Flag Registers In 8086

Flag Register:-

It is a 16-bit register with each bit corresponding to a flip-flop. It changes its status according to the stored output in the accumulator. A flag can control certain operation of the EU.

Active flag:- There are nine active flags out of 16, in the 8086 flag register. The remaining are undefined flag.

Control Flag:- Out of nine active flags, six are conditional flags and the remaining three are called as the control flag. The three control flags are:

1. The Trap flag(TF)

2. The interrupt flag(IF)

3. The direction flag(DF)

The Trap Flag:

· Setting TF puts the processor into single step mode for debugging, In single stepping microprocessor executes a instruction and enters into single step ISR.
· If TF=1, the CPU automatically generates an internal interrupt after each instruction, allowing a program to be inspected as it executes instruction by instruction.

The Interrupt Flag:-

· If IF=1, the CPU will recognize external interrupt request (Interrupt Disabled). If IF=0, then interrupt disabled.
· Clearing IF disables these interrupts.
· IF has no effect on either non-maskable external or internally generated interrupt.

The Direction Flag:-

· This bit is specially for string instructions.
· If DF=1, the string instruction will automatically decrement the pointer. If DF=0,the string instruction will automatically increment the pointer.