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[ The PC Guide | Systems and Components Reference Guide | Motherboard and System Devices | System Buses | System Bus Types | Peripheral Component Interconnect (PCI) Local Bus ]

PCI IDE Bus Mastering

The PCI bus also allows you to set up compatible IDE/ATA hard disk drives to be bus masters. Under the correct conditions this can increase performance over the use of PIO modes, which are the default way that IDE/ATA hard disks transfer data to and from the system. When PCI bus mastering is used, IDE/ATA devices use DMA modes to transfer data instead of PIO; IDE/ATA DMA modes are described in detail here.

Since this capability was made available to newer machines, it has been one of the most talked about (and most misunderstood) functions of the modern PC. There is a lot of confusion amongst PC users about what PCI IDE bus mastering does and how it works. In particular, there are a lot of misconceptions about its performance advantages. In addition, there have been a lot of problems with compatibility in getting this new technology to work.

IDE bus mastering requires all of the following in order to function at all:

  • Bus Mastering Capable System Hardware: This includes the motherboard, chipset, bus and BIOS. Most newer motherboards using the Intel 430 Pentium chipset family (FX, HX, VX, TX) or the Intel 440FX Pentium Pro chipset, will support bus mastering IDE.
  • Bus Mastering Hard Disk: Normally this means that the drive must be capable of at least multiword DMA mode 2 transfers. All Ultra ATA hard disks also support bus mastering.
  • 32-Bit Multitasking Operating System: This means usually Windows NT, Windows 95, Linux, or similar (but see below for caveats.)
  • Bus Mastering Drivers: A special driver must be provided to the operating system to enable bus mastering to work.

Getting this all set up can be a great deal of work. In particular, the following are common problems encountered when trying to set up bus mastering:

  • Driver bugs and incompatibility issues, especially with older drivers that are "generic" and not tuned to a particular motherboard.
  • Older hard disk drives not working properly.
  • Problems with dissimilar drives connected to the same IDE channel as master and slave.
  • Problems when using a CD-ROM drive alone on an IDE channel without a hard drive.
  • Bus mastering drivers that don't work on certain motherboards; also, some motherboards or systems just will not work with bus mastering at all.

Assuming that you get bus mastering IDE to work, you will see improvement if you are using a true multi-tasking operating system, and you are running multiple applications that are disk-access-intensive. This would not generally include most regular Windows 95 users, for example. Bus mastering IDE will not help at all in the following situations:

  • It will not make that 100 MB transfer from C: to D: that you are sitting and watching go much faster at all.
  • It will not speed up DOS games.
  • It will not make applications load more quickly (unless you somehow are loading more than one at a time).
  • It will not speed up single applications.

Especially: IDE bus mastering will not really speed up Windows 95 in general. Windows 95 does not do "true" multitasking and in many cases the processor will be held up waiting for the transfer to complete even if bus mastering is employed. So even though the processor in theory is freed up to do other things, it doesn't really do other things. Also, most people multitask by switching between applications that are open, but rarely have anything running in two or more simultaneously.

For most people, IDE bus mastering is not worth the effort and problems, and I now do not bother with it on new installs of Windows 95. This may be somewhat controversial, but in my opinion it is very overrated as a potential system improvement, given how much effort it requires. You're better off working overtime for a few hours and buying another 16 MB of RAM. :^) If you feel like trying it, contacting the company that made your motherboard for a driver set is a good place to start. You can also try Intel for a generic driver that may work on your Intel-chipset system. I'd recommend that you back up your hard disk first before trying any of these... refer to this section of the Troubleshooting Expert for more help resolving problems with these drivers if you have difficulties with them.

I am hopeful that in time, bus mastering over the IDE/ATA interface will be improved and these problems will be just a distant memory. With the creation of Ultra ATA and the DMA-33 high-speed transfer mode, it appears that the future lies in the use of PCI bus mastering with the IDE/ATA interface. There is just some work to do until this support is both universal and well-implemented.

Next: PCI Plug and Play


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