In an 8085 based system, how is the programmable interrupt controller 8259 typically interfaced to the microprocessor?

Introduction / Context:
The Intel 8259 programmable interrupt controller is designed to manage multiple interrupt sources and present them in a prioritized way to a microprocessor such as the 8085 or 8086. Understanding how the 8259 connects to the interrupt pins of the 8085 is important for designing interrupt driven systems. This question focuses on which interrupt input is appropriate for interfacing the 8259, especially in relation to the special TRAP pin.

Given Data / Assumptions:

• The microprocessor is the Intel 8085, which has several interrupt pins including TRAP, RST 7.5, RST 6.5, RST 5.5, and INTR.
• The peripheral is the Intel 8259 programmable interrupt controller.
• The 8259 can generate a maskable interrupt request to the CPU.
• TRAP on the 8085 is a non maskable and edge or level sensitive interrupt with special properties.

Concept / Approach:
The 8259 is designed to present prioritized maskable interrupt requests to the CPU. On the 8085, the INTR pin is the general interrupt request input for external devices that are serviced through the external interrupt acknowledge and restart instruction mechanism. The 8259 output INT is therefore connected to INTR. TRAP, on the other hand, is a non maskable interrupt line with the highest priority that is not intended to be driven by a programmable interrupt controller. Using TRAP for 8259 output would defeat the masking and prioritization features of the controller.

Step-by-Step Solution:
Step 1: Recall that the 8085 INTR pin is used for general maskable interrupts from external hardware.Step 2: Recognize that the 8259 is built to consolidate multiple maskable interrupts into one prioritized output signal.Step 3: Understand that the 8259 output INT should therefore be wired to the 8085 INTR input, allowing the CPU to respond with interrupt acknowledge cycles.Step 4: Note that TRAP is a non maskable interrupt with fixed highest priority and is usually reserved for critical error or emergency conditions.Step 5: Conclude that the correct interfacing method is to connect the 8259 output to INTR, not to TRAP.

Verification / Alternative check:
Standard reference designs and textbooks on microprocessor interfacing show the 8259 connected to INTR for the 8085. They also describe how the interrupt acknowledge cycle allows the 8259 to place an appropriate restart instruction on the data bus. TRAP is described separately as a non maskable interrupt that is often tied to signals such as power failure or watchdog timer triggers, not to general interrupt controllers. These circuit examples confirm the correct connection.

Why Other Options Are Wrong:
Option B is incorrect because TRAP is non maskable and should not be used as the normal input for a programmable controller that manages maskable interrupts. Option C is wrong because the READY pin is used to insert wait states when slow memory or peripherals need more time; it is not used for interrupts. Option D is incorrect because the 8259 was specifically designed to work with microprocessors like the 8085, and many systems successfully interface them.

Common Pitfalls:
A frequent misunderstanding is to think that the highest priority interrupt, TRAP, should be used for any important interrupt controller signal. However, this ignores the distinction between maskable and non maskable interrupts. Another pitfall is confusing the functions of INTR and the vectored RST interrupt pins. INTR is the correct input for a programmable controller that supports vectoring via interrupt acknowledge cycles.

Final Answer:
In an 8085 system, the 8259 is normally connected to the 8085 INTR input pin as a maskable interrupt request source, not to the TRAP pin, which is option A.