MS-DOS 6.22 Execution Calls to Missing Drive Letters

This article examines how MS-DOS 6.22 handles system execution calls directed at non-existent or unmapped drive letters. It covers the internal validation mechanisms of DOS, the behavior of low-level execution interrupts such as INT 21h Function 4Bh, the specific return codes generated, and how the command interpreter presents these failures to the user.

Drive Letter Allocation and Structure Validation

MS-DOS 6.22 manages drive availability through an internal array of structures known as the Current Directory Structure (CDS). The total number of addressable drive letters is bounded by the LASTDRIVE directive in CONFIG.SYS (defaulting to the drive letter after the last physical/logical drive detected, or drive E: if fewer are present).

When an execution request occurs, DOS evaluates the target drive letter through two primary criteria:

  1. Out-of-Bounds Drives: Drive letters that exceed the configured LASTDRIVE limit.
  2. Unassigned In-Bounds Drives: Drive letters within the LASTDRIVE range that lack an associated Device Parameter Block (DPB), network redirector, or virtual drive assignment (such as via SUBST).

Low-Level Execution Handling (INT 21h, Function 4Bh)

Program execution requests in MS-DOS are initiated via system call INT 21h, Function 4Bh (EXEC). The calling process passes an ASCIIZ string defining the program path and an execution parameter block.

When the path string contains a drive letter (e.g., Z:\PROGRAM.EXE):

  1. Drive Parsing: DOS parses the path prefix to identify the logical drive number (where 1 = A:, 2 = B:, 3 = C:, etc.).
  2. Bounds Verification: If the drive number exceeds the system's LASTDRIVE ceiling, DOS fails the operation immediately. The processor's Carry Flag (CF) is set to 1 to signify an error, and register AX returns error code 0Fh (Invalid drive).
  3. CDS Inspection: If the drive is within LASTDRIVE limits but lacks a mapped physical or logical device, the drive’s entry in the CDS has its attributes checked. If the entry is marked invalid/inactive, the call halts immediately without dispatching any hardware-level commands.
  4. Return Code: In this case, DOS clears the hardware access pipeline, sets the Carry Flag (CF = 1), and loads AX with either error code 0Fh (Invalid drive) or 03h (Path not found), depending on the specific execution sub-path and parser stage.

Critical Error Handler (INT 24h) Bypassing

Unlike scenarios involving a valid drive with missing media (such as an empty floppy drive, which triggers an INT 24h Critical Error like "Abort, Retry, Fail?"), execution attempts on missing or unmapped drive letters completely bypass INT 24h.

Because no device driver header is linked to the requested drive letter, there is no underlying hardware device to query or fail on. MS-DOS treats the request purely as a logical path failure and returns control directly to the parent process via software error flags.

User Interface and Shell Response (COMMAND.COM)

When an execution call is invoked directly from the standard MS-DOS 6.22 shell (COMMAND.COM) via the command line or a batch file, the shell evaluates the path before invoking INT 21h, AH=4Bh.

  • Missing Target Drive: If a user specifies an execution path targeting a missing drive, COMMAND.COM intercepts the invalid drive designator during path normalization. It outputs the standard diagnostic message: 
    Invalid drive specification
  • Execution Failure: If a script or wrapper attempts the execution directly, the return code is stored in the system's exit status (ERRORLEVEL), allowing batch processes to detect that the target binary failed to launch due to an invalid path.