4 Before buying...

There are a number of 'EIDE' features which can be desirable in a new interface. Some features, however, may already be present on your system or be unimportant to you.

• The least a modern interface should provide is PIO mode 3 transfers (up to 11.1MB/s) for drives that support it. It should also be able to use slower PIO modes (0, 1 and 2) to ensure compatibility with older drives.
• True direct memory access (DMA) is found mostly on interfaces integrated into mainboards, Intel 430*X based boards being the most common example. This will improve system performance in multitasking operating systems. Ultra-ATA will give you DMA/33, which actually isn't all that much faster but a lot safer for your data.
• Since ATAPI CD-ROMs have rapidly become very popular in the low-end market, and ATAPI tapestreamers are similarly taking off, a secondary port allowing you to connect a total of four ATA* devices is hardly a luxury. Note that a few modern soundcards provide a secondary or tertiary ATA/IDE interface instead of the traditional proprietary CD-ROM connectors. Beware of conflicts in combination with a dual-ported interface. If you intend to connect harddisks to the second port and use them with DOS or Windows 3.x, remember that many older BIOSes have no support for the secondary channel and many interfaces do not ship with the required drivers.
• You will want an on-card Enhanced BIOS too if your mainboard BIOS doesn't support translation or if its support is buggy or outdated. The interface BIOS will override the mainboard BIOS' harddisk routines. A BIOS ROM will, just like a mainboard with integrated EIDE, often have the added advantage that you don't need separate DOS drivers. The BIOS on an interface will occupy 8 to 16k of UMB space, though, and you will still want drivers for every other operating system you use.
• Drivers! Without well-designed drivers most interfaces could as well be old-fashioned ISA cards for all the good they'll do. Even if the card has a BIOS, which usually removes the need for a driver under DOS, you will still need drivers for other operating systems, including Windows and Win95. Newer versions of Windows (Win95 OEM2 and beyond) ship with a large amount of drivers.

Note that some 'EIDE' interfaces which used to be popular were slightly to very buggy. This ranged from minor problems with ATA-2 compliance to obscure things like the use of a single buffer for both primary and secondary channel or a badly designed prefetch buffer, both of which may cause data corruption under very specific circumstances.

Good question.

Unfortunately, I know no easy answer. The mere ability to specify more than 1,024 cylinders in the BIOS setup is not conclusive. In your BIOS setup, drive related settings like "LBA", "ECHS" or even something silly like "Large" are telltale signs of a BIOS with translation support, which should be good for disk capacities of up to 8GB. A copyright before 1994, on the other hand, reduces your chances to something close to zero :-(

It should be, but there are problematic cases.

It has been observed that some controllers will base their I/O cycle times on the bus clock of the computer. This means that with a 50MHz bus, the cycle time will be faster than with a 33MHz bus. This could lead to undesirable results if these cycle times are faster than the drive can handle. Make sure your controller supports multiple bus speeds in the software setup, and if necessary program it correctly.

This should work fine.

Older drives do not support the high-speed advanced transfer modes defined by ATA-2 and, later, Ultra-ATA. To retain compatibility with these drives, ATA-2 and Ultra-ATA interfaces have programmable timing. The driver or BIOS queries the drive regarding its maximum transfer rate and will configure the interface to match. A few interfaces are jumper configurable in which case the responsibility is yours.

If you intend to put an older drive on the same cable as an ATA-2 drive, you may want to verify that your interface is capable of using independent timing for master and slave device. Otherwise, your old drive might end up slowing down your newer one because the controller is forced to use the lowest common speed. Modern interfaces like the Intel PIIX3 (430HX and VX and 440FX chipset), PIIX4 (430TX chipset) and, to a lesser extent PIIX (430FX chipset) don't suffer from this problem.

Fast-ATA and EIDE devices can be used without fear on regular ISA or VLB IDE controllers. The thing about advanced modes is that the drive is ready for the data in the fastest mode; the slower the controller, the easier it is on the drive. Of course you should expect the transfer rates to come out lower than on a full ATA-2 interface.

Likewise, an Ultra-ATA drive will do fine on an ATA-2 (EIDE) interface. Better even, using DMA mode 2, its performance will be hardly any slower than it would be on an Ultra-ATA interface.

It is not necessary to use an EIDE interface for harddisks of over 504MB. An Enhanced BIOS is all you need for DOS and Windows; for many other operating systems such as OS/2 or Linux you won't need even that.

Yes. These devices were designed to be compatible with ATA (IDE) from the very start, and should work fine.

No. These CD-ROMs ship with a driver that provides complete support. The BIOS doesn't need to support them and usually doesn't even know they're there. Some modern BIOSes are ATAPI aware and have a special setting; barring those, you can generally leave the corresponding harddisk entry at 'Not Installed'.

The options are: a new mainboard BIOS, add-on BIOS, or software.

• To upgrade your mainboard BIOS, either with a newer version of your current BIOS or using a custom made BIOS from firms such as Microid Research (MR BIOS), is probably the best option. It may or may not be more effective to replace the entire mainboard and get all the latest bells and whistles for $100 more.
• The next option is to purchase a card with an add-on BIOS that effectively replaces the harddisk portion of your mainboard BIOS. An example is LBA Pro from Storage Technologies, sold in the USA by Unicore software, or AMI's AMIDisk Extender. DataTechnology (DTC) also sell such a card.
• Most BIOSes on ATA-2 interfaces, if they have a BIOS at all, also do this. That may be a nice way to go if your present interface isn't up to snuff. For the ISA bus, there are a few "EIDE" interfaces that have a BIOS: examples are the Promise EIDEMax and the SIIG IDE Enhancer.

