diff --git a/boot/mbr.asm b/boot/mbr.asm
index 135763f..af44954 100644
--- a/boot/mbr.asm
+++ b/boot/mbr.asm
@@ -1,6 +1,7 @@
 	cpu 8086
 
-%define seg 0x060
+%define seg 0
+%define off 0x700
 
 	org 0x7C00 - (seg << 4)
 
@@ -16,24 +17,12 @@ init:	cli
 .retry:	mov ax, 0x0208
 	mov cx, 2
 	mov dh, 0
-	mov bx, 0x100
+	mov bx, off
 
 	int 0x13
 	jc .retry
 
-	xor di, di
-	mov ax, 0x18CD
-	stosw
-	int 0x12
-	mov cl, 6
-	shl ax, cl
-	stosw
-	mov al, 2
-	stosb
-	mov ax, 0x18CD
-	stosw
-
-	jmp seg:0x100
+	jmp seg:off
 
 	times (0x1BE - ($-$$)) db 0
 
diff --git a/src/@rdos.asm b/src/@rdos.asm
index 973a134..ccd2e3c 100644
--- a/src/@rdos.asm
+++ b/src/@rdos.asm
@@ -1,15 +1,15 @@
+	org 0x700
+cseg	equ 0x0
 	jmp init
 
 %include "bpb.inc"
 %include "farptr.inc"
 
-	; drive actually selected for I/O
-dsknum	db 0xFF
-
-dap	db 0x10, 0
-	dw 1
-dapbuf	dw dskbuf, 0
-dskseek	dw 0,0,0,0
+	; dflags bitfields
+DRVLOG	equ 0x01 ; bit 0 (1) - drive logged in (see drvnum)
+DRVRST	equ 0x02 ; bit 1 (2) - controller configured
+DRVEXT	equ 0x04 ; bit 2 (4) - EBIOS supported
+DIRTY	equ 0x08 ; bit 3 (8) - dskbuf dirty
 
 	; initial setup for disk i/o
 	; copy previously set DPT to our data area
@@ -23,8 +23,6 @@ dinit	xor ax, ax
 	mov es, cx
 	mov word [es:4*0x1E], dpt
 	mov word [es:4*0x1E+2], ds
-	; set segment addr in DAP
-	mov [dapbuf+2], cs
 	ret
 
 	align 16
@@ -36,6 +34,8 @@ i13isp	jmp short int13i
 	iret
 	align 16
 
+	; Entry point when user program calls int13h
+	; TODO: flush buffer and log out drive
 int13i	call int13
 	iret
 
@@ -86,7 +86,7 @@ getprm	test byte [dskflag], 4
 	; select a drive for io
 	; IN	dl	drive number
 	; dont do anything if drive already selected
-select	cmp dl, [dsknum]
+select	cmp dl, [drvnum]
 	je logfdd.ret
 	; clear out current contents
 	push dx
@@ -140,7 +140,7 @@ loghdd	sub dl, 2
 	sbb bx, 0xAA55
 	jnz .chs
 	; enable ebios and assume controller is configured
-	or byte [dskflag], (2+4)
+	or byte [dflags], DRVCTL | DRVEXT
 	; get chs data (needed or we cant load vbr)
 .chs	call getprm
 	; read mbr
@@ -181,74 +181,68 @@ logerr	stc
 	ret
 
 	; map sector into dskbuf
-	; sector number from partition start
-	; IN	dx:ax	sector number
-map	add ax, [bpb+BPBHS]
-	add dx, [bpb+BPBHS+2]
-	; absolute sector count
 	; skip doing a read if sector number matches
-	; IN	dx:ax	sector number
-mapabs	cmp ax, [dskseek]
-	jne l003
-	cmp dx, [dskseek+2]
-	je l002
-	; flush and read other sector
-l003	push ax
+	; IN	dx:ax	absolute sector number
+map	cmp ax, [drvseek]
+	jne .do
+	cmp dx, [drvseek+2]
+	jne .do
+	ret
+	; flush previous contents
+.do	push ax
 	push dx
 	call flush
 	pop dx
 	pop ax
-	; store the sector number
-	mov [dskseek], ax
-	mov [dskseek+2], dx
-	jmp _read
+	; set sector number
+	mov [drvseek], ax
+	mov [drvseek+2], dx
+	; issue read cmd
+	mov ch, 2
+	jmp diskio
 
 	; mark dskbuf as containing unwritten changes
-dirty	or byte [cs:dskflag], 1 ; dirty
+dirty	or word [cs:dflags], DIRTY
 l002	ret
 
