diff --git a/bios/rombios.c b/bios/rombios.c

index c4f6ccd..c4bfe60 100644

--- a/bios/rombios.c

+++ b/bios/rombios.c

@@ -10196,22 +10196,43 @@ no_serial:

   ret

 rom_checksum:

-  push ax

-  push bx

-  push cx

+  pusha

+  push ds

+

   xor  ax, ax

   xor  bx, bx

   xor  cx, cx

+  xor  dx, dx

+

   mov  ch, [2]

   shl  cx, #1

+

+  jnc checksum_loop

+  xchg dx, cx

+  dec  cx

+

 checksum_loop:

   add  al, [bx]

   inc  bx

   loop checksum_loop

+

+  test dx, dx 

+  je checksum_out

+

+  add  al, [bx]

+  mov  cx, dx

+  mov  dx, ds

+  add  dh, #0x10

+  mov  ds, dx

+  xor  dx, dx 

+  xor  bx, bx

+

+  jmp  checksum_loop

+

+checksum_out:

   and  al, #0xff

-  pop  cx

-  pop  bx

-  pop  ax

+  pop  ds

+  popa 

   ret