when it is cleared, interrupts are allowed. Another flag called the decimal flag allows the processor to follow a more advanced binary mode to perform perfect arithmetic equations. When the flag is set, it uses this advanced binary mode. Another register bit is the break flag bit, which is set when the Force Break (BRK) command is executed.
An overflow flag is set during arithmetic operations if an operation produces an invalid result. The negative flag is set if the result of the most recent operation has it set to 1. One last flag, called bit 5, has no name and is always set to 1. This particular bit is essentially not used by programmers.
The FLAGS register is an example of a status register that was used in certain central processing units and contained the current states of a processor. It was 16 bits wide and was succeeded by EFLAGS and RFLAGS, a 32-bit register and a 64-bit register, respectively. The FLAGS register, however, had some flags that were different from the original 8-bit register, including the parity flag, auxiliary flag, and capture flag.
