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In other words, this breaks the 528MB barrier in essentially the same way as Extended CHS does. Because it is somewhat simpler to use a single linear number to address a sector on the hard disk compared to a CHS type address, this is the preferred method if the drive supports LBA.

A word of warning with these BIOS translation settings: If you switch between standard CHS, Extended CHS, or LBA, the BIOS may change the (translated) geometry. The same thing may happen if you transfer a disk that has been formatted on an old, non-LBA computer to a new one that uses LBA. This will cause the logical CHS geometry seen by the operating system to change, and will cause the date to appear in the wrong locations from where it actually is! This can cause you to lose access to your data if you are not careful. I always recommend recording the CMOS Setup screens associated with the hard disk configuration so that you can properly match the setup of a drive to the settings it was originally at.

Faster Data Transfer

ATA-2/EIDE defines several high-performance modes for transferring data to and from the drive. These faster modes are the main part of the new specifications and were the main reason they were initially developed. Most of the faster drives on the market today will support either PI/O transfer Mode 3 or Mode 4, which results in a very fast transfer. The following section discusses these modes.

The PIO mode determines how fast data is transferred to and from the drive. In the slowest possible mode, PIO mode 0, the data cycle time cannot exceed 600 nanoseconds (ns). In a single cycle, 16 bits are transferred in or out of the drive making the theoretical transfer rate of PIO Mode 0 (600 ns cycle time) 3.3MBps. Most of the high-performance ATA-2 (EIDE) drives today support PIO Mode 4, which offers a 16.6MBps transfer rate.

The following table shows the PIO modes, with their respective transfer rates:

PIO Mode Cycle Time (ns) Transfer Rate (M/sec) Specification

0 600 3.3 ATA
1 383 5.2 ATA
2 240 8.3 ATA
3 180 11.1 ATA-2
4 120 16.6 ATA-2

To run in Mode 3 or 4 requires that the IDE port on the system be a local bus port. This means that it must operate through either a VL-Bus or PCI bus connection. Some newer motherboards with ATA-2/EIDE support have dual IDE connectors on the motherboard, with only the primary connector running through the system's PCI local bus. The secondary connector usually runs through the ISA bus, and therefore supports up to Mode 2 operation only. Make sure you get clarification of this before you purchase that new motherboard!

When interrogated with an Identify Drive command, a hard disk returns, among other things, information about the PIO and DMA modes it is capable of using. Most enhanced BIOSs will automatically set the correct mode to match the capabilities of the drive. If you set a mode faster than the drive can handle, data corruption will result.

ATA-2 drives also perform Block Mode PIO, which means that they use the Read/Write Multiple commands that greatly reduce the number of interrupts sent to host processor. This lowers the overhead, and the resulting transfers are even faster.

DMA Transfer Modes

Although it is not used by most operating system or BIOS software, ATA-2 drives also support Direct Memory Access transfers. DMA means that the data is transferred directly between drive and memory without using the CPU as an intermediary, as opposed to PIO.

There are two distinct types of direct memory access: DMA and busmastering DMA. Ordinary DMA relies on the DMA controller on the system's mainboard to perform the complex task of arbitration, grabbing the system bus, and transferring the data. In the case of busmastering DMA, all this is done by logic on the interface card itself. Of course, this adds considerably to the complexity and the price of a busmastering interface.

Unfortunately, the DMA controller on ISA systems is ancient and slow, and out of the question for use with a modern hard disk. Today, proper software support for DMA is still rare.

The newest development in DMA transfer modes is Ultra-DMA which can speed data transfer up to 33M/sec with an Ultra-DMA drive.

Many of these latest IDE versions require not only correct drive and BIOS support but chipset support as well. While there are many companies who make chipsets, Intel chipsets are found in most modern motherboards. The following table lists the recent Intel chipsets, which processors they are used with, and which recent IDE ATA extensions they support.

Chipset Processor ATA IDE Extensions Supported

430VX Pentium Bus Mastering
430TX Pentium Ultra DMA 33
430HX (Triton 2) Pentium Bus Mastering
430FX (Triton) Pentium Bus Mastering
430MX Pentium
440FX Pentium Pro Bus Mastering
or Pentium II
450GX Pentium Pro *
450KX Pentium Pro *
440LX Pentium II Bus Mastering and DMA 33

* Some motherboards with this chipset support bus mastering IDE

ATAPI (ATA Packet Interface)

ATAPI is a standard designed for devices such as CD-ROMs and tape drives that plug into an ordinary ATA (IDE) connector. The principal advantage of ATAPI hardware is that it's cheap and works on your current adapter. For CD-ROMs, it has a somewhat lower CPU usage compared to proprietary adapters, but there's no performance gain otherwise. For tape drives, ATAPI has potential for superior performance and reliability compared to the popular "floppy" tape devices.

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