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Choice of E5 for a deleted entry: #1 E5 is a "sync byte" it cannot be rotated and mis-interpreted: E5 E5 E5 ... can be used to synchronize a bitstream. Note th
by ratboy666 8y ago
Choice of E5 for a deleted entry:
#1 E5 is a "sync byte" it cannot be rotated and mis-interpreted: E5 E5 E5 ... can be used to synchronize a bitstream. Note that floppy discs are read bit-wise.
#2 Empty 8" floppy discs came pre-formatted with E5 written everywhere.
#3 05 is a "control-e" in "extended 8 bit ASCII (IBM encoding)". E5 was a usable character.
With CP/M, the disc bitmap was produced when the disc was "loaded". All directory entries were scanned, and the allocation bitmap was produced. A freshly formatted disc would have E5 fill, and thus would have no files. MS-DOS had a separate allocation table, which was also the file linkage table. So, the strategy of a fresh E5 filled disc being taken as empty no longer works. But, the key of a deleted entry having E5 as the initial character was still used.
Hope this helps.
- benj111 8y agoThanks for that. I hadn't come across the idea of a sync byte. For anyone else, the binary representation of E5 is 11100101. No matter where you start reading you're always going to know whether you are reading from the start of the byte or not. Contrast with null (00000000) its impossible to know where you've started from. This has a bit of info ftp://ftp.apple.asimov.net/pub/apple_II/documentation/misc/disk_encoding.doc.txt