MS-DOS FORMAT Program Writes Over ROM BIOS on 80486 PC (62566)

The information in this article applies to:
    Microsoft MS-DOS operating system 3.3
    Microsoft MS-DOS operating system 4.01
    Microsoft MS-DOS operating system 5.0

This article was previously published under Q62566


Under certain conditions, the MS-DOS versions 3.3 and 4.01 FORMAT program writes over the ROM BIOS.

MS-DOS changes the number of sectors per track (FLOPPY PARAMETER TABLE entry + 4) when formatting a disk. While this really has no effect on systems with ROM or write-protected shadow RAM, it affects 80486 systems that cache the system BIOS. There is no way to write-protect the BIOS ROM (in the cache) when it is cached on an 80486 PC.

The only workaround we recommend is to not use the caching for ROM BIOS in 486s with the FORMAT command in MS-DOS versions 3.3 and 4.01.

Microsoft has confirmed this to be a problem in MS-DOS versions 3.3 and 4.01. This problem does not occur in later versions of MS-DOS.

Modification Type: Major Last Reviewed: 11/17/2003
Keywords: KB62566