www.pcguide.com/ref/hdd/if/ide/modesUDMA-c.html
As a result, focus was placed on the use of direct memory access (DMA) mod es. In particular, bus mastering DMA on the PCI bus became mainstream du e to its efficiency advantages.
read the description of the various DMA modes and how bus mastering DMA work s; Of course, hard disks get faster and faster, and the maximum speed of mul tiword DMA mode 2, 167 MB/s, quickly became insufficient for the fastes t drives. However, the engineers who went to work to speed up the interf ace discovered that this was no simple task. The IDE/ATA interface, and the flat ribbon cable it used, were designed for slow data transfer--abo ut 5 MB/s. Increasing the speed of the interface (by reducing the cycle time) caused all sorts of signaling problems related to interference. So instead of making the interface run faster, a different approach had to be taken: improving the efficiency of the interface itself. The result was the creation of a new type of DMA transfer modes, which were called Ultra DMA modes. The key technological advance introduced to IDE/ATA in Ultra DMA was doub le transition clocking. Before Ultra DMA, one transfer of data occurred on each clock cycle, triggered by the rising edge of the interface clock (or "strobe"). With Ultra DMA, data is transferred on both the rising a nd falling edges of the clock.
In order to improve the integrity of this now faster interface, Ultra DMA also introduced the use of cyclical redundancy checking or CRC on the i nterface. The device sending data uses the CRC algorithm to calculate re dundant information from each block of data sent over the interface. On the other end of the interf ace, the recipient of the data does the same CRC calculation and compare s its result to the code the sender delivered. If there is a mismatch, t his means data was corrupted somehow and the block of data is resent.
The first implementation of Ultra DMA was specified in the ATA/ATAPI-4 st andard and included three Ultra DMA modes, providing up to 33 MB/s of th roughput. Several newer, faster Ultra DMA modes were added in subsequent years. This table shows all of the current Ultra DMA modes, along with their cycle times and maximum transfer rates: Ultra DMA Mode Cycle Time (nanoseconds) Maximum Transfer Rate (MB/s) Defining Standard Mode 0 240 167 ATA/ATAPI-4 Mode 1 160 250 ATA/ATAPI-4 Mode 2 120 333 ATA/ATAPI-4 Mode 3 90 444 ATA/ATAPI-5 Mode 4 60 667 ATA/ATAPI-5 Mode 5 40 1000 ATA/ATAPI-6?
I'm not sure why they even bothered with mode 0, pe rhaps for compatibility. Ultra DMA mode 5 is the latest, and is implemen ted in all currently-shipping drives.
regular DMA" multiword mode 2, which has half that maximum. N ow, you may be asking yourself: if they had to go to double transition c locking to get to to 333 MB/s, how did they get to 66 MB/s, and then 10 0 MB/s? Well, they did in fact speed up the interface after all. Without double transition clocking , Ultra DMA mode 5 would have required a cycle time of 20 nanoseconds in stead of 40, making implementation much more difficult.
To use Ultra DMA modes over 2, a special, 80-conductor IDE c able is required. This cable uses the same 40 pins as the old cables, bu t adds 40 ground lines between the original 40 signals to separate those lines from each other and prevent interference and data corruption.
There are several specific requirements for running Ultra DMA: 1 Hard Disk Support: The hard disk itself must support Ultra DMA In ad dition, the appropriate Ultra DMA mode must be enabled on the drive. This can be either the interface controller built into the motherboard, or an add-in IDE/ATA interface card. If an 80-conductor cable is not detected by the system, 66 MB/s or 100 MB/s operation will be disabled.
On new systems there are few issues with running Ultra DMA, because the h ardware is all new and designed to run in Ultra DMA mode. In theory, new drives should be bac kwards compatible with older controllers, and putting an Ultra DMA drive on an older PC should cause it to automatically run in a slower mode, s uch as PIO mode 4 Unfortunately, certain motherboards don't function we ll when an Ultra DMA drive is connected, and this may result in lockups or errors. A BIOS upgrade from the motherboard manufacturer is a good id ea, if you are able to do this. Otherwise, you may need to use a special Ultra DMA software utility (available from the drive manufacturer) to t ell the hard disk not to try to run in Ultra DMA mode.
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