This item is from the Yet Another Enhanced IDE/Fast-ATA/ATA-2 FAQ, by John Wehman and Peter den Haan with numerous contributions by others. (v1.92).
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.
o 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.
o 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.
o 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.
o 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will want an on-card Enhanced BIOS too if your mainboard BIOS doesn't support translation or if its support is buggy or outdated.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.
o 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.