Either way, you will have to repartition and reformat the (large) harddisk.

Using a large harddisk is usually no big problem, even if you don't have an Enhanced BIOS. However, some OSs don't understand translation, which makes the combination with DOS, Windows and Win95 problematic.

With operating systems such as NetWare, Unix, Win/NT and OS/2, the only thing you need to use large ATA disks is a BIOS that allows more than 1024 cylinders in the drive type setup. There is one caveat though: the BIOS is still used to boot the operating system, so you will have to ensure that everything necessary to get the OS running in the first place resides below cylinder 1024. Remember that if you have an Enhanced BIOS, drives up to 8GB will appear to have no more than 1024 cylinders, so in those cases these boot restrictions are removed.

Once running, these operating systems use their own software to control the disks (Win95 also does this, but has a special position; see below). That way, they are not subject to the BIOS' restrictions such as the capacity limit. Unfortunately, this also means that if you have a translating EBIOS or software driver, the OS has to be aware of the translation scheme used, or conflicts will arise between the operating system and DOS/Win/Win95. If you can set up partitions so that all DOS and boot partitions reside below the first 1024 cylinders (504MB), you can avoid translation altogether and all the hassles with it.

Older operating systems don't understand translation at all. Newer ones (OS/2 3.x, Linux 1.2 or better, Win95, SCO 5.0.x) will handle standard translation schemes out of the box, but not always those employed by some software drivers (EZDrive, Disk Manager v6.x or older). In the case of DM 6, fixes or updates are available for some operating systems (see above).

Novell has a NetWare driver IDE.DSK version 3.0 dated September 2, 1994. This version of the driver uses the Identify Device ATA command to get the drive parameters and ignores the BIOS parameters. This means that Novell now works with big IDE drives.

A final remark: OS/2 enforces DOS compatibility for FAT partitions. That means that without an Enhanced BIOS, only HPFS partitions can extend beyond cylinder 1024.

True, but it doesn't necessarily mean you can actually benefit from that support in all cases.

Despite a large number of significant changes, in its way of handling harddisks Win95 largely resembles Windows for Workgroups. Just like OS/2, Linux and other operating systems, Win95 uses 32-bit protected mode drivers for the harddisks (unless it's using compatibility mode), and is happy to handle harddrives of respectable size: well over 100GB. However, unlike OS/2 and Linux, and like Windows for Workgroups, Win95 boots from the same old DOS we all love to hate. Even once Win95 has booted, DOS hasn't gone. Win95 always keeps it behind the scenes and uses it to run devices in compatibility mode. In 'safe mode', important for troubleshooting, Win95 completely relies on DOS.

What all of this implies is that even though Win95's protected-mode disk driver may support gargantuan drives, it will never change the setup it inherits from DOS in order to retain compatibility with the latter. If you can only get at the first 504MB of your drive in DOS, it will be the same in Win95. You will still need one of the usual > 504MB solutions mentioned elsewhere in this FAQ. For details, see also Microsoft KnowledgeBase article Q126855, "Windows 95 Support for Large IDE Hard Disks".

For DOS and Windows you probably want BIOS support for all your harddisks. Older BIOSes support drives on the primary channel only, just two; newer ones add support for the secondary channel for a total of four. Few BIOSes support more than that; MR BIOS is an exception.

The current versions of most operating systems support ATAPI CD-ROMs. That includes OS/2 3.0, Linux 1.2 and above, and Win95.

Unfortunately, hardware manufacturers have started designing these CD-ROM units while the ATAPI standard was still evolving, so there are now a couple of versions implemented in the real world. Moreover, the complexity and novelty of ATAPI means that there are some variations even among implementations of the same revision. Writing an OS ATAPI driver that works with all CD-ROMs, therefore, is a daunting task and not all have succeeded equally well. So although these OSs all support ATAPI, they do not work with all ATAPI equipment.

There is a problem with PCI and multi-I/O or dual-ported IDE interfaces. Such interfaces need two or more predetermined interrupts which cannot be shared with other PCI devices, which is probably impossible with your PCI slots. The usual workaround consists of either integration of I/O functions on the mainboard, or the use of a tiny 'paddle' board that plugs into an ISA slot.

This is not so much a weakness in the design of PCI, but a conflict between PCI's plug'n'play philosophy and the requirement that these interfaces be fully compatible with oldfashioned I/O cards.

Some vendors use PCI interfaces that rely on a proprietary extension of the PCI bus. This is obviously not portable; you often can't even move the card to a different slot in the same machine. Moreover, such extensions may cause compatibility problems since they use PCI signals reserved for other purposes.

A new interface may or may not help; it is possible to make a rough prediction if a better interface would really speed things up.

Hardware vendors and marketing people would love to see everyone rush out and buy the latest generation of 'Ultra-ATA' adapters. To achieve this noble goal they tend to juggle with too-good-to-be-true performance figures. The relation between this advertising hype and the real world is shaky at best.

The main point to remember is: a slow drive is a slow drive no matter how good the interface is. If the speed at which the drive physically transfers the data to/from the media is the limiting factor in performance, and it often is, the only way to make things go significantly faster is to purchase a better drive. Note that the transfer modes supported by modern drives (those 33MB/s figures) have little to do with their real-world performance.

In addition, an 'ordinary' ATA-2 (EIDE) interface already offers respectable bandwidth---the fastest ATA-2 transfer mode is theoretically 16.6MB/s, which is more than any Ultra-ATA drive on the market today can sustain. In addition it usually supports the CPU-cycle-saving DMA modes. An IDE interface, on the other hand, makes a much better candidate for replacement since it rarely has a bandwidth over 2.5MB/s, which is cramped by today's standards, and doesn't support DMA.