compute checksum for roms bigger than a segment

Commit Message

Glauber Costa March 25, 2009, 3:06 p.m. UTC

Some option roms (e1000 provided by gpxe project as an example)
are bigger than a segment. The current algorithm to compute the
checksum fails in such case. To proper compute the checksum, this
patch deals with the possibility of the rom's size crossing a
segment border.

We don't need to worry about it crossing more than one segment
border, since the option roms format only save one byte to store
the image size (thus, maximum size = 0xff = 128k = 2 segments)

[ including improvements suggested by malc ]

Signed-off-by: Glauber Costa <[e-mail]>
---
 bios/rombios.c |   33 +++++++++++++++++++++++++++------
 1 files changed, 27 insertions(+), 6 deletions(-)

Patch

diff --git a/bios/rombios.c b/bios/rombios.c
index bc43251..5e7ad24 100644
--- a/bios/rombios.c
+++ b/bios/rombios.c
@@ -10162,22 +10162,43 @@  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