Windows 98 kernels, the WIN.COM file emulates a Windows 3.x kernel for NTVDM, which runs the 16-bit DOS-based Windows applications on Windows NT. Shimming Application compatibility issues, notably around long filenames, multiple users and the concept of least privilege, may prevent some applications from working. For example, they may incorrectly assume full write access to the whole file system whereas NTFS security is in place. When the Windows 95 line of operating systems was designed, a key requirement was for the file system to keep backward compatibility with 8.3 filenames to allow legacy applications to continue to work on the platform. Windows 95 and later operating systems therefore support a compatibility mode whereby both a long filename and a short filename are stored in the File Allocation Table. Furthermore, legacy applications that attempt to access hardware directly cannot do so in user mode. Legacy applications may also fail if system configuration files from the DOS and Windows 9x era are not present in Windows NT based kernels, hence the reason for zero-length versions of files like AUTOEXEC.BAT and CONFIG.SYS having to be carried forward on operating systems that do not use them. A considerable number of shims are present in the application compatibility layer of later versions of Windows to intercept and modify API calls made by legacy applications that were written with a different set of assumptions and operating system best practices in mind. These fixes are updated from time-to-time as issues are discovered in popular legacy applications that are still in use. See also Wine (software) References External links Windows NT subsystems What are NTVDM and WOW? Optimize How Windows 7 Runs 16-Bit and MS-DOS-Based Programs Discontinued Windows components 1993 software