DMA Check Box Does Not Remain Checked

ID: Q159560

The information in this article applies to:

SYMPTOMS

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.


CAUSE

The hard disk may not support a multiple-word DMA protocol.


RESOLUTION

To determine whether your IDE hard disk supports multiple-word DMA protocol, follow these steps:

Testing the Primary IDE Drive:

  1. Restart the computer. For Windows 95, when you see the "Starting Windows 95" message, press the F8 key and then choose Command Prompt Only from the Startup menu.

    For Windows 98, restart your computer, press and hold down the CTRL key after your computer completes the Power On Self Test (POST), and then choose Command Prompt Only from the Startup menu.


  2. At the command prompt, type:

    debug


  3. At the hyphen prompt, type the following lines, pressing ENTER after each line. Do not type the semicolon (;) or the comment after the semicolon.

    NOTE: The first character of each line is the letter o, not the numeral zero. 
     
      o 1f6 a0 ; a0 (a-zero) is for a master drive, use b0 for a slave.
      o 1f2 22 ; 22 is for DMA mode 2, use 21 for DMA mode 1.
      o 1f1 03 ; 03 (zero-3) is to program the hard disk timing.
      o 1f7 ef ; ef is the set feature command for the hard disk.
      i 1f1    ; Reads in the error status; a value is returned.


  4. To quit Debug, type the letter "q" and press ENTER.

    If the number returned after entering "i 1f1" is 00, the hard disk accepts the DMA protocol timing that you have entered with the "o 1f2" statement, and the hard disk supports DMA. A return value of 04 indicates that the hard disk does not support a DMA multiple-word protocol. If the value returned is not 00 or 04, you may not have typed the characters correctly, or you may need to quit Windows.

    PIO mode 3 hard disks may support multiple-word DMA mode 1. PIO mode 4 hard disks should support multiple-word DMA mode 2. If you have a PIO mode 4 drive that does not support multiple-word DMA mode 2, it is possible that the hard disk has a firmware problem. Contact the hard disk's manufacturer, and verify the firmware version.


Testing the Secondary IDE Drive:
  1. Restart the computer. When you see the "Starting Windows 95" message, press the F8 key and then choose Command Prompt Only from the Startup menu.


  2. At the command prompt, type:

    debug


  3. At the hyphen prompt, type the following lines, pressing ENTER after each line. Do not type the semicolon (;) or the comment after the semicolon.

    NOTE: The first character of each line is the letter o, not the numeral zero. 
    
      o 176 a0 ; a0 (a-zero) is for a master drive, use b0 for a slave.
      o 172 22 ; 22 is for DMA mode 2, use 21 for DMA mode 1.
      o 171 03 ; 03 (zero-3) is to program the hard disk timing.
      o 177 ef ; ef is the set feature command for the hard disk.
      i 171    ; Reads in the error status; a value is returned.


  4. To quit Debug, type the letter "q" and press ENTER.

    If the number returned after entering "i 171" is 00, the hard disk accepts the DMA protocol timing that you have entered with the "o 172" statement, and the hard disk supports DMA. A return value of 04 indicates that the hard disk does not support a DMA multiple-word protocol.

    If your drive does support a DMA multiple-word protocol and the DMA check box will not remain enabled, the IDE controller may not be compatible with the Microsoft IDE bus mastering driver.



MORE INFORMATION

DMA (also referred to as bus mastering) reduces CPU overhead by providing a mechanism for data transfers that do not require monitoring by the CPU. The transfer rate for a particular data transfer event will not noticeably increase. However, overall CPU overhead should be reduced using DMA mode.

A disadvantage of implementing DMA data transfer operations has been that the PC/AT and IDE hard disk controller evolved around PIO data transfer methods. As a result, the system Int 13h BIOS and native operating system device drivers evolved around PIO transfers instead of DMA transfers. Modifications to the BIOS, as well as external device drivers, have been necessary to achieve the incremental performance that DMA offers. Windows 95 OEM Service Release 2 offers native operating system support for many bus mastering IDE controllers.

Additional query words: 95 


Keywords          : kbhw kbui osr2 win95 win98 
Version           : 95
Platform          : WINDOWS 
Issue type        : kbprb

Last Reviewed: January 29, 1999