## Virtual Device Driver Reference for OS/2

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### Virtual Direct Memory Access Device Driver

The virtual direct memory access device driver (VDMA) provides per-DOS session per-direct memory access (DMA) channel virtualization. In addition, the virtual DMA device driver provides the following:

Serializes the DOS session's DMA requests on a per-DMA channel basis (but allows simultaneous requests on different DMA channels)

Converts the DOS session's linear addresses to physical addresses, and handles all the data movement required

Handles all the restrictions of a DMA chip

Supports the enhanced DMA operations and commands of a personal computer

Optimizes DMA virtualization on personal computers by using the enhanced mode

Supports INT 4Bh - Function 81h virtual DMA services

Supports virtualized DMA in auto-initialization mode

In addition to the above features, the virtual DMA device driver also synchronizes the operations with other virtual device drivers registered on different DMA channels. A virtual device driver for a piece of hardware, which uses a DMA channel, registers with the virtual DMA device driver. The virtual device driver informs these virtual device drivers before starting and stopping the DMA operation on their channels.