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.

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  1. explain each register briefly with examples if any.



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