section .bss

chsspt: equ 0 ; word sectors per track
chsnos: equ 2 ; word number of sides
chspo:  equ 4 ; dword partition offset
chssiz: equ 8

chstab: resw (4*chssiz)

	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

fndchs:	; find CHS table entry
	; IN	dl	DOS drive number (A=0, B=1, ...)
	; OUT	cs:si	ptr to chs table entry
	push ax
	mov ax, chssiz
	mul dl
	add ax, chstab
	mov si, ax
	pop ax
	ret

	; compare sector number in disk access packet with zero
	; sets zero flag accordingly
iszero:	push ax
	mov ax, [bx]
	or ax, [bx+2]
	pop ax
	ret

	; calculate CHS from dap and CHS table entry
	; IN	ds:bx	disk access packet
	;	cs:si	chstab table entry
	; OUT	cx,dh	chs data for int 13h
calchs:	push ax
	; load linear sector number
	mov dx, [bx+2]
	mov ax, [bx]
	; if any word of it is non-zero, we need to do calculation
	call iszero
	jz .zero
	; ax is track number (lba / spt)
        ; dx is sector (lba % spt) + 1
	div word [cs:si+chsspt]
	inc dx
	; sector number
	mov cl, dl
	; ax is cylinder (track / heads)
	; dx is head (track % heads)
	xor dx, dx
	div word [cs:si+chsnos]
	; set up cylinder and head number
	mov ch, al
	mov dh, dl
	pop ax
	ret
	; set up CHS data for int 13h for reading sector zero
.zero:	mov cx, 1 ; C=0, S=1
	mov dh, 0 ; H=0
	pop ax
	ret

	; ABSOLUTE DISK READ / DISK WRITE
int25h:	mov ah, 2
	jmp adisk
int26h:	mov ah, 3
adisk:	push bp
	push ds
	push es
	mov bp, sp
	cmp cx, 0xFFFF
	je .islrg

	; build a disk access packet on the stack
	push ds ; dword transfer buffer
	push bx
	push cx ; word number of sectors
	push cs ; dword starting sector number (CS always 0)
	push dx

	; set our freshly created dap as DS:BX
	mov bx, sp
	push ss
	pop ds

.islrg: mov dl, al
	call fndchs
	call calchs
	mov dl, al
	mov al, [bx+4]
	mov es, [bx+8]
	mov bx, [bx+6]
	int 0x13
	jc .ret

.ret:	mov sp, bp
	pop es
	pop ds
	pop bp
	retf