AGI 1.xx directory (logdir...) format

byte    +0       +1       +2
bit     76543210 76543210 76543210
meaning vvtttttt ssssssho oooooooo

v       volume number (0-2, 3 means empty slot)
t       track number (0-63)
s       sector number (0-3)
h       head number (0-1)
o       offset (0-511)
