Hard Disk CHS and LBA Addressing quick notes.
by David Sudjiman ~ February 16th, 2006. Filed under: Tech.Hard Disk (HD) using CHS addresing or so-called 3D notation. Each data is saved on HD on particular Sector(s) (normally) as big as 512 bytes.
C : Cylinders. Track on of disk which has the same value for each plate on each side. Cylinders are numbered from 0 to (nC-1)
H : Head. Numbered from 0 to (nH-1)
S : Sector. Numbered from 1 - nS. (Sector 0 is MBR, 512 bytes size)
Using int13h bios interface, CHS numbering has 24 bits to address its data location.
10 bits for Cyclinder; Max. 1024.
8 bits for Head; Max. 256.
6 bits for Sector; Max. 63
It means that using C * H * S * 512 bytes is equal to 8,455,716,864 bytes (8.5 GB).
To locate a data beyond the 24bits limitation, BIOS uses LBA (Logical Block Addressing). LBA basically still using CHS numbering with different calculation.
LBA = Cylinder * nH * nS + Head * nS + (S-1)
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