Logical Block Addressing And Cylinder Head Sector

Logical Block Addressing (LBA)

Logical block addressing (LBA) is a common scheme used for specifying the location of blocks of data stored on computer storage devices, generally secondary storage systems such as hard disks.

Cylinder-head-sector

Cylinder-head-sector, also known as CHS, was an early method for giving addresses to each physical block of data on a hard disk drive. In the case offloppy drives, for which the same exact diskette medium can be truly low-level formatted to different capacities, this is still true.

If you look at most sites they will give you this set of formulas:
Sector = (LBA mod SectorsPerTrack)+1
Cylinder = (LBA/SectorsPerTrack)/NumHeads
Head = (LBA/SectorsPerTrack) mod NumHeads

Simple Assembly example:

(This is not likely to work if you cut and paste it and is here to show the principle only, look at the next section for a complete example)

LBACHS:
; Set up the registers ready for the divide
MOV ax, [LBAvalue] ; []'s means value at memory location LBAvalue.
; Make the divide
DIV [SectorsPerTrack] ; Carry out the division of ax.
; Put the returned Number Of Tracks some where
MOV [NumTracks], ax   ; Put the quotient into a memory variable
; Sort out the sector value
INC dx        ; Add 1 to the remainder
MOV [Sector], dx     ; Put the remainder into a memory variable

; Set up the registers ready for the divide
MOV ax, [NumTracks]    ; Put the number of tracks in to ax
; Make the divide
DIV [NumHeads]  ; Divide NumTracks (ax) by NumHeads
; Stash the results in some memory locations
MOV [Cylinder], ax     ; Quotient value, the Number of heads to be moved from ax
MOV [Head], dx        ; Remainder value, the cylinder value to be oved from dx