Interrupts 8259 PIC and 8086 Interrupts Quiz

Interrupts and Interrupt Responses

• Interrupts break the normal sequence of execution of instructions, diverting the CPU to an Interrupt Service Routine (ISR) and then returning to the main program.
• There are three sources of interrupts:
  • External signals applied to the Non-Maskable Interrupt (NMI) input pin or the Interrupt (INTR) input pin (hardware interrupts)
  • Execution of the Interrupt instruction, INT (software interrupts)
  • Error conditions produced by the execution of an instruction (e.g. divide by zero error)

Interrupt Procedure

• The interrupt procedure involves:
  • Pushing flags and clearing IF and TF
  • Pushing CS and IP on the stack
  • Fetching the ISR address
  • Executing the ISR procedure
  • Popping IP, CS, and flags from the stack
  • Returning to the main program using IRET

8259A Interrupt Operation

• To implement an interrupt, the interrupt enable flip-flop in the microprocessor should be enabled by writing the EI (8085) or STI (8086) instruction.
• The 8259A should be initialized by writing control words in the control register, including Initialization Command Words (ICWs) and Operational Command Words (OCWs).
• ICWs set up the proper conditions and specify vector address, while OCWs perform functions such as masking interrupts and setting up status-read operations.

Single 8259 Operation

• A single 8259 can accept 8 interrupts.
• When a device interrupts 8259, 8259 interrupts the microprocessor on the INTR pin.
• The sequence of events when an interrupt occurs includes:
  • Setting the corresponding bit in the Interrupt Request Register (IRR)
  • The Priority Resolver determining the highest priority interrupt
  • Storing the ISR address in the Interrupt Vector Table (IVT)

Non-Maskable Interrupt (NMI)

• The 8086 automatically performs a type 2 interrupt response when it receives a low-to-high transition on its NMI input pin.
• The type 2 interrupt response cannot be disabled (masked) by any program instructions.
• NMI is used to signal the 8086 that some condition in an external system must be taken care of.

Breakpoint Interrupt (INT 3)

• The type 3 interrupt is used to implement a breakpoint function in a system.
• A breakpoint is used to examine the CPU and memory after the execution of a group of instructions.
• When a breakpoint is inserted, the system executes the instructions up to the breakpoint and then goes to the breakpoint procedure.