Why did dos only have a three digit file extension naming system?

[yoda313]

I know that old dos had a max of 8 characters and a 3 digit extension. I guess what I want to know is how was it determined to use three letters for the extension? Why not two or four? Is there some computational restriction that made three a logical choice? Or was it as simple as three was easier to make abrreviations for long names???

I know NT and XP no longer have the 8 character restriction and you see four digit extensions sometimes - mainly .divx (is that really a four digit extension or is it just a fake .avi extension?). What is the difference between dos and NT that allows the filename increase in letter usage from 8 to essentially infintity?? Is it faster processors or more memory? Or did they redesign the code specifically to allow for longer file names??

I'd be interested in the answers. No real reason but just idle curiousity.

[kschang]

Neither, actually. I don't have a definite answer, but you have to keep in mind that MS-DOS was essentially a copy of the UNIX system shell, with somewhat renamed commands (out of command.com) and similar file system. Instead of actually allowing you to define a "filetype", the idea of "file extension" was introduced instead, and I guess they felt 2 letters is too limiting.

Technically the LFN (long file name), introduced with Win95, have taken away the 8.3 limit, but the truth is it's NOT really gone. If you do a DIR you can see that "Program Files" subdir is really PROGRA~1. The OS will generate a 8.3 name for each LFN you use and keep hashing until it generates a unique one. And that's still true today, even on XP.

[jagabo]

MS-DOS was a clone of CP/M. In those days computers typically had 16 KB of memory, 180 KB floppy disks, and single digit MHz CPUs. The use of 8.3 filenames was simply a matter of space (on disk and in memory) savings and processing power.

[hunter99]

Hardware. 3 letters = 24 Bits. 24 Bit registers were very common.
Programers wrote for the chips available.
A lot more to it, but best use of hardware and memory.

[yoda313]

Ok so it was a space limitation then.

MS-DOS was a clone of CP/M

What's CP/M?? I thought MS-DOS was its own creation?

[jagabo]

CP/M was the popular O/S used on 8080 and Z80 CPUs before the IBM PC was introduced.

http://en.wikipedia.org/wiki/CP/M

MS-DOS was designed to look and work a lot like CP/M.

[yoda313]

Thanks jagabo.

[Nelson37]

It was 16 bits total including the attribute bits hidden, system, read-only, and archive. Yes, the period counted. IIRC the actual length of the extension was designed for particular extensions like "exe" and "com", with a seperator, (the period) leaving eight characters for the file name.

[hunter99]

I belive some are confusing Bits with Bytes.
8 bits = 1 byte

Following from "Upgrading and Repairing PCs 13th Ed."

Table 25.6 FAT Directory Format
Offset Offset Field
(Hex) (Dec) Length Description Bit Length
00h 0 8 bytes Filename 8X8= 64
08h 8 3 bytes File extension 3X8= 24
0Bh 11 1 byte File attributes 1X8= 8
0Ch 12 10 bytes Reserved (00h) 10X8= 80
16h 22 1 word Time of creation
18h 24 1 word Date of creation
1Ah 26 1 word Starting cluster
1Ch 28 1 dword Size in bytes

Filenames and extensions are left-justified and padded with spaces, (which are represented as ASCII
32h bytes). In other words, if your filename is “AL”, it is really stored as “AL------”, where the hyphens
are spaces. The first byte of the filename indicates the file status for that directory entry, shown in

[offline]

DOS was essentially a copy of the UNIX system

Nope. The main difference being that in Unix, file name extensions were always optional for executables. This was not the case for CP/M, AppleDOS or early MSDOS flavours.