Enhanced IDE
There may be some formating problems
I am working on that
EIDE Written by Harry Romell 3-11-95
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So you've got a local bus or PCI system and would like to make it go as fast as it possibly can.
Well, lets take a look at the disk subsystem. Typical AT bus only offered transfer rates of 2-3 MB
PIO to 5.5 MB with local bus. This seemed a little slow in comparison to SCSI so enter in Western
Digital who wrote up a new spec called EIDE (Enhanced IDE).
EIDE gives us quite a few additions provided you are using local bus or PCI.
1) Support for drives greater than 528MB
This limitation comes about as a combination of things. Look at the following table for a highlight:
BIOS IDE Limitation
Max Sectors/Track 63 255 63
Numbers of Heads 255 16 16
Number of Cylinders 1024 65536 1024
Maximum Capacity 8.4GB 136.9GB 528MB
As you can see by taking the Maximum that each can do imposes the limitation. To get around this
WD created LBA to work with the Int 13h BIOS. In a nutshell LBA allows the BIOS to see drives
larger than 528MB drives.
2) Faster Host Transfers
There are 2 basic methods of 16 bit host transfers, PIO and DMA. PIO uses the CPU to perform the
data transfer (Processor I/O), while DMA does not. Instead it uses another system item to perform
data transfers (Direct Memory Access). Using DMA frees up the CPU to execute instructions from
its L1 and L2 caches during DMA data transfer. Below is a list of the different transfer modes:
PIO Transfers
MODE Cycle Time Burst Transfer Rate
0 600ns 3.33 Mbyte/sec
1 383ns 5.22 Mbyte/sec
2 240ns 8.33 Mbyte/sec
Single DMA Transfers
MODE Cycle Time Burst Transfer Rate
0 960ns 2.08 Mbyte/sec
1 480ns 4.16 Mbyte/sec
2 240ns 8.33 Mbyte/sec
Multiword DMA Transfers
MODE Cycle Time Burst Transfer Rate
0 480ns 4.16 Mbyte/sec
IORDY Mode 3
IDE Cycle Time Memory Cycle Time Total Mbyte/sec
120ns 60ns 180ns 11.11
120ns 90ns 210ns 9.52
120ns 120ns 240ns 8.33
A mode 4 has been put together that supports transfer rates up to 16.6 Mbyte/sec.
3) Dual IDE Channels
With dual IDE channels you can support up to 4 IDE drives, or if you choose a IDE CDROM and/or
a tape drive. To use the 2nd channel keep in mind it usually uses IRQ 15.
4) Non Disk IDE Peripherals
As mentioned above you can use IDE CDROM and/or a tape drive on the secondary channel.
So now you got a Mode 3 Drive and a controller to match and you get them hooked up and you
don't notice a difference. Depending on your operating system (Windows, DOS, OS/2, and NT) you
are using, you need to install the matching driver to enable the faster transfer rates. Once this is
done you should notice a difference. The biggest difference I have seen is in Windows. I use loading
times for the control panel for comparison. Without the driver, loading the control panel is noticably
slower.
This is a quick overview. If you want more in depth information try the following sites for more information.