On classic IBM compatible PCs, which hardware interrupt request (IRQ) line is commonly used by the system timer to generate regular clock ticks?

Introduction / Context:
Older IBM compatible PCs used a fixed set of interrupt request lines, called IRQs, to allow hardware devices to signal the CPU. Each important device was assigned a specific IRQ number. The system timer is a fundamental device that generates periodic clock ticks so that the operating system can manage time slices, scheduling and timekeeping. This question checks whether you remember which IRQ line the system timer uses in this classic architecture, a common fact in computer hardware general knowledge.

Given Data / Assumptions:

• The context is classic PC hardware with a programmable interval timer.
• The question asks which IRQ line the system timer commonly uses.
• Options list IRQ 0, IRQ 1, IRQ 3, IRQ 4 and IRQ 7 with typical device associations.
• We assume the standard mapping used in many DOS era and early Windows systems.

Concept / Approach:
In traditional PC designs, the system timer is connected to IRQ 0. This timer generates regular interrupts, often around 18.2 times per second in very old systems, allowing the operating system to update the system clock and manage multitasking. IRQ 1 is normally assigned to the keyboard, IRQ 3 and IRQ 4 are used for serial ports COM2 and COM1, and IRQ 7 is often associated with the first parallel printer port. Therefore, when the question asks which IRQ the system timer uses, the correct answer is IRQ 0, the dedicated timer interrupt line.

Step-by-Step Solution:

Verification / Alternative check:
PC hardware reference tables list standard IRQ assignments. In those tables, IRQ 0 is consistently labelled system timer, IRQ 1 is keyboard, IRQ 3 and IRQ 4 are used by COM ports, and IRQ 7 is associated with LPT1, the first printer port. Numerous training materials and A plus style certification courses teach these mappings as facts to memorise. This stable and widely documented mapping confirms that the system timer uses IRQ 0, not any of the other listed options.

Why Other Options Are Wrong:

• IRQ 1 (keyboard interrupt): Reserved for the keyboard controller, not for the system timer.
• IRQ 3 (serial port interrupt): Typically used by the COM2 serial port rather than by the timer.
• IRQ 4 (serial port interrupt): Usually assigned to the COM1 serial port, not to the system timer.
• IRQ 7 (printer interrupt): Commonly associated with the first parallel printer port (LPT1).

Common Pitfalls:
Because many devices and IRQ numbers must be memorised, students sometimes mix up which device uses which line. A useful mnemonic is to associate zero with the fundamental timer that drives everything, making IRQ 0 easy to remember as the system time keeper. Keyboard events then become the next priority on IRQ 1. By keeping the most important devices at the lowest numbers in your mind, you can quickly recall that the system timer belongs to IRQ 0, and avoid confusing it with keyboard or serial port interrupts in exam questions.

Final Answer:
On classic PCs, the system timer commonly uses IRQ 0 (system timer interrupt) to generate regular clock ticks.