diff --git a/kernel/bpb.asm b/kernel/bpb.asm
deleted file mode 100644
index f0213e5..0000000
--- a/kernel/bpb.asm
+++ /dev/null
@@ -1,31 +0,0 @@
-; IN	DL	drive number
-;	BP	ptr to bpb_size'd buffer
-bpb_load:
-  push ax
-  push dx
-
-  mov [bp+(bpb_size-1)], dl
-
-  ; load first sector
-  xor dx, dx
-  xor ax, ax
-  call drive_read
-
-  push cx
-  push si
-  push di
-
-  ; copy BPB to BP
-  mov si, (diskbuf+0x0B)
-  mov di, bp
-  mov cx, (bpb_size-1)
-  rep movsb
-
-  pop di
-  pop si
-  pop cx
-
-  pop dx
-  pop ax
-  ret
-
diff --git a/kernel/drive.asm b/kernel/drive.asm
index aa4dfc0..cdb1600 100644
--- a/kernel/drive.asm
+++ b/kernel/drive.asm
@@ -1,6 +1,32 @@
+; Load a sector from drive.
+; IN    DX:AX   sector number (lba)
+;       BX      ptr to FCB
+; OUT	CL	Sector number
+;	CH	Cylinder number
+;	DH	Head number
+drive_chs_calc:
+  push bx      ; backup fcb ptr
+  mov bx, [bx+fcb_spt] ; nos is in BH now
+  push bx      ; backup nos
+  xor bh, bh   ; zero out nos so spt is left
+  div word bx  ; ax:temp     = (lba / spt)
+  inc dx       ; dx:sector   = (lba % spt) + 1
+  mov cl, dl   ; sector number
+
+  pop bx       ; restore nos
+  xchg bl, bh  ; put nos into bl
+  xor bh, bh   ; zero out spt
+  div word bx  ; ax:cylinder = (temp / nos)
+               ; dx:head     = (temp % nos)
+  mov ch, al   ; cylinder number
+  mov dh, dl   ; head number
+
+  pop bx       ; restore fcb ptr
+  ret
+
 ; Load a sector from drive.
 ; IN    DX:AX   sector number
-;	BP	ptr to BPB
+;	BX	ptr to FCB
 drive_read:
   push ax
   push cx
@@ -12,19 +38,14 @@ drive_read:
   or cx, ax
   jz .fast
 
-  div word [bp+13] ; ax:temp     = (lba / spt)
-  inc dx           ; dx:sector   = (lba % spt) + 1
-  mov cl, dl       ; sector number
+  call drive_chs_calc
 
-  div word [bp+15] ; ax:cylinder = (tmp / heads)
-                   ; dx:head     = (tmp % heads)
-  mov ch, al       ; cylinder number
-  mov dh, dl       ; head number
   mov si, 5        ; retry count
 .try:
-  mov dl, [bp+(bpb_size-1)] ; drive number
-  mov bx, diskbuf
+  mov dl, [bx+fcb_drv] ; drive number
+  dec dl
   mov ax, 0x0201
+  mov bx, diskbuf
   int 0x13
 .giveup:
   pop si
diff --git a/kernel/exec.asm b/kernel/exec.asm
index 2e416b5..2632196 100644
--- a/kernel/exec.asm
+++ b/kernel/exec.asm
@@ -7,7 +7,7 @@ exec:
   mov bx, sp
 
