What is dma hard drives ?

[NAEVA]

The direct memory access or DMA (acronym for Direct Memory Access) is a computer where data traveling to or from a device (communication port, hard disk) is transferred directly by a controller adapted to the main memory of the machine, without intervention of microprocessor if it is to initiate and conclude the transfer. The conclusion of the transfer or availability of the device can be indicated by interruption.


The DMA is required to maintain the fluidity of using a multi-tasking when accessing devices fast as hard drives. In the absence of DMA, the system is almost blocked during data transfers. In addition, devices for rapid, it is impossible to transfer each data by an interrupt. An alternative is that the device has a memory buffer shared with the system, whose filling is indicated by an interruption. This arrangement, used on some network controllers, however, induces an additional cost due to the additional copy to be done.

[Taban]

During the DMA operation, it enters sometimes in conflict with the processor, memory can not be carried out by defining an access cycle (since this is the very definition of the cycle). DMA can not necessarily wait until the processor (which is unlikely, however, the loss of information theft), it has priority access technique is called cycle stealing.

[Galvin]

When the device that makes the DMA can send its own data on the bus system, and scatter-gather when a transfer may involve a succession of several memory areas. This last feature is useful for systems with virtual memory as a buffer located in contiguous addresses in virtual memory is usually composed of several blocks of physical memory.

[HAMAKO]

Direct Memory Access (DMA) is the theoretical data access rate of a Drive measured in megabytes per second. Thus, an Ultra DMA-100 Hard Drive will theoretical access data at a rate of 100 megabytes per second and an Ultra DMA-133 Drive will theoretical access data at a rate of 133 megabytes per second. Therefore, this technical brief will show you how to enable Direct Memory Access (DMA) on your system.