Final Year Project: Using Linux Filesystems Under Windows   Chris Bryden BEng. Electronics and Software Engineering    School of Computer Science   University of Birmingham 3 2.  Hard Disk Structure This section aims to outline several concepts related to hard disk access that are necessary to understand the low level disk access routines that provide the foundations for accessing any filesystem on disk.   2.1  Physical Disk Access: Cylinders, Heads and Sectors Information stored on a disk is divided into small parts (usually 512 bytes for hard disks) called sectors. When a disk I/O operation is carried out this is the smallest  amount  of  information  that  can  be  read  or  written  in  one  go.  These sectors  are  known  as  physical  sectors.  A  hard  disk  consists  of  one  or  more platters    on    which    information    is stored. These are magnetically coated  disks  stacked  one  on  top  of another  on  a  central  rotating  shaft. These  platters  are  read  by  a  head suspended a fraction of a millimetre away from both sides of each platter. The head is free to move across the platter  from  it’s  centre  to  it’s  outer edge.  The  sectors  are  arranged  in concentric   circles   called   tracks   on both sides of the platters. A cylinder describes  all  tracks  at  a  given  head position across all platters.   A   three   figure   reference   is used to uniquely identify each sector on  a  disk:  a  cylinder  number,  head number and sector number (CHS). The cylinder number gives the cylinder to be read,  and  the  head  number  gives  the  head,  it  can  be  seen  that  these  two numbers  are  sufficient  to  position  the  correct  head  above  the  desired  cylinder. The sector number then gives the individual sector to be read from the disk. It should  be  noted  that  although  these  are  referred  to  as  physical  sectors,  the cylinder, head and sector numbers no longer relate to the physical position of the sector on modern hard disks, as internal translation is carried out to overcome certain design limitations.    So,  to  conclude,  reading  or  writing  a  physical  sector  on  a  disk  involves specifying  its  CHS  co-ordinates.  This  level  of  disk  access  pays  no  attention  to what is actually on the disk, this must be done by the operating system or user program.