LASTDRIVE Directive in MS-DOS 6.22 CONFIG.SYS

The LASTDRIVE directive in the MS-DOS 6.22 CONFIG.SYS file specifies the maximum drive letter that the operating system can recognize and assign. By defining this upper alphabetical limit (from A through Z), MS-DOS sets up the internal data structures required to access local hard drives, CD-ROM drives, RAM disks, network shares, and logical drives created with utility commands.

Purpose and Functionality

When MS-DOS boots, it scans the system's hardware to detect physical floppy drives and hard drive partitions, automatically assigning sequential drive letters starting with A. However, software-based drives initialized later in the boot sequence—such as mapped network drives, RAM disks created with RAMDRIVE.SYS, substituted drives via SUBST, and optical drives managed by MSCDEX.EXE—require drive letters beyond the detected physical partitions.

The LASTDRIVE directive ensures that MS-DOS prepares enough drive access structures in advance to accommodate these additional devices.

Syntax

The command is placed as a single line within the CONFIG.SYS file:

LASTDRIVE=x

The parameter x represents a single letter between A and Z, case-insensitive.

Default Behavior

If LASTDRIVE is omitted from CONFIG.SYS, MS-DOS 6.22 defaults to the drive letter immediately following the last physical or logical drive detected at boot time. If the system has fewer drives, it typically defaults to E. If you need to mount network shares or access a CD-ROM that requires a drive letter past E, the software will fail or return an "invalid drive specification" error unless LASTDRIVE is explicitly defined to a higher letter.

Memory Impact

For every drive letter defined up to the specified LASTDRIVE, MS-DOS allocates a small data structure known as the Current Directory Structure (CDS) in conventional memory. Each letter consumes approximately 88 bytes.

While allocating up to Z (LASTDRIVE=Z) provides maximum flexibility for assigning drive letters, it consumes over 1.8 KB of memory that could otherwise be preserved for running DOS programs. For optimal conventional memory management, set LASTDRIVE only to the highest letter actually required by your hardware and software setup.