Hard drive technologies

Hard drive technologies

Five types of hard drive technologies have been used in PCs over the years:

l ST506

l ESDI (Enhanced Small Device Interface)

l IDE (Integrated Drive Electronics)

l EIDE (Enhanced Integrated Drive Electronics)

l SCSI (Small Computer System Interface)

ST506 and ESDI are outdated hard drive technologies, along with the AT computer in which they were used. Most of the PCs in use today use either an IDE/EIDE or a SCSI hard drive.

IDE protocols and modes

Remember The ATA IDE interface standard defines a variety of features and translation modes that are used to interact with the disk drive and the internal systems of the PC. Here are the two you should be aware:

l PIO (programmed input/output) modes: This is the standard protocol used to transfer data over an ATA IDE interface. Five different PIO rates or modes exist, each with a different maximum data transfer rate, expressed in megabytes per second (MBps). Transfer rates range from 3.3 MBps (Mode 0) to 11.1 MBps (Mode 3) to 16.6 MBps (Mode 4). Modes 0, 1 (5.2 MBps), and Mode 2 (8.3 MBps) are supported by all ATA IDE standards. Modes 3 and 4 are supported only by the ATA-2.

l DMA (direct memory access) modes: This data transfer protocol, which is also called bus mastering, allows the hard drive's built-in controller to control the transfer of data into the PC's main memory without involving the CPU, as is the case with a PIO transfer. Don't confuse this with the DMA function of the same name, used with the ISA (industry standard architecture) bus structure usually included in the system chipset. The IDE DMA requires a PCI bus and works independently of any other DMA functions of the PC. IDE DMA is defined in a series of modes that each support a different maximum data transfer rate that range from the 2.1 MBps of the Single Word 0 mode to the 13.3 MBps of the Multiword 1 mode to the 33.3 MBps of the Multiword 3 or DMA-33 mode. All ATA IDE standards support DMA modes with speeds up to 4.2 MBps (Multiword 0), but DMA modes with speeds of 13.3 and above are supported only by ATA-2 and above standards (see "Moving ultra fast" later in this section).

A drive typically uses either PIO or DMA and rarely both. In fact, using both would be very inefficient because both the CPU and the disk controller vie to move data to and from RAM.