rdos/kernel/drive.asm

; drive table
%define drivetab_type    0x00 ; BYTE 0=unassigned, 1=BIOS/CHS, 2=BIOS/LBA
%define drivetab_num     0x01 ; BYTE DL number
%define drivetab_heads   0x02 ; CHS heads
%define drivetab_spt     0x03 ; CHS sectors per track
%define drivetab_sectors 0x04 ; total number of sectors
%define drivetab_offset  0x08 ; partition offset
%define drivetab_size    0x0C

drive_table:
  dw 0
.count:
  dw 4

; DI is incremented for the space taken
; IN	ES:DI
drive_setup:
  xor cx, cx
  mov [drive_table], di
  mov al, drivetab_size
  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, drivetab_size
  mul dl
  add si, ax
  pop ax

  ret
.err:
  stc
  ret