%define drive_struct_len 0x0C

drive_table:
  dw 0
.count:
  dw 4

; Drive table
; 00  BYTE     DL number
; 01  BYTE     file system type
; 02  BYTE     CHS heads (ignored if LBA)
; 03  BYTE     CHS sectors per track (ignored if LBA)
; 04  DWORD    total number of sectors
; 08  DWORD    partition offset

; DI is incremented for the space taken
; IN	ES:DI
drive_setup:
  xor cx, cx
  mov [drive_table], di
  mov al, drive_struct_len
  mov ah, BYTE [drive_table.count]
  mul ah
  xchg cx, ax
  ; this increments DI and also fills table with zeros
  rep stosb
  ret

; Load a drive table entry
; IN    DL      drive number
; OUT   DS:SI   ptr to table item
drive_load:
  ; bail out if number too high
  cmp dl, [cs:drive_table.count]
  jnc .err

  push cs
  pop ds
  mov si, [drive_table]

  push ax
  mov al, drive_struct_len
  mul dl
  add si, ax
  pop ax

  ret
.err:
  stc
  ret