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)

	; 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

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

	push ax

	cmp cx, 0xFFFF
	je .lrg

	; build ebios DAP from registers supplied
	; qword sector number
	push cs
	push cs
	push cs
	push dx
	; dword target buffer
	push ds
	push bx
	; word number of sectors
	push cx
	jmp .psz

.lrg:	; build ebios DAP from int 25h DAP
	; qword sector number
	push cs
	push cs
	mov ax, [bx+2]
	push ax
	mov ax, [bx]
	push ax
	; dword target buffer
	mov ax, [bx+8]
	push ax
	mov ax, [bx+6]
	push ax
	; word number of sectors
	mov ax, [bx+4]
	push ax
.psz:	; word packet size
	mov ax, dapsiz
	push ax

	; DS:SI = SS:SP (ptr to dap)
	push ss
	pop ds
	mov si, sp

	; set up int 13h subfunction number
	mov ax, [si+dapsiz]
	add ah, 0x40

	; get BIOS drive number
	mov dl, al
	call dnconv

	int 0x13

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