WDMA (computer)

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The Word DMA (WDMA) interface was the fastest method used to transfer data between the computer (through the Advanced Technology Attachment (ATA) controller) and an ATA device until Ultra Direct Memory Access (UDMA) was implemented. Single/Multiword DMA took over from Programmed input/output (PIO) as the choice of interface between ATA devices and the computer. The WDMA interface is grouped into different modes.

In single transfer mode, only one word (16-bit) will be transferred between the device and the computer before returning control to the CPU, and later it will repeat this cycle, allowing the CPU to process data while data is transferred. In multiword transfer mode (block mode), once a transfer has begun it will continue until all words are transferred.

Two additional Advanced Timing modes have been defined in the CompactFlash specification 2.1. Those are Multiword DMA mode 3 and Multiword DMA mode 4. They are specific to CompactFlash. Multiword DMA is only permitted for CompactFlash devices configured in True IDE mode.

DMA Modes
ModeMax Transfer Rate (MB/s)BitsMin cycle time Defining standard
Single 02.116960 ns ATA-1
Single 14.216480 ns ATA-1
Single 28.316240 ns ATA-1
Multi 04.216480 ns ATA-1
Multi 113.316150 ns ATA-2
Multi 216.716120 ns ATA-2
Multi 32016100 ns CompactFlash 2.1
Multi 4251680 ns CompactFlash 2.1

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