section .bss

disk_buffer:
	resb 512

disk_current:
	resb 1

lba_supported:
	resb 1

disk_chs: ; DL, DH, CL, CH for int 13h CHS functions
	resb 4
disk_dap: ; disk access packet for int 13h EBIOS functions
	resb 0x10

disk_tracks: ; heads * spt
	resw 1
disk_spt:
	resb 1

	; Disk access packet for EBIOS extensions
dapps:	equ 0 ; byte packet size
dapnum: equ 2 ; word number of sectors to transfer
dapbuf: equ 4 ; dword transfer buffer
dapsec:	equ 8 ; qword absolute sector number
dapsiz:	equ 16

	section .text

	; Convert between drive number and BIOS dl
	; Bidirectional mapping
	; 0 <-> 0, 1 <-> 1, 2 <-> 0x80, 3 <-> 0x81
dnconv:	mov cx, 7
	ror dx, 1
	rol dl, 1
dncl:	rcl dl, 1
	rcr dh, 1
	loop dncl
	xchg dh, dl
	ret

	; Select a drive for I/O
	; IN	dl	0=A, 1=B, 2=C, 3=D
select:	mov byte [disk_current], 0xFF
	mov byte [lba_supported], 0
	cmp dl, 0x02
	jc select_floppy
	; detect EBIOS/LBA extensions
	call dnconv
	mov byte [disk_chs], dl
	mov ah, 0x41
	mov bx, 0x55AA
	int 0x13
	test cx, 1
	jz .nolba
	cmp bx, 0xAA55
	jnz .nolba
	mov byte [lba_supported], 1
.nolba:	call seek_zero
	ret

select_floppy:
	xor dx, dx
	xor ax, ax
	call seek
	call read
	ret

	; Set absolute sector number
	; IN	dx:ax	32-bit sector number
seek:	push ax
	or ax, dx
	pop ax
	jz seek_zero

	; dx:ax = lba
	div word [disk_tracks]
	xchg ax, dx
	; dx = cylinder, ax = head * spt + sector
	div byte [disk_spt]
	; dx = cylinder, al = head, ah = sector
	xchg dl, dh
	ror dl, 1
	ror dl, 1
	or dl, ah
	;inc dx
	; dh bit 0-7: cylinder 0-7
	; dl bit 0-5: sector number 0-5
	; dl bit 6-7: cylinder 8-9
	; store
	mov byte [disk_chs+1], al
	mov word [disk_chs+2], dx
	int 3
	ret

seek_zero:
	mov byte [disk_chs+1], 0
	mov word [disk_chs+2], 1
	ret

	; Read a sector into buffer
read:	mov ax, 0x0201
	mov dx, [disk_chs]
	mov cx, [disk_chs+2]
	lea bx, [disk_buffer]
	push cs
	pop es
	int 3
	int 0x13
	int 3
	ret

	; Write a sector into buffer
write:
	ret