DMA Resource for Floppy Controller On legacy PCs, the floppy disk controller (FDC) uses which ISA DMA channel for data transfers?
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A5
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B4
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C1
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D9
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E2
Answer
Correct Answer: 2
Explanation
Introduction / Context:ISA-era systems assign fixed DMA channels to certain controllers. Recognizing these defaults helps diagnose resource conflicts and configuration issues. The floppy disk controller is one of the classic examples with a well-known DMA assignment.
Given Data / Assumptions:
- Standard PC/AT mapping with dual 8237 DMA controllers (8-bit and 16-bit channels).
- A conventional floppy disk drive and controller are present.
- We are referring to ISA-based systems.
Concept / Approach:
The FDC conventionally uses 8-bit DMA channel 2 for data transfers. Other standard mappings include DMA channel 3 for LPT (in some modes) and higher channels for 16-bit devices. Remembering “FDC → DMA 2” is a common A+ and hardware exam fact.
Step-by-Step Solution:
Recall ISA DMA assignments: channel 2 is reserved for floppy operations.Cross-check typical I/O base of FDC (3F0–3F7) alongside the DMA channel.Select channel 2 as the correct choice.Verification / Alternative check:
View Device Manager on old systems or BIOS setup to confirm the FDC uses DMA 2. Vendor manuals also list this mapping.
Why Other Options Are Wrong:
Channels 1, 4, 5, and 9 are not the default for the floppy controller in ISA PCs. Hence, option “2” is uniquely correct.
Common Pitfalls:
Confusing IRQs (e.g., FDC uses IRQ6) with DMA channels, or mixing up printer DMA usage with floppy.
Final Answer:
2