Date   : Sun, 13 Mar 1988 15:30:22 GMT
From   : rochester!ur-tut!cwwj@bbn.com (Clarence Wilkerson)
Subject: CP/M disk directories

I'm a little rusty at this, but let try any way. Here's an explanation
that seems to fit your observations:
1) The number of extents controlled by a single directory entry is 2
16k extents.
2) The total number of sectors covered by a single directory entry is
at most 80h in RC + 1*80h from the extent byte.
3) Thus your 2 directory entries cover 256 sectors for first entry
and 255 for second = 511.
  That is, the first directory entry in your system will have an extent
byte 0 if you have 80h or fewer sectors in the file. If you have between
81h and 256 sectors, the extent byte is 1, and the RC byte is the rest.

n
I'm a little rusty, but here's an exp