PC architecture (AT class): How many DMA (Direct Memory Access) channels does an AT-class PC implement in total?

Introduction / Context:
Direct Memory Access (DMA) allows peripherals to transfer data to or from system memory without continuous CPU intervention. Legacy PC designs standardized the count and usage of DMA channels, knowledge of which is still useful when dealing with retro hardware or certification exams.

Given Data / Assumptions:

• The question refers to IBM PC/AT architecture.
• Standard dual 8237-compatible DMA controller arrangement is used.
• Some channels are reserved for specific hardware tasks.

Concept / Approach:

An AT-class PC combines two cascaded 8237 DMA controllers to provide eight channels numbered 0 through 7. Channels are divided into low and high byte/word transfer groups, and certain channels are typically reserved (for example, channel 4 often serves as the cascade link). Understanding the total channel count (8) and their typical use cases helps diagnose conflicts and configure legacy sound or storage devices.

Step-by-Step Solution:

Verification / Alternative check:

AT documentation and OS device manager listings from the era enumerate eight channels. Setup screens in legacy BIOS also display channels 0–7 when configuring ISA devices.

Why Other Options Are Wrong:

• 3 / 4 / 7: Do not represent the full, standard AT total.
• None of the above: Incorrect because the standard total is eight.

Common Pitfalls:

Confusing total channels with user-available channels; assuming modern PCs still expose ISA-style DMA in the same way.

Final Answer:

8