HOWTO: Determine Whether a Drive Is a RAM Drive

• Microsoft C for MS-DOS, versions 6.0, 6.0a, 6.0ax
• Microsoft C/C++ for MS-DOS, version 7.0
• Microsoft Visual C++ for Windows, versions 1.0, 1.5
• Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SUMMARY

There are no C language functions or C run-time functions to determine whether or not a drive is a RAM drive. The GetFileType() API may be called by Win32 applications to obtain the type of a particular drive from the operating system. More information may be found about GetFileType() in the Win32 SDK Reference of the Visual C++ 32-bit Edition Books Online.

For MS-DOS and Win16 applications, although there are no definitive methods available, several possible ways exist to determine that a given drive is "probably" a RAM drive. The remainder of this article will briefly discuss each of four methods to check for a RAM drive. Note that for all methods each drive of the host system must be checked.

MORE INFORMATION

Method 1

Check the drive's volume label. This method is probably the most common. The Microsoft RAMDRIVE.SYS uses a volume label of "MS-RAMDRIVE". However, this is not guaranteed to be the volume label and will probably not be the volume label on RAM drives from other companies. A user may also change this by using the label command.

For additional information on finding the disk volume, please see the following article(s) in the Microsoft Knowledge Base:

   ARTICLE-ID: Q58800
   TITLE     : Using _dos_findfirst() Function to Retrieve Disk Volume

Method 2

Call Interrupt 21h Function 44h subfunction 0Dh, which is a generic I/O control function for block devices. Most devices support this function. However, RAMDRIVE.SYS currently does not support it. This lack of support can be used to identify a RAM drive. Load the CX register with 0860h to get disk drive device parameters; if the function fails, leaving the carry flag set, the drive is probably a RAM drive.

Method 3

Check how many FAT tables the drive has; if it has only 1, it is probably a RAM drive. Because RAMDRIVE.SYS does not support the IOCTL functions, the only way to determine the number of FAT tables is to use INT 25, Absolute Disk Read. Make note of the warnings that this function has been superseded and is not supported under Windows. For more information and a sample showing how to use INT 25, please see the following article(s) in the Microsoft Knowledge Base:

ARTICLE-ID: Q102870

TITLE     : SAMPLE: Reading the Boot Sector of a Drive

Method 4

Finally, call Interrupt 21h Function 44h subfunction 09h to check whether the drive is remote. If the value returned in DX is 0x0800, having only bit 11 set, the drive is probably a RAM drive. This is known to be a RAM drive by deduction. Because bits 6 and 7 are clear, it is known that this device does not accept IOCTL functions, like most devices. Because bit 12 is clear, this is not a network drive, and so on.

Because none of the above methods is guaranteed, it is best to use a combination of these methods. First eliminate the obvious, such as network drives, local hard drives, CD-ROM drives, and so forth. Then try to determine whether or not the given drive is a RAM drive.