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In MS-DOS (and FAT16), you could only have 8.3 file names (like QUESTION.TXT) with only single-case letters and numbers (and a few symbols). The MS-DOS Editor also defaulted to 8.3 - typing edit questiontxt in MS-DOS 6.22 would edit QUESTION.TXT.
While everything used 8.3, were 9.2 names possible?
For example, instead of the example above (QUESTION.TXT), would QUESTIONT.XT be an allowed filename (although it would not be practical)?
No. The FAT12/16/32 filesystems store the filename and extension together in a fixed-length 11 character field. The first 8 characters of this field is the filename and the last 3 is the extension. Trailing space characters in both the filename proper and its extension are ignored. The '.' character between the filename and extension is implicit. Additionally, the '.' character is not a valid character in either the filename or the extension. Spaces, however, are permitted. Note that the above does not apply to VFAT which is its own kettle of worms.
on disk: on screen:
│AUTOEXECBAT│ <=> AUTOEXEC.BAT
│CONFIG SYS│ <=> CONFIG.SYS
Setting aside the file system (other file systems besides FAT were possible on MS-DOS using redirector technology or via the shell approach (ie NetWare)), 8.3 was still a requirement due to the fact that a number of INT 21h functions used the FCB (file control block). (The FCB has hardcoded locations for filename and for the extension.) In particular functions 0Fh through 17h all used FCB(s). (See Ralf Brown's interrupt list.) So a program using FCB functions would not be able to specify a 9 character filename or a 4 character extension.
Additionally the Program Segment Prefix contained two FCBs, so any program examining PSP would need be able to interpret a non 8.3 filename.
DIRand various other commands look for a
DESCRIPT.IONfile and display file descriptions (long file names) stored there. Since this relies on a regular file, it works with any archiver, but few programs can actually work with the descriptions.