rdos/kernel/drive.asm

section .bss

		; drive actually selected for I/O
dsknum:		resb 1
		; current sector number
dskseek:	resd 1

		; disk buffer for I/O operations
dskbuf:		resb 512

dpt:		resb DPTSIZE
bpb:		resb BPBSIZ4

section .text

ret:		ret

		; Convert drive number into DL for int 13h
		; A=0, B=1, C=0x80
		; IN	dl	dos drive number
		; OUT	dl	bios drive number
getdl:		cmp dl, 2
		jc ret
		mov dl, 0x80
		ret

		; Ask BIOS for drive parameters
		; IN	dl	dos drive number
		; OUT	CX,DH	chs data
		;	ES:DI	DPT if floppy
qrychs:		mov ah, 8
		les di, [0x1E*4]
		int 0x13
		jc .defs
		test cl, cl
		jz .defs
		ret

.defs:		mov cx, 0x2709 ; max track = 39, max sec = 9
		mov dh, 1 ; 2 heads
		ret

		; select drive
		; IN	dl	drive number
dsksel:		cmp dl, byte [dsknum]
		je ret

		call dskrst

		call getdl
		push dx
		call qrychs

		; store CHS data in BPB
		xor ax, ax
		mov al, cl
		mov [bpb+BPBSPT], ax
		mov al, dh
		inc ax
		mov [bpb+BPBNOS], ax

		; get dpt from bios if not hdd
		pop dx
		test dl, 0x80
		jnz .nodpt
		push dx
		mov dx, dpt
		mov bx, di
		mov cx, DPTSIZE
		call lod
		mov ax, [bpb+BPBSPT]
		mov [dpt+DPTSPT], al
		pop dx
.nodpt:


		ret

dskrst:		call flush
		mov ax, 0xFFFF
		mov [dsknum], al
		mov [dskseek], ax
		mov [dskseek+2], ax
		ret

		; flush buffer if dirty
flush:		ret