What is function of status register?

The status register holds the values of "flags" - bits indicating information about the state of the processor. Usually the bits indicate one of three possible outcomes of an arithmetic function: zero, carry, or overflow.

A "Zero" flag means that the result of an operation was "zero" - for example adding equal positive and negative numbers or that a logical evaluation returned a FALSE result.

A "Carry" flag can be used to allow operations on a data element comprised of more than one "Word" by allowing an increment or decrement to be "carried" between a "Word" of lower significant value and a "Word" of higher significant value.

An "Overflow" flag is used to indicate that the results of an operation will not fit within the limits of a register width using twos complement representation.

Many systems also use the status register to indicate whether the result of an operation is negative or positive.

Some systems also have flags for overflow between 'nibbles' (half a byte), odd or even results, whether an operation is executing in 'supervisor mode', or interrupt enable bits.

All of these flags are normally set or cleared when an operation is completed. The register values can then be used to test for jumping/branching conditions.