8086 Processor Control Instruction

8086 Processor Control Instruction:

The 8086 Processor Control Instruction are namely,

  • STC
  • CLC
  • CMC
  • STD
  • CLD
  • STI
  • CLI

STC Instruction :

This instruction sets the carry flag, STC does not affect any other flag.

CLC Instruction :

This instruction resets the carry flag to zero. CLC does not affect any other flag.

CMC Instruction :

This instruction complements the carry flag. CMC does not affect any other flag.

STD Instruction :

This instruction is used to set the direction flag to one so that SI and/or DI can be decremented automatically after execution of string instructions. STD does not affect any other flags,

CLD Instruction :

This instruction is used to reset the direction flag to zero, so that SI and/or DI can be incremented automatically after execution of string instructions. CLD does not affect any other flag.

STI Instruction :

This 8086 Processor Control Instruction sets the interrupt flag to one. This enables INTR interrupt of the 8086. STI does not affect any other flag.

CLI Instruction :

This instruction resets the interrupt flag to zero. Due to this 8086 Processor Control Instruction will not respond to an interrupt signal on its INTR input. CLI does not affect any other flag.

8086 Interrupt Instructions :

INT Instruction : INT Type

This instruction causes the 8086 to call a far procedure. The term type in the instruction refers to a number between 0-255 which identifies the interrupt. The address of the procedure is taken from the memory whose address is four times the type number.

INTO Instruction :

If the overflow flag is set, this instruction will cause the 8086 to do an indirect far call to a procedure you write to handle overflow condition. To do call the 8086 will read a new value for IP from address 00010H and a new value of CS from address 00012H.

IRET Instruction :

The IRET instruction is used at the end of the interrupt service routine to return execution to the interrupted program. The 8086 copies return address from stack into IP and CS registers and the stored value of flags back to the flag register.

Note : The RET instruction does not copy the flags from the stack back to the flag register.

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