How can I calculate CHS from the LBA?

Question

I'm writing a bootsector, and for it I want to access a particular logical block address on a floppy disk. I have all the parameters of the disk, such as total sectors, sectors per track, head count, etc. And I know the formula to go from CHS to LBA, but I'm in need of a formula to calculate the Cylinder, Head, and Sector from a given LBA address.

I actually found such a function in the MikeOS source code:

l2hts:          ; Calculate head, track and sector settings for int 13h
            ; IN: logical sector in AX, OUT: correct registers for int 13h
    push bx
    push ax

    mov bx, ax          ; Save logical sector

    mov dx, 0           ; First the sector
    div word [SectorsPerTrack]
    add dl, 01h         ; Physical sectors start at 1
    mov cl, dl          ; Sectors belong in CL for int 13h
    mov ax, bx

    mov dx, 0           ; Now calculate the head
    div word [SectorsPerTrack]
    mov dx, 0
    div word [Sides]
    mov dh, dl          ; Head/side
    mov ch, al          ; Track

    pop ax
    pop bx

    mov dl, byte [bootdev]      ; Set correct device

    ret

But I can't work out how this works. Take the Sector for example, from what I can work out the formula for the Sector here is (logical sector % sectors per track) + 1. Is this correct? It doesn't make much sense to me, for some reason. Perhaps I just don't fully understand how logical sectors are layed out.

Answer 1

I found the answer:

 Cylinder = LBA / (HPC * SPT)
 Head     = (LBA / SPT) % HPC
 Sector   = (LBA % SPT) + 1

LBA = logical block address
HPC = Heads Per Cylinder (reported by disk drive, typically 16 for 28-bit LBA)
SPT = Sectors Per Track (reported by disk drive, typically 63 for 28-bit LBA)

'%' is a modulus operation
'*' is a multiplication operation
'/' is a division operation