Ambiguous References to Sector One and Sector Zero

• Microsoft Windows NT operating system version 3.1
• Microsoft Windows NT Advanced Server version 3.1
• Microsoft Windows NT Workstation version 3.5
• Microsoft Windows NT Server version 3.5

This article discusses some inconsistencies in sector numbering.

Sector counting starts at 1 when referring to the physical disk geometry. The first sector is reference by head 0, cylinder 0, sector 1. When people refer to absolute sectors on a physical volume, however, they typically refer to sector 0 as the first sector. Many applications (for example, Norton Disk Doctor) refer to the first sector as sector 0. Other tools use different conventions.

Documentation often references sector 0, as it is less ambiguous than using sector 1. Sector 0 is clearly the first sector, even if you disagree with the convention. Sector 1 is the first sector to some people, and the second sector to others.

On a disk with 37 sectors per track, 8 heads, and 512 cylinders, Windows NT would look at the numbering as follows:

                                   absolute
cylinder     head      sector   sector address

   0          0           1              0
   0          0           2              1
   0          0           3              2
   0          0           4              3

   0          0          36             35
   0          0          37             36
   0          1           1             37
   0          1           2             38

   0          1          36             72
   0          1          37             73
   0          2           1             74
   0          2           2             75

   0          7          36            294
   0          7          37            295
   1          0           1            296
   1          0           2            297

 511          7          36         151550
 511          7          37         151551