MS-DOS 6.22 Non-System Disk Startup Error

When a computer boots with an MS-DOS 6.22 floppy disk or storage medium that lacks the required operating system files, the bootloader halts the startup sequence immediately. This article explains how MS-DOS 6.22 detects a non-bootable disk, the exact error messages it outputs to the screen, the underlying technical mechanism controlling this behavior, and the steps required to resume normal system operation.

The Boot Detection Mechanism

During the initial boot phase, the computer's ROM BIOS performs the Power-On Self-Test (POST) and queries the boot sequence (typically starting with floppy drive A:, followed by hard drive C:). If a formatted floppy disk is detected in drive A:, the BIOS reads the first physical sector—the Master Boot Record (MBR) or Volume Boot Record (VBR)—into memory at address 0000:7C00h and transfers control to it.

In MS-DOS 6.22, this boot sector contains a small bootstrap program. The code searches the root directory of the disk specifically for the first two contiguous system files: IO.SYS and MSDOS.SYS. If these files are absent, damaged, or not positioned correctly within the file system structure, the bootstrap program cannot load the kernel.

The Displayed Error Message

When the boot sector fails to locate IO.SYS and MSDOS.SYS, the bootstrap code halts the load sequence and prints the standard error message directly to the screen via BIOS interrupt INT 10h:

Non-System disk or disk error
Replace and press any key when ready

(Note: Older revisions of DOS used the phrasing "Replace and strike any key when ready," but MS-DOS 6.22 uses "press any key.")

Machine State and User Intervention

Once the error message appears, MS-DOS 6.22 enters a waiting loop. The bootstrap program invokes BIOS interrupt INT 16h (keyboard services), waiting for a keystroke event. During this state:

  • The system does not automatically fall back to the next device in the BIOS boot order (such as drive C:).
  • No further DOS drivers, memory managers (HIMEM.SYS), or configuration files (CONFIG.SYS, AUTOEXEC.BAT) are processed.
  • The system will remain indefinitely at this prompt until physical input is received.

Resolving the Prompt

To proceed, the user must perform one of two actions:

  1. Remove the Non-Bootable Disk: Eject the non-system floppy disk from drive A: and press any key. The bootstrap code will issue BIOS interrupt INT 19h (Bootstrap Loader), which re-initiates the boot process from scratch. With drive A: empty, the BIOS proceeds to the next designated boot device, typically booting MS-DOS 6.22 from the primary hard disk.
  2. Insert a Valid System Disk: Insert a bootable floppy disk containing IO.SYS, MSDOS.SYS, and COMMAND.COM into drive A: and press any key. The bootstrap program restarts the read cycle, loads the operating system files into memory, and continues startup normally.