rdos/kernel/drive.asm

; Load a sector from drive.
; IN    DX:AX   sector number (lba)
;       BX      ptr to FCB
; OUT	CL	Sector number
;	CH	Cylinder number
;	DH	Head number
drive_chs_calc:
  push bx      ; backup fcb ptr
  mov bx, [bx+fcb_spt] ; nos is in BH now
  push bx      ; backup nos
  xor bh, bh   ; zero out nos so spt is left
  div word bx  ; ax:temp     = (lba / spt)
  inc dx       ; dx:sector   = (lba % spt) + 1
  mov cl, dl   ; sector number

  pop bx       ; restore nos
  xchg bl, bh  ; put nos into bl
  xor bh, bh   ; zero out spt
  div word bx  ; ax:cylinder = (temp / nos)
               ; dx:head     = (temp % nos)
  mov ch, al   ; cylinder number
  mov dh, dl   ; head number

  pop bx       ; restore fcb ptr
  ret

; Load a sector from drive.
; IN    DX:AX   sector number
;	BX	ptr to FCB
drive_read:
  push ax
  push cx
  push dx
  push bx
  push si

  mov cx, dx
  or cx, ax
  jz .fast

  call drive_chs_calc

  mov si, 5        ; retry count
.try:
  mov dl, [bx+fcb_drv] ; drive number
  and dl, 0x7F
  dec dl
  mov ax, 0x0201
  mov bx, diskbuf
  ; try read
  int 0x13
  jnc .end
  ; reset disk
  mov ah, 0x00
  int 0x13
  ; loop with si
  dec si
  test si, si
  jnz .try
  stc
.end:
  pop si
  pop bx
  pop dx
  pop cx
  pop ax
  ret
.fast: ; fast lane for boot sector
  inc cl
  jmp .try