diff --git a/kernel/fcb.asm b/kernel/fcb.asm
index b945d0a..a299e51 100644
--- a/kernel/fcb.asm
+++ b/kernel/fcb.asm
@@ -1,25 +1,25 @@
 ; diskbuf needs to be defined
 
 ; User-accessible part of FCB (12 bytes)
-%define fcb_drv   0 ; 1 byte, 1=A: 2=B: 3=C:
-%define fcb_fn    1 ; 8 bytes
-%define fcb_ext   9 ; 3 bytes
+fcb_drv:   equ 0  ; 1 byte, 1=A: 2=B: 3=C:
+fcb_fn:    equ 1  ; 8 bytes
+fcb_ext:   equ 9  ; 3 bytes
 
 ; Drive & FS data (6 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: 0=128, 1=256, 2=512, ...
-%define fcb_cs    15 ; byte    cluster size: 0=128, 1=256, 2=512, ...
-%define fcb_co    16 ; word    start sector for theoretical cluster 0
+fcb_spt:   equ 12 ; byte    sectors per track
+fcb_nos:   equ 13 ; byte    number of sides/heads
+fcb_ss:    equ 14 ; byte    sector size: 0=128, 1=256, 2=512, ...
+fcb_cs:    equ 15 ; byte    cluster size: 0=128, 1=256, 2=512, ...
+fcb_co:    equ 16 ; word    start sector for theoretical cluster 0
 
 ; Read/Write pointer (6 bytes)
-%define fcb_left  18 ; word    number of bytes left to read from diskbuf
-%define fcb_off   20 ; dword   offset in disk (not in file)
+fcb_left:  equ 18 ; word    number of bytes left to read from diskbuf
+fcb_off:   equ 20 ; dword   offset in disk (not in file)
 
 ; Link to directory item (4 bytes)
-%define fcb_ds    24 ; dword   offset of directory item
+fcb_ds:    equ 24 ; dword   offset of directory item
 
-%define fcb_end   32
+fcb_end:   equ 32
 
 fcb_open_rootdir:
   mov dl, 0x01
@@ -40,8 +40,11 @@ fcb_open_rootdir:
 
   ; copy sector size
   mov ax, [diskbuf + 0x0B]
+  ; ... save as number of valid bytes in diskbuf
+  mov WORD [bx+fcb_left], ax
   call log2
   sub ax, 7
+  ; ... save as sector size
   mov byte [bx+fcb_ss], al
 
   ; copy cluster size
@@ -51,17 +54,14 @@ fcb_open_rootdir:
   add al, [bx+fcb_ss]
   mov byte [bx+fcb_cs], al
 
-  ; transfer sectors per track
+  ; copy sectors per track
   mov al, [diskbuf + 0x18]
   mov byte [bx+fcb_spt], al
 
-  ; transfer number of heads/sides
+  ; copy number of heads/sides
   mov al, [diskbuf + 0x1A]
   mov byte [bx+fcb_nos], al
 
-  ; first 128 bytes are always valid
-  mov WORD [bx+fcb_left], 0x80
-
   ret
 
 fcb_get_sector_mask:
diff --git a/kernel/main.asm b/kernel/main.asm
index 72efe25..92da89f 100644
--- a/kernel/main.asm
+++ b/kernel/main.asm
@@ -1,24 +1,23 @@
 cpu 8086
 
-; Memory layout (starting from top)
-%define self    0xF000
-
-; A single sector for deblocking
-%define diskbuf (self-0x200)
-
-%define stack   (diskbuf)
-
-%define default_drive BYTE [0x4]
+self:    equ 0xF000
+diskbuf: equ (self-0x200) ; deblocking
+stack:   equ (diskbuf) ; grows down
+defdrv:  equ 4
 
 org self
 jmp init
+pad 3
+
+self_offset:
+  db (self >> 8)
 
 banner:
   db "RDOS 2019-09", 0x0A, 0x0D, '$', 0
 
 init:
   mov sp, stack
-  mov default_drive, 0x01
+  mov byte [defdrv], 0x01
 
   mov dx, banner
   call print_string