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Machine Control Instructions (Part-II) - EI, DI, NOP, HLT

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EI:
  • This instruction is used to set the interrupt enable flip flop so that all maskable interrupts are enabled.
  • After an interrupt is acknowledged, the interrupt enable flip flop is reset.
  • This instruction is used to re-enable the interrupts.
OPERATION: IE (FF) = 1
  • No flags are affected.
  • It is a one byte instruction.
  • Implied addressing mode is used.
DI:
  • This instruction is used to reset the interrupt enable flip flop so that all maskable interrupts are disabled.
  • This instruction cannot disable TRAP since TRAP is a non-maskable interrupt.
OPERATION: IE (FF) = 0
  • No flags are affected.
  • It is a one byte instruction.
  • Implied addressing mode is used.
NOP:
  • This instruction shows that no operation is performed.
OPERATION: PC = PC + 1
  • No flags are affected.
  • It is a one byte instruction.
  • Implied addressing mode is used.
HLT:
  • This instruction is used to halt the processor.
  • The processor can be restarted by a valid interrupt or by applying a RESET signal.
OPERATION: Stop
  • No flags are affected.
  • It is a one byte instruction.
  • Implied addressing mode is used.
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