diff --git a/src/sys.asm b/src/sys.asm
index bc565e2..7dcfcbf 100644
--- a/src/sys.asm
+++ b/src/sys.asm
@@ -15,7 +15,12 @@ main	mov di, 0x81
 	mov cl, [0x80]
 	repe scasb
 	repne scasb
-	mov si, di
+	test cx, cx
+	jnz rddrv
+	call putsh
+	db "BIOS drive number not specified", 0x0A, 0x0D, 0
+	int 0x20
+rddrv	mov si, di
 	call lodnum
 	mov dx, ax
 	test dx, ~0x83
@@ -37,17 +42,23 @@ floppy	mov ax, 0x0201
 	int 0x13
 	jc drverr
 
+	mov ax, [prog+0x016]
+	cmp ax, 12
+	jbe .gok
+	call putsh
+	db "Unsupported FAT size", 0x0A, 0x0D, 0
+	int 0x20
+.gok	nop
+
 	; calculate start of cluster area
 	mov ax, [prog+0x011] ; number of dir entries
 	mov cl, 5
 	shl ax, cl
 	mov bx, [prog+0x0B] ; bytes per sector
 	div bx
-	; TODO: abort if dx is not zero
 	mov cx, ax
 	mov ax, [prog+0x016] ; logical sectors per fat
-	; TODO: abort if ax is zero (=large FAT32)
-	mov bl, [prog+0x010]
+	mov bl, [prog+0x010] ; number of fats
 	mov bh, 0
 	mul bx
 	add ax, cx
@@ -55,24 +66,36 @@ floppy	mov ax, 0x0201
 	mov si, ax
 
 	; calculate sector offset of target track
-	mov ax, [prog+0x018]
-	mul word [prog+0x01A]
+	mov ax, [prog+0x018] ; sectors per track
+	mul word [prog+0x01A] ; number of heads
 	mul word [track]
 
-	; calculate cluster offset of target track
+	; calculate cluster offset of target track -> SI
 	sub ax, si
 	mov bl, [prog+0x00D] ; sectors per cluster
 	mov bh, 0
-	call debug
+	xor dx, dx
 	div bx
+	mov si, ax
+
+	; calculate cluster length of target track -> DI
+	mov ax, [prog+0x018] ; sectors per track
+	add ax, dx ; remainder
+	; add sectors per cluster minus one
+	add ax, bx
+	dec ax
+	; divide by sectors per cluster and floor
+	xor dx, dx
+	div bx
+	mov di, ax
+
+	; load fat table
 	call debug
 
 	ret
+
 ; TODO:
-; calculate cluster offset of target track
-; calculate cluster length of target track
 ; calculate offset into fat table
-; load part of fat table spanning our track
 ; check that all relevant clusters are 0 or 0xFFF7
 ; set all relevant clusters to 0xFFF7
 ; track is now available for direct writing