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Direct Memory Access (DMA) improves system performance by
allowing external devices to transfer information directly to or from
the PC's memory without using the CPU. The PCM-3718H/3718HG's
DMA capability significantly improves the system performance in
high-speed A/D applications.
The 8237 DMA controller chip on the PC system board handles the
DMA operation. This chip has four prioritized direct memory access
channels. Channel 0 is reserved by the PC system to refresh its
dynamic RAM. Channel 2 supports floppy disk operations. Channel
3 is generally used for hard disk operations. Channel 1 is not for any
internal operations and is reserved for your applications.
Each channel has two associated control signals. The DMA Request
Signal (DRQ) triggers a DMA operation, and the DMA Acknowledge
Signal (DACK) authorizes the 8237 to start the data transfer.
The 8237 DMA chip has four operating modes (single, demand, block
and cascade) and four control registers. These registers are:
1. Operation mode register (set mode of operation)
2. Address register (specify memory segment starting address)
3. Word count register (specify the number of transfers)
4. Initialization register (enable and disable DMA channels)
You must properly set all four registers before requesting the DMA
operation.
DMA transfer is a powerful but complicated operation. Related
descriptions of the DMA transfer have been covered in other para-
graphs of this manual ( for details please see pages 38~40). The
following steps summarize how to use DMA transfer with the PCM-
3718H/3718HG:
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PCM-3718H/3718HG User's Manual
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