Legacy PC serial ports (I/O mapping): Which I/O base address is traditionally assigned to COM4 on IBM PC/AT–compatible systems?

Introduction / Context:
Classic DOS/Windows systems mapped serial ports to fixed I/O base addresses. Understanding these defaults is useful when configuring legacy hardware, embedded consoles, or troubleshooting IRQ and port conflicts.

Given Data / Assumptions:

• IBM PC/AT–style serial port conventions are assumed.
• COM ports are RS-232 interfaces mapped to standard base addresses.
• Hexadecimal notation uses suffix H.

Concept / Approach:

The conventional assignments are: COM1 = 3F8H, COM2 = 2F8H, COM3 = 3E8H, and COM4 = 2E8H. These addresses pair with typical IRQs (COM1/3 often IRQ4, COM2/4 often IRQ3), though systems could reconfigure them. Therefore, COM4 corresponds to 2E8H.

Step-by-Step Solution:

Verification / Alternative check:

Legacy BIOS setup screens, DOS MODE command outputs, and motherboard manuals confirm these canonical base addresses for serial ports.

Why Other Options Are Wrong:

• 2F8H and 3F8H: Assigned to COM2 and COM1 respectively.
• 3E8H: Assigned to COM3.
• None of the above: Incorrect because 2E8H is correct.

Common Pitfalls:

Mixing up COM3 and COM4 addresses; assuming modern systems still use fixed ISA mappings; overlooking that some multi-I/O cards allowed remapping.

Final Answer:

2E8H