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Win9x sets DMA to OFF by default. You have to switch it ON.

In theory, enabling DMA increases UDMA Hard Disk access to 33.3 MB/sec. In practice, speed will increase substantially from the old standard of 16MB/sec.

DMA uses only 25% of CPU resources (normally 40%).
Nearly all 5400rpm / 7200rpm, and SOME lower speed, drives, support DMA. Enabling DMA in Device Manager requires just a 'click'.

You can substantially improve performance by enabling DMA for all devices that support it.

It is dangerous to use DMA for a device not supporting it.
Fortunately this option will normally be greyed-out (unavailable) if the device is not DMA-capable.
Nevertheless, read the device manual, or Help files, carefully.

Using DMA under older versions of Win95 is possible, if problematic.


98+ Win98 or WinSE or WinME

If you have DMA capability (likely, but read "DMA needs") you must ensure it is enabled.

To check if you are already using BUS Mastering, select Control Panel • System • Device Manager • Disk Drives • (select a drive) • Settings. If the DMA check box is ticked, you are already using DMA for the drive.

If the drive or controller isn't DMA capable, or if the installed driver does not support DMA, the DMA check box will be grayed out or absent.

1. Backup the Registry
2. Select Device Manager • Disk Drives • 'your drive' • Properties • Settings
3. Remember, if there is no check box (or it is grayed out), your motherboard chipset does not support the bus master interface (or perhaps the specific BUS Master driver is not installed).
4. Enable the DMA box
5. Click OK to any caution
6. Reboot when prompted
7. Repeat for all your hard drives & CD-ROM drive (ONLY if DMA-capable)

Should a problem ensue then disable DMA for that device or, if necessary, reinstall the Registry.

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What's DMA, UDMA, Bus Master

IDE hard disks are accessed in one of two ways: PIO (Programmed Input/Output) or DMA (Direct Memory Access). There are 5 different PIO modes, from slowest (0) to fastest (4). The highest is normally used when the drive is automatically detected. Best check in your BIOS.

PIO: Traditionally, the transfer method for IDE devices has been PIO where data is transferred to/from IDE devices using the CPU. PIO requires a relatively large amount of dedicated CPU time (40%), especially when transferring large amounts of data to/from hard disk drives. Transfer speeds may be in the region of 16MB/sec.

DMA: DMA allows for the transfer of data to/from IDE devices without heavy dependence on the CPU and increases transfer speeds. Its operation requires a Bus Master driver.(It uses Bus Master IDE (BM-IDE) logic contained within the PIIXn IDE controller. The PIIXn IDE controller acts as a PCI Bus Master on behalf of IDE DMA slave devices, performing DMA transfers to/from devices on primary or secondary IDE channels).

UDMA: Newer Intel chipsets (PIIX3 and later) can perform fast DMA transfers using the UltraDMA/33 hardware interface. UDMA is an abbreviation of UltraDMA. Its operation requires its own Bus Master driver. Speeds of 33MB/sec can, theoretically, be achieved.

These can be a boost to speed - but mostly under certain circumstances. Disk-intensive applications will benefit from UDMA's increased throughput, and applications stored on the UDMA hard drive will load faster. Bootups should be faster.

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Basic requirements for using DMA

1. Your device (usually a Hard Disk) must be capable of using DMA (read below)
2. Your motherboard chipset must support DMA (read below)
3. Your BIOS must support DMA (look in the BIOS, and turn it ON)
4. Your OS must have multitasking capability (that's Win9x)
5. You must have the BUS Mastering driver installed (they're in Win98+, but read below)
6. You must enable DMA in Device Manager (the default is OFF)

The vast majority of Win systems built in 1999+ meet the above requirements, except the last.

1. To determine if your device supports Bus Mastering look in its manual, or visit the manufacturer's Web site, or try this Windows-Help page, or use the test described at this Microsoft page

2. The Intel chipsets from about the 430HX PCIset, and later, all support DMA.

5. The BUS Master drivers usually come with a newer motherboard (last few years) or with the OS (Win98+) or can be downloaded. Check for the latest UDMA driver.

6. Win9x does not enable DMA, and most vendors do not enable it. Therefore you need to activate it yourself via Device Manager (but ONLY for devices that are DMA-capable).

