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The Complete Guide on DMA, UDMA |
DMA
Abbreviation of direct memory access, a technique
for transferring data from main memory to a device without passing it through
the CPU. Computers that have DMA channels can transfer data to and from
devices much more quickly than computers without a DMA channel can. This
is useful for making quick backups and for real-time applications.
Some expansion boards, such as CD-ROM cards, are capable of accessing the computer's DMA channel. When you install the board, you must specify which DMA channel is to be used, which sometimes involves setting a jumper or DIP switch.
A means of data transfer between the device and host memory without processor intervention. There are two DMA modes, Single and Multi-Word. Since single word is slower than PIO mode, no one uses it. It will be ignore it here. Multi Word DMA is used in EISA, VLB, and PCI equipped systems. They are capable of the very fast transfer rates, utilizing cycle times of 480ns or faster. The current ATA specification limit is 150ns. This flowchart show the virtual elimination of the CPU from the transfer process.
1.The system specifies where the data transfer is to begin, and how many sectors to transfer.
2.The system sends a Read/Write DMA command to transfer data.
3.The drive asserts DERQ to signify it is ready to transfer data.
4.The DMA controller asserts DACK- to indicate the bus is free to transfer data, and enables the memory address where the data needs to either come from or go to.
5.When IOR- or IOW- are asserted, the drive transmits or receives a word of data.
6.If the next word of data is ready to be transmitted/received, step 5 is repeated. If the data is not ready, the following occurs:
A.The drive negates DREQ to tell the DMA controller to temporarily halt the data transfer.
B.The DMA responds by negating DACK- and freeing up the bus to the CPU. Once the drive is ready to proceed, the process returns to step 3.
7.Once the entire data transfer is complete, the drive issues an interrupt
to tell the CPU the data is where it belongs.
Ultra DMA
A protocol developed by Quantum Corporation and Intel that supports
burst mode data transfer rates of 33.3 MBps. This is twice as fast as the
previous disk drive standard for PCs, and is necessary to take advantage
of new, faster Ultra ATA disk drives.
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The latest improvement of the EIDE device interface. Also known as
ATA/ATAPI-4, Ultra-ATA, or Ultra-DMA/33, this is the latest advancement
to the ATA specifications (draft): ATA/ATAPI-4. Which, among other improvements,
supports Ultra DMA. This interface has a theoretical maximum data transfer
rate (burst, not sustained) of 33MB/sec.
Ultra-DMA doubles the maximum transfer speed of the ATA-3 standard.
ATA-4 Extensions
Synchronous DMA Mode for Ultra DMA/33
Mode Cycle Times Transfer
Rate
0
235ns
16MB/s
1
160ns
24MB/s
2
120ns
33MB/s
As you can see, Ultra DMA/33 achieves nearly double the transfer rate of the existing ATA interface while not increasing the cycle time of the ATA bus clock over the rate currently used for PIO Mode 4.
This apparent miracle is achieved by:
1.Improving timing windows in the protocol on the ATA bus;
2.Reducing propagation delays by pipelining data transfers;
3.Transferring data in synchronous (strobed) mode;
In addition, Ultra DMA/33 improves data integrity by using a Cyclic Redundancy Check (CRC) to flag any data transfer errors that may be made over the ATA bus (Note: In this application CRC is only used to improve data integrity for ATA bus transfers, it is not used to improve the data integrity of either disk drives or host systems).
Ultra DMA/33 requires no physical change to the cable, receivers, or drivers of the ATA bus. Furthermore, Ultra DMA/33 protocol and commands have been designed to be compatible with existing ATA devices and systems. Thus, new Ultra DMA/33 devices will be backwards compatible with most older (non Ultra DMA/33) systems. Older ATA-1/2/3 devices will also be forwards compatible with systems that have Ultra DMA/33 capability.
Ultra DMA FAQ
http://www.maxtor.com/technology/q&a/30026.html
DMA Check Box Does Not Remain Checked
http://support.microsoft.com/support/kb/articles/q159/5/60.asp