WDMA (computer)

The Word DMA (WDMA) interface was the fastest method used to transfer data between the computer (through the ATA controller) and an ATA device until UDMA. Single/Multiword DMA took over from PIO as the choice of interface between ATA devices and the computer. The WDMA interface is grouped into different modes.

The explanation for the difference between single and multiword DMA can be found in how the Intel 8237 DMA chip works. 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 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 nsATA-1
Single 14.216480 nsATA-1
Single 28.316240 nsATA-1
Multi 04.216480 nsATA-1
Multi 113.316150 nsATA-2
Multi 216.716120 nsATA-2
Multi 32016100 nsCompactFlash 2.1
Multi 4251680 nsCompactFlash 2.1

See also

  • PIO - The first interface type used between devices (mainly hard disks) and the computer.
  • ATA the category
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