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UDMA has some clear advantages over PIO

UDMA & Bus Mastering will definitely help your system complete tasks faster especially if you have multiple disk-intensive programs running simultaneously. It should also shorten bootup/restart times somewhat. Basically anything that involves that device, especially hard disk access, should work faster.

Many users claim good results with DMA-compliant CD-ROMs.

Aside from that, DMA will probably not greatly benefit you if you typically run DOS games, or possibly if you work with only one program is running, or you have many programs running but they are not disk intensive.

The benefits of DMA in the home computer will depend on your own type of usage. Faster bootup should be a help if nothing else!

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95 Win95b or Win95c

If you are using the Win95 OSR 2, OSR 2.1, or OSR 2.5 releases, you should not download the Intel BM-IDE driver, as these already contain a BM-IDE driver from Microsoft.

To enable UDMA2 on Windows 95b/c (OSR2.x) using Microsoft BM drivers for the Intel chipsets:

1. Do not install Intel's BM drivers if doing this

2. Fresh installation of OSR2 if a previous DMA attempt failed




6. Now enable the DMA box in Device Manager ONLY for DMA-capable devices (reboot required)

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95 Win95 or Win95a

You may still use DMA even though your Windows does not have the BM driver. Use Intel's BM-IDE driver for Windows 95. You MUST NOT use this with Windows 95b (OSR2) or later.

Use with some caution in Win95, 95a. As you have an older system it is more likely you have a motherboard chipset that does not support DMA.

Requirements :
  • Intel Triton chipset (PIIXn; its in most Pentium motherboards)
  • A Ultra DMA/33 interface is not required, but may enhance
  • a DMA-capable device (disk drive or CD-ROM)
  • Windows 95 or 95a (OSR1) ONLY
  • BM-IDE device driver (bmide_95.exe provided by Intel)
  • System BIOS need not be BM-aware
  • the driver enabled for that drive !

If you are using the Windows 95 retail or OSR 1 (95a) release, you should download and install the Intel BM-IDE driver for Windows 95. Enable it only for DMA-capable devices.

Note:   Many CD-ROMs cannot deal with DMA bus mastering drivers, and problems may result if you try to enable Bus Mastering for a hard disk on a channel shared with a CD-ROM.

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Some caution is advisable

NOTE I: Avoid any 'try it and see' attitude to enabling DMA; the consequence could be severe if the device is not capable of using DMA. There is no guarantee that the presence of a DMA check box confirms that you should enable it! Enable DMA ONLY for your devices that are DMA-capable.

NOTE II: You do not need to install Bus Master drivers if you have Win98 (they are build into the Win98 system). However you may have to install a BM driver for UDMA if you have a UDMA device.

NOTE III: If the DMA check box does not remain checked, look for a fix at:

NOTE lV: Users with the original or OSR1 (95 or 95a) versions of Windows 95 must read further down (early versions of Windows do not have the BUS mastering driver).

NOTE V: Using a modern UDMA device in an older system does not create any problems - it will simply run as a standard DMA, or PIO mode 4, device.

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NOTE VII: You may have BUS mastering capability, but do not have the appropriate driver.
. First check your OEM's home page for updated drivers and files.
. Download the latest Intel Bus Mastering Drivers for your chipset. Each chipset requires a different driver. The set-up program will only install on PC's capable of this feature.
. Run set-up, and follow the on-screen instructions. Restart your computer.

NOTE VIII: Ultra DMA capable and standard IDE drives can be used in the same system. Ultra DMA drives are also fully backward compatible with existing IDE drives and CD-ROM drives.

NOTE IX: You can toggle Bus Mastering ON and OFF. This is especially useful for a CD-ROM, or an old hard drive, that may not work correctly with it switched on.

NOTE X: If you upgrade to Windows 98 over a system using the Intel BM driver, you may have to (re)install the Windows BM driver.

NOTE XI: Ultra DMA PCI controller cards are available for users who do not have the required chipset - the advantage for most home users does not justify the cost.

Intel's site at is a good source of further information.

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The DMA Check Box may not remain Checked
When you enable DMA support on the Settings tab in an IDE hard disk's properties, the DMA check box in Device Manager may not remain checked even though the IDE controller reportedly supports bus mastering and DMA.
This is a multiple-word DMA protocol problem. Read the at Microsoft's site.

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Please remember that you alone are responsible for the consequences of any changes you make to your computer hardware or software.

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