The status register – also known as the flag register, program status word, and condition code register – is defined as a collective of flag bits within a processor. A register is a processor circuit and is much like a memory location, which means that data can be written to and read from it. Unlike a memory location, the status register usually does not have an address because the microprocessor uses it internally. In an 8-bit central processing unit (CPU), a status register bit can be set, equal to the number 1, or cleared, equal to the number 0, 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 record has a unique purpose. The transport flag is set if a previous operation overflowed the seventh bit – or underflow – or underflowed the carry flag. It is set during logic, comparison and arithmetic changes. The zero flag is set if the most recent operation result was 0.
A flag called “disable interrupt” operates by allowing or disallowing 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 interrupt flag bit, which is set when the Force Interrupt (BRK) command is executed.
An overflow flag is set during arithmetic operations if an operation yields 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, is unnamed and is always set to 1. Programmers essentially have no use for this particular bit.
The FLAGS register is an example of a status register that was used on certain central processor 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 trap flag.