Calculating Cylinder / Head / Sector
If a particular disk has 6 surfaces and 50 sectors/track,
what is the C/H/S (Cylinder/Head/Sector) location of absolute
disk sector 454 (all values in decimal)?
Method: Calculate how many full cylinders and tracks
precede the given sector.
Our desired sector is number 454, therefore 454 sectors
precede it (those preceding disk sector numbers are #0 through
- Calculate the number of full cylinders: C
Subtract the sectors contained in full cylinders that
precede given sector.
Each cylinder contains 6
surfaces times 50 sectors = 300 sectors/cylinder.
Dividing 454 by 300 gives: 1 full cylinder with a remainder
of 154 sectors.
Therefore, one full cylinder precedes sector number 454.
That cylinder is numbered zero, because cylinders number from
Cylinder zero is full, so the cylinder that contains our
sector must be the next (not full) cylinder after cylinder
zero. That cylinder is number 1.
Therefore: C = 1
- Calculate the number of full tracks: H
track corresponds to a different surface and a different
head. Track = head = surface.)
Subtract, from the sectors that remain, the sectors
contained in full tracks that precede the given
(These full tracks are in cylinder 1, calculated above.)
Each track contains 50 sectors.
Dividing the remaining 154 sectors by 50 gives: 3 full tracks
with a remainder of 4 sectors.
Therefore, in cylinder 1, 3 full tracks precede sector number
Those 3 tracks are numbered 0,1,2, because the heads (tracks)
number from zero.
Tracks 0-2 are full, so the track that contains our sector
must be the next (not full) track after track 2. That track
is number 3.
Therefore: H = 3
- Calculate the number of full track sectors: S
(Reprise: We calculated that cylinder 0 is full; tracks 0,1,2
of cylinder 1 are also full.)
How many sectors remain in cylinder 1, track 3 that
precede the given sector?
The remainder of 4 above tells us that, in track 3 of
cylinder 1, four track sectors precede sector number 454.
Those four track sectors are numbered 1,2,3,4, because track
sectors number from one (not from zero).
Track sectors 1-4 are full, so the track sector that contains
our sector must be the next sector after track sector 4. That
track sector number is 5.
Therefore: S = 5
Therefore, disk sector 454 is located at (decimal)
C/H/S = 1/3/5
This is head #3, which is the bottom head on the
Other calculations using the same disk geometry (for fun and
123 => 0/2/24 top
200 => 0/4/1 top
300 => 1/0/1 top
454 => 1/3/5 bottom
567 => 1/5/18 bottom
600 => 2/0/1 top
678 => 2/1/29 bottom
789 => 2/3/40 bottom
900 => 3/0/1 top
1200 => 4/0/1 top
8910 => 29/4/11 top