 	; flush buffer if dirty
-flush	test byte [cs:dskflag], 1
+flush	test word [cs:dflags], DIRTY
 	jz l002
-	; low level read and write
-	; call again to retry, no input registers
-	; read or write is configured in cx
-_write	mov ch, 3
-	db 0x3D ; cmp ax, imm16: causes next 2 bytes to be skipped
-_read	mov ch, 2
-	mov cl, 1 ; read len
+	; issue write cmd
+	mov ch, 3
+
+	; Do disk I/O
+	; IN	ch	2 = read
+	;		3 = write
+	; Sector number is read from [drvseek]
+diskio	mov cl, 1 ; read len
 	; DS := ES := CS
-	mov ax, cs
-	mov ds, ax
-	mov es, ax
+	call rstseg
 	; check if ebios supported
-	test byte [dskflag], 4
-	jz .l00
+	test word [dflags], DRVEXT
+	jz .noext
+	; set up regs for ebios call
 	mov ax, cx
 	or ah, 0x40
 	mov si, dap
 	jmp .do
 	; check if we can skip controller reset
-.l00	test byte [dskflag], 2
-	jnz .l01
+.noext	test word [dflags], DRVRST
+	jnz .norst
 	; do controller reset
 	mov dl, [biosnum]
 	mov ah, 0
 	call int13
-	or byte [dskflag], 2
-	; put sectors per cylinder into bx
-.l01	mov ax, [bpb+BPBSPT]
-	mul word [bpb+BPBNOS]
-	mov bx, ax
+	or word [dflags], DRVRST
 	; put linear sector num into dx:ax
-	mov ax, [dskseek]
-	mov dx, [dskseek+2]
-	; dx:ax = linear count, bx = sectors / cylinder
-	div bx
+.norst	mov ax, [drvseek]
+	mov dx, [drvseek+2]
+	; dx:ax = linear count
+	div word [drvspc]
+	; TODO: is it possible to get an overflow here?
 	xchg ax, dx
 	; dx = cylinder, ax = head * spt + sector
-	mov bl, [bpb+BPBSPT]
-	div byte bl
+	div byte [drvspt]
 	; dx = cylinder, al = head, ah = sector
 	xchg dl, dh
 	ror dl, 1
@@ -274,34 +268,57 @@ _read	mov ch, 2
 	call int13
 	jc .err
 	; clear dirty flag on success
-	and byte [dskflag], 0xFE
+	and word [dflags], ~DIRTY
 	clc
 	ret
 	; assume controller is misconfigured
-.err	and byte [dskflag], 0xFD
+.err	and word [dflags], ~DRVRST
 	; exit with carry flag set
 	stc
 	ret
 
-init	mov ax, 0x0e38
-	int 0x10
+init	mov ax, cs
+	; boot failure if loaded wrong
+	cmp ax, cseg
+	je $+4
+	int 0x18
 halt	hlt
 	jmp halt
 
-section .bss
+dap	db 0x10, 0
+	dw 1
+	dw dskbuf, cseg
+drvseek	dw 0,0,0,0
 
-	; bios drive we end up using
-biosnum	resb 1
-
-	; bit 0 (1) - dirty flag for dskbuf
+	; bit 0 (1) - drive logged in (see drvnum)
 	; bit 1 (2) - controller configured
 	; bit 2 (4) - EBIOS supported
-dskflag	resb 1
+	; bit 3 (8) - dskbuf dirty
+dflags	dw 0
 
-dpt	resb 11
+section .bss
 
-bpb	resb BPBSIZ
+	; drive currently logged in
+drvnum	resb 1
+	; number, just for bios
+biosnum	resb 1
+
+	; DPT tables for 4 floppy drives
+dpt	resb 4*14
+
+; Information for logged in drive
+drvss	resb 1 ; sector size, 2^(7+n) bytes
+drvcs	resb 1 ; cluster size, 2^(7+n) bytes
+	alignb 4
+drvspt	resb 2 ; sectors per track
+drvspc	resb 2 ; sectors per cylinder
+drvoff	resb 4 ; partition offset
+drvend	resb 4 ; first sector after the partition
+drvfat	resb 4 ; offset to fat table
+drvrd	resb 4 ; offset of root directory
+drvcla	resb 4 ; offset to cluster area
+drvfn	resb 2 ; sectors per fat table
 
 	; disk buffer for I/O operations
 	alignb 2
-dskbuf	resb 512
+dskbuf	resb 1024