Sometime on 4/5/2011, Ron Wilson wrote: >Interestingly, Microsoft choose control-Z as end-of-file, >rather than any of the other defined control values that might have >been better. My guess is that that was because Z is the last >letter of >the alphabet, and Z being closest to the lower left corner of the >keyboard.
For some reason, that sets off an urge to reminisce about old computers... Ctrl+Z was used as an end of file mark in CP/M-80, and DOS 1.0 had a lot of CP/M heritage and inspiration. That is where drive letters and 8.3 file names come from, for instance. Ctrl+Z (or some marker, at least) was needed because the CP/M file system stored files in whole disk blocks, and had no other way of marking the end of a file to single-byte accuracy. CP/M wasn't alone in this choice. Digital's RT-11 also stored files in whole block increments and used Ctrl+Z as EOF. IIRC, very early versions of DOS followed the same conventions, and many old utilities would pad files out to block boundaries or at least insert a Ctrl+Z after the last byte written. To this day, files opened in Text on Windows will report EOF at the first Ctrl+Z encountered, although all versions of the FAT file system and its descendents know the file size in bytes. As a further bit of trivia, PNG files include a Ctrl+Z in their header to explicitly catch the case that the file was opened with text-mode translations enabled. In that case, the file will appear to end after six bytes are read. The header is: 0x89 0x50 0x4E 0x47 0x0d 0x0a 0x1a 0x0a. This sequence catches a bunch of incorrect ways to process the file including 7-bit only transmission, CRLF to NL, NL to CRLF, Ctrl+Z as EOF, and byte order. And if you use TYPE at the command prompt to display a PNG file, it will show the string PNG before stopping. Incidentally, the use of Ctrl+D in Unix derivatives is a property of the tty driver and it does not get translated as EOF when read from files at all. Ross Berteig r...@cheshireeng.com Cheshire Engineering Corp. http://www.CheshireEng.com/ _______________________________________________ fossil-users mailing list fossil-users@lists.fossil-scm.org http://lists.fossil-scm.org:8080/cgi-bin/mailman/listinfo/fossil-users
