Step 4. Determine Unused Space

Unused space on the last track of the track index depends on the number of tracks required for track index entries, which in turn depends upon the number of tracks per cylinder and the number of track index entries per track. You can use any unused space for any prime data records that will fit.

   Unused space = (Number of index entries per track)
                  - (2 (Number of tracks per cylinder
                  - Number of overflow tracks per cyl.) + 1)
                  (Number of bytes per index)

For variable-length records, or when a prime data record will not fit on the last track of the track index, the last track of the track index is not shared with prime data records. In this case, if the remainder of the division is less than or equal to 2, drop the remainder. In all other cases, round the quotient up to the next integer.

Example: The 3380 disk pack from the 3380 Model AD4 has 15 tracks per cylinder. You can fit 57 track index entries into one track. Therefore, you need less than 1 track for each cylinder.

   Number of trk index = (2 (15 - 5) + 1) / (57 + 2)
   trks per cylinder   = 21 / 59

The space remaining on the track is 47968 - (21 (832)) = 30496 bytes.

This is enough space for 16 blocks of prime data records. Because the normal number of blocks per track is 26, the blocks use 16/26ths of the track, and the effective number of track index tracks per cylinder is therefore 1 - 16/26 or 0.385.

Space is required on the last track of the track index for a dummy entry to show the end of the track index. The dummy entry consists of an 8-byte count field, a key field the same size as the key field in the preceding entries, and a 10-byte data field.