How Windows 95 Handled CD AutoRun Installations

When Microsoft introduced Windows 95, it transformed software setup from a manual command-line process into an automated experience via the AutoRun feature. This article explains how Windows 95 detected optical media insertion at the hardware and driver level, how it read the root configuration file, and how the operating system shell executed automated setup programs.

Detection via Hardware Polling and Drivers

Windows 95 relied on continuous communication between the CD-ROM storage subsystem and the Windows shell to detect when a user inserted a disc. Because legacy ATAPI and proprietary CD-ROM interfaces often lacked interrupt-driven media-change notifications, the 32-bit protected-mode CD-ROM file system driver (CDFS.VXD) and the associated virtual device drivers periodically polled the drive for state changes.

If the drive hardware supported the Media Status Notification (MSN) specification, it reported a NOT_READY to READY transition. Once the operating system confirmed that a new disc was present and mounted, the file system sent a broad system message (WM_DEVICECHANGE) notifying the Windows shell (Explorer) that new media had arrived.

Parsing the autorun.inf File

Upon receiving the notification, Windows Explorer scanned the root directory of the newly mounted volume specifically for a plain-text configuration file named autorun.inf.

This file followed the standard INI file syntax and required an [autorun] header section. The primary directives used for software installation included:

• open=: Pointed to the executable binary or batch script to launch immediately (for example, open=setup.exe or open=install.exe /welcome).
• icon=: Defined the visual icon path to represent the drive in "My Computer" and Windows Explorer, replacing the default optical drive icon.
• shell\action\command=: Allowed developers to define custom right-click context menu actions in the shell.

If the file did not contain an open= line, or if the file was missing entirely, the operating system simply mounted the drive silently without running any program.

Executing the Installer

Once Windows 95 verified the presence of the open= directive in autorun.inf, Windows Explorer called the Win32 CreateProcess or ShellExecute API to start the designated program in the context of the currently logged-in user. The installer ran immediately in the foreground, allowing the setup wizard to present welcome screens, license agreements, and configuration options without requiring the user to locate the drive letter or manually double-click a file.

Bypass and Disabling Mechanisms

To allow users to bypass automatic execution—such as when inserting an audio CD or running a secondary disc—Microsoft included a built-in override. Holding down the Shift key while inserting the disc instructed the shell to ignore the autorun.inf trigger. Additionally, users and system administrators could disable the feature permanently through the Windows Registry by altering the NoDriveTypeAutoRun key or through Device Manager by unchecking the "Auto insert notification" checkbox under the CD-ROM drive's properties.