Legacy COM Port I/O Base Addresses What is the standard I/O base address for COM1 on IBM PC–compatible systems?

Difficulty: Easy

Correct Answer: 3F8

Explanation:


Introduction / Context:
Identifying default I/O base addresses and IRQs for serial ports is essential when configuring or troubleshooting legacy hardware and embedded systems. COM1/COM2 assignments remain common knowledge in certification exams and service work on older equipment.


Given Data / Assumptions:

  • IBM PC–compatible hardware conventions.
  • COM1 and COM2 are the first two serial ports.
  • We are interested in the hexadecimal I/O base for COM1.


Concept / Approach:

By convention, COM1 uses base address 3F8h (IRQ4), and COM2 uses 2F8h (IRQ3). Other addresses like 3E8h and 5F8h are sometimes used for additional ports (COM3, COM4) but not the primary COM1. 3BCh corresponds to a legacy parallel port (LPT1 on some early systems).


Step-by-Step Solution:

Recall standard mapping: COM1 → 3F8h; COM2 → 2F8h.Scan options for 3F8.Select 3F8 as the correct I/O base.


Verification / Alternative check:

BIOS setup screens and OS device managers on legacy systems display COM1 at 3F8h. Technical references (datasheets for 16550 UART) confirm this mapping.


Why Other Options Are Wrong:

3E8 and 5F8 are often assigned to COM3/COM4. 3BC belongs to a parallel port. “008” is not a valid serial base address.


Common Pitfalls:

Confusing COM and LPT addresses or mixing decimal and hexadecimal representations.


Final Answer:

3F8

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