Flag Registers of 8086

Posted By on September 21, 2014

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Assignment AP
Instruction formats of 8086

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.
Assignment AP
Instruction formats of 8086

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Posted by Akash Kurup

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