XT-Class 8-bit ISA Bus: Number of IRQ Levels How many standard interrupt levels are provided on the original 8-bit ISA bus (XT-class computer)?

Introduction / Context:
Early IBM PC/XT systems used the 8-bit ISA bus with a single Programmable Interrupt Controller (PIC). Understanding the number of IRQ levels available clarifies device limitations and resource conflicts on vintage systems.

Given Data / Assumptions:

• IBM PC/XT architecture with one 8259A PIC.
• No cascaded second PIC as in later AT-class systems.
• We are counting standard, usable IRQ lines.

Concept / Approach:

The XT-class machine provides 8 interrupt request lines, IRQ0 through IRQ7, managed by a single 8259A PIC. Later AT-class systems (16-bit ISA) added a second PIC cascaded on IRQ2, providing IRQ0–IRQ15. Therefore, the correct number for the 8-bit XT system is 8.

Step-by-Step Solution:

Verification / Alternative check:

Technical references list IRQ0–IRQ7 for XT, and IRQ0–IRQ15 for AT due to the cascaded PIC pair. This distinction is foundational in legacy PC hardware documentation.

Why Other Options Are Wrong:

4: Too few; the PIC provides 8. 12 or 16: Reflect expanded systems (16 for AT), not XT. None of the above: Incorrect because 8 is correct.

Common Pitfalls:

Confusing XT and AT capabilities; forgetting that IRQ2 in AT is used for PIC cascade and remapped to IRQ9 for devices.

Final Answer:

8