On Jan 15, 6:45 pm, jonnosan <jonno...@gmail.com> wrote: > AF AF <- sector # in 4-4 encoding=3 3 (51) One way of 4&4 decoding is ((n1 << 1) | 1) & n2. AF << 1 => 5E 5E | 1 => 5F 5F & AF => 0F So it's sector 15. If in doubt, look at a DOS disassembly: http://www.textfiles.com/apple/ANATOMY/rwtsdrvr.read.txt (Search for "B96D" and ignore the flawed comment above it!) Cheers, Nick.