   call fcb_parse
-  call fcb_open
+  call fcb_open_rootdir
 
   add sp, fcb_size
   pop bx
diff --git a/kernel/fcb.asm b/kernel/fcb.asm
index 892e893..ddc6185 100644
--- a/kernel/fcb.asm
+++ b/kernel/fcb.asm
@@ -1,70 +1,58 @@
-; Parse ASCIIZ string into FCB
-; IN	SI	ptr to filename
-;       BX	ptr to FCB
-fcb_parse:
-  push di
-  push ax
-  mov di, bx
-  xor ax, ax
-  stosb
-.cleanout:
-  push di
-  mov cx, 0x0A
-  mov al, 0x20
-  rep stosb
-  pop di
-.base_loop:
-  call .read
-  cmp al, 0x2E
-  je .ext_start
-  cmp al, 0x20
-  je .ret
-  stosb
-  jmp .base_loop
-.ext_start:
-  mov di, bx
-  add di, 9
-.ext_loop:
-  call .read
-  cmp al, 0x20
-  je .ret
-  stosb
-  jmp .ext_loop
-.read:
-  lodsb
-  test al, al
-  jz .eret
-  cmp al, 0x0D
-  je .eret
-  ret
-.eret:
-  dec si
-  pop ax
-.ret:
-  pop ax
-  pop di
-  ret
+; User-accessible part of FCB (12 bytes)
+%define fcb_drv   0 ; 1 byte
+%define fcb_fn    1 ; 8 bytes
+%define fcb_ext   9 ; 3 bytes
 
-fcb_open:
+; Drive & FS data (4 bytes)
+%define fcb_spt   12 ; byte    sectors per track
+%define fcb_nos   13 ; byte    number of sides/heads
+%define fcb_ss    14 ; byte    sector size: 2^ss
+%define fcb_cs    15 ; byte    cluster size: 2^(ss+cs)
+
+; Read/Write pointer (12 bytes):
+; Will be overwritten by new filename for CL=17
+%define fcb_fsize 16 ; dword   file size
+%define fcb_fptr  20 ; dword   handle position
+%define fcb_clus  24 ; word    current cluster fptr points in
+%define fcb_co    26 ; word    start sector for theoretical cluster 0
+
+; Link to directory item (3 bytes)
+%define fcb_ds    28 ; word    directory sector
+%define fcb_do    30 ; byte    directory offset
+
+fcb_open_rootdir:
   cmp byte [bx], 0
   jne .drivedone
   mov al, default_drive
   inc al
   mov [bx], al
 .drivedone:
-  push bp
-  sub sp, bpb_size
-  mov bp, sp
-
   mov dl, [bx]
   dec dl
 
-  call bpb_load
-  mov ax, [bp]
-  mov cx, [bp+2]
-  mov dx, [bp+4]
-  mov bx, [bp+6]
+  xor ax, ax
+  xor dx, dx
+  ; set directory sector to 0 (=rootdir)
+  mov [bx+fcb_ds], ax
+  ; load first sector
+  call drive_read
+
+  ; transfer sector size
+  mov ax, [diskbuf + 0x0B]
+  call log2
+  mov byte [bx+fcb_ss], al
+
+  ; transfer cluster size
+  mov al, [diskbuf + 0x0D]
+  call log2
+  mov byte [bx+fcb_cs], al
+
+  ; transfer sectors per track
+  mov al, [diskbuf + 0x18]
+  mov byte [bx+fcb_spt], al
+
+  ; transfer number of heads/sides
+  mov al, [diskbuf + 0x1A]
+  mov byte [bx+fcb_nos], al
 
-  add sp, bpb_size
-  pop bp
   ret
diff --git a/kernel/main.asm b/kernel/main.asm
index e5154c1..648c02c 100644
--- a/kernel/main.asm
+++ b/kernel/main.asm
@@ -13,8 +13,7 @@ cpu 8086
 
 %define default_drive BYTE [0x5B]
 
-%define fcb_size 16
-%define bpb_size 18
+%define fcb_size 31
 
 org self
 jmp init
@@ -70,5 +69,6 @@ putc:
 
 %include "exec.asm"
 %include "fcb.asm"
-%include "bpb.asm"
+%include "fcbparse.asm"
 %include "drive.asm"
+%include "log2.asm"
diff --git a/lib/log2.asm b/lib/log2.asm
new file mode 100644
index 0000000..cdc2bb8
--- /dev/null
+++ b/lib/log2.asm
@@ -0,0 +1,14 @@
+; Calculate dual logarithm (int)
+; IN	AX	number
+; OUT	AX	dual logarithm
+log2:
+  push cx
+  mov cx, 16
+.loop:
+  sar ax, 1
+  loopnz .loop
+  xchg ax, cx
+  pop cx
+  neg ax
+  add ax, 15
+  ret