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Chapter 8 Direct Memory Access (DMA)
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. PCM-3718's DMA capability sig-
nificantly improves the system performance in high-speed A/D applica-
tions.
8.1 Introduction to the 8237 DMA Controller
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 Sig-
nal (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.
PCM-3718 Series User Manual
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Operation
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