Can now not create a file with open_file

ponydos.asm

@@ -62,6 +62,7 @@ load_shell:
 	push word 0x1000
 	pop es
 	mov si, shell_name
+	xor dx, dx
 	call 0:open_file
 	xor bx, bx
 	xor di, di ; read
@@ -90,7 +91,7 @@ draw:
 	; Draw windows
 	push cs ; Return segment
 	push word mainloop ; Return offset
-	mov bx, [window_chain_head]
+	mov bx, [di - mouse_column + window_chain_head]
 	mov ax, 0xf000
 	and ax, bx
 	push ax ; Call segment
@@ -100,7 +101,7 @@ draw:
 	retf
 
 mainloop:
-	cmp byte [redraw], 0
+	cmp byte [di - mouse_column + redraw], 0
 	jne draw
 
 	mov bx, [di - mouse_column + mouse_x]
@@ -299,6 +300,7 @@ shell_name db 'shell.bin', 0
 ; out:
 ;  ax = LBA of first sector, 0 if no space left
 ;  cx = length in sectors
+;  dx = non-zero => do not create new file
 ;  di = dirent address (in GLOBAL_DIRENTS)
 ; [Far calls only]
 open_file:
@@ -354,6 +356,9 @@ open_file:
 	retf
 
 	.create_file:
+		test dx, dx
+		jnz .return
+
 		; TODO: zero out the sector for this file?
 		inc word [es:di]
 

shell.asm

@@ -44,6 +44,7 @@ process_event:
 initialize:
 	; Set wallpaper
 	mov si, wallpaper_name
+	xor dx, dx
 	call PONYDOS_SEG:SYS_OPEN_FILE
 
 	mov bp, PONYDOS_SEG