In legacy MS-DOS environments, which executable must be loaded to enable CD-ROM (ISO-9660) drive access via a DOS drive letter?

Introduction / Context:
On classic MS-DOS systems, optical drives did not work out of the box. Accessing a CD-ROM required a two-layer driver model: a low-level hardware driver and a high-level redirector that exposed the disc as a DOS drive letter (for example, D:). Knowing which utility performs this role is essential for configuring legacy PCs and understanding boot disks used by technicians and retro-computing enthusiasts.

Given Data / Assumptions:

• We are in an MS-DOS context (not Windows NT/2000+).
• Objective: enable reading ISO-9660 CDs from DOS using a standard drive letter.
• Typical CONFIG.SYS and AUTOEXEC.BAT workflow is assumed.

Concept / Approach:

The standard stack is: load a device-specific CD driver (e.g., OAKCDROM.SYS) in CONFIG.SYS, then load Microsoft CD-ROM Extensions in AUTOEXEC.BAT. The latter, named MSCDEX.EXE, is a redirector that interfaces with the device driver and integrates the CD file system into DOS, assigning a letter and enabling directory/file access. Other executables listed in the options serve unrelated purposes: VLM.EXE is a NetWare client component, CDDEX.EXE is not the Microsoft utility, and DOSKEY.EXE provides command history and macros only.

Step-by-Step Solution:

Verification / Alternative check:

Typical AUTOEXEC.BAT lines include: MSCDEX /D:MSCD001 /L:D which confirms the required component and its parameters (/D device name, /L drive letter).

Why Other Options Are Wrong:

• VLM.EXE: NetWare DOS requester module, unrelated to CD-ROM file systems.
• CDDEX.EXE: nonstandard name; the Microsoft tool is MSCDEX.
• DOSKEY.EXE: keyboard macro/history helper, no file-system function.
• None of the above: incorrect because MSCDEX.EXE is present.

Common Pitfalls:

• Confusing the low-level device driver (.SYS) with MSCDEX; both are needed for full functionality.
• Assuming Windows 9x native support applies equally to pure DOS mode—DOS still needs MSCDEX.

Final Answer:

MSCDEX.EXE.