Architecture of 8259A: Diagram

Discuss the actions performed by 8086 when an interrupt is encountered by it? How 8259A can be used for multiple interrupts priority management? Draw and briefly explain the internal architecture of 8259A programmable interrupt controller.

Actions performed by 8086 when an interrupt is encountered

When the 8086 processor encounters an interrupt, it follows a series of steps to handle it. First, it completes the execution of the current instruction and then saves the flags and the contents of the registers onto the stack. After that, it fetches the address of the interrupt service routine (ISR) from the interrupt vector table (IVT). The processor then transfers control to the ISR to execute the specific task associated with the interrupt. Once the ISR is completed, the processor restores the saved register values and flags from the stack and resumes normal program execution.

Using 8259A for multiple interrupts priority management

The 8259A programmable interrupt controller is employed to manage multiple interrupts with different priorities. It allows for the prioritization of interrupts so that higher priority interrupts can be serviced before lower priority ones. The 8259A has multiple interrupt request lines (IRQs), and each IRQ can be assigned a specific priority level. By programming the 8259A, you can control which interrupts are enabled, their priority levels, and the interrupt service routine addresses associated with each interrupt.

Internal architecture of 8259A programmable interrupt controller

The 8259A consists of several key components, including:

  • Interrupt Request Register (IRR): This register stores the status of the interrupt lines, indicating which interrupts are currently pending.
  • In-Service Register (ISR): It keeps track of interrupts that are currently being serviced.
  • Interrupt Mask Register (IMR): This register allows you to enable or disable specific interrupt lines.
  • Control Word Register: Used for programming the 8259A. It includes information about the operating mode, whether it is edge or level triggered, and how the priorities are set.
  • Data Bus Buffer: Facilitates communication between the CPU and the 8259A.
  • Cascade Buffer: Used when multiple 8259A chips are connected to handle more than eight interrupts.

The 8259A can be cascaded to manage up to 64 interrupts by connecting multiple chips. Each 8259A can handle eight interrupts, and the master-slave configuration allows for the expansion of interrupt handling capabilities.