3 ms·

In what world are files required to have an extension?

tom_ 8y ago
The space for the name is pre-booked, so in a sense all files always have a full 8-char name and 3-char extension, whether they like it or not. If you want to support <8-char names, <3-char extensions, or no extension at all, you need to decide on values that indicate this, because those bytes will still need filling with something. (On 6502-based systems, 0 would be a reasonable choice for a padding value, because you get a free zero check every time you read a byte. The 8086 doesn't work like this... I don't think any particular value suggests itself. Perhaps they thought picking a printable character would be useful.)
- red_admiral 8y ago
  Picking the space character, apart from being I think an established convention already (the CHAR(n) type in databases), definitely has the advantage you can just "blit" the filename onto the video buffer in text mode when doing a DIR command. You don't have to bother with checking its length or anything, just copy 11 bytes and insert a space (not dot in the original DIR listing format) after the 8th one. If the file doesn't actually have an extension, this just prints the right number of spaces so the other columnms are aligned. EDIT: see https://en.wikipedia.org/wiki/Dir_(command)#/media/File:SpartaDOS_X_prompt.png https://en.wikipedia.org/wiki/Dir_(command)#/media/File:Spar... for an example.
red_admiral 8y ago
They weren't required to, but the convention for the shell was, unlike unix with its X bit: extension COM: just load the file into memory and start executing from the very beginning (I think the max size of these might have been 64KB or something). extension EXE: this is a "full" executable with different segments - load it properly extension BAT: this is a "shell" script, run it with the interpreter The advantages of COM over EXE were slightly smaller file size on disk and slightly faster startup as you didn't have to set up segments and stuff first. Some versions of DOS even had a program called EXE2BIN that converted an EXE file into a COM file, if this was possible. The joys of the 16-bit "tiny" memory model ...

tom_ 8y ago

The space for the name is pre-booked, so in a sense all files always have a full 8-char name and 3-char extension, whether they like it or not. If you want to support <8-char names, <3-char extensions, or no extension at all, you need to decide on values that indicate this, because those bytes will still need filling with something. (On 6502-based systems, 0 would be a reasonable choice for a padding value, because you get a free zero check every time you read a byte. The 8086 doesn't work like this... I don't think any particular value suggests itself. Perhaps they thought picking a printable character would be useful.)

red_admiral 8y ago
Picking the space character, apart from being I think an established convention already (the CHAR(n) type in databases), definitely has the advantage you can just "blit" the filename onto the video buffer in text mode when doing a DIR command. You don't have to bother with checking its length or anything, just copy 11 bytes and insert a space (not dot in the original DIR listing format) after the 8th one. If the file doesn't actually have an extension, this just prints the right number of spaces so the other columnms are aligned. EDIT: see https://en.wikipedia.org/wiki/Dir_(command)#/media/File:SpartaDOS_X_prompt.png https://en.wikipedia.org/wiki/Dir_(command)#/media/File:Spar... for an example.

red_admiral 8y ago

They weren't required to, but the convention for the shell was, unlike unix with its X bit: extension COM: just load the file into memory and start executing from the very beginning (I think the max size of these might have been 64KB or something). extension EXE: this is a "full" executable with different segments - load it properly extension BAT: this is a "shell" script, run it with the interpreter The advantages of COM over EXE were slightly smaller file size on disk and slightly faster startup as you didn't have to set up segments and stuff first. Some versions of DOS even had a program called EXE2BIN that converted an EXE file into a COM file, if this was possible. The joys of the 16-bit "tiny" memory model ...