ISA/AT architecture — How many DMA channels does a standard AT-class PC provide (combined 8-bit and 16-bit controllers)?

Introduction / Context:
Direct Memory Access (DMA) channels enable devices to transfer data to/from memory without heavy CPU involvement. AT-class systems combined two DMA controllers to support both 8-bit and 16-bit transfers.

Given Data / Assumptions:

• Legacy ISA/AT architecture with cascaded DMA controllers.
• Counting all channels, including the cascade line.
• Standard IBM-AT mapping.

Concept / Approach:

The original PC/XT used an 8237 DMA controller with four channels (0–3). The AT added a second controller for 16-bit DMA (5–7), with channel 4 used for cascading, yielding eight numbered channels in total (0–7).

Step-by-Step Solution:

Verification / Alternative check:

Hardware references and OS device settings from the era enumerate these channels explicitly. Many sound cards and SCSI cards used 5, 6, or 7.

Why Other Options Are Wrong:

3 and 4 correspond to subsets; 7 ignores the cascade channel’s presence in the numbering; “None” is incorrect.

Common Pitfalls:

Confusing the number of “usable” channels with the total numbered channels; forgetting that channel 4 is reserved for cascading.

Final Answer:

8.