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Bug in stage2 loader (stage2/start.S)

From: Herbert Nachtnebel
Subject: Bug in stage2 loader (stage2/start.S)
Date: Mon, 9 Oct 2000 12:24:08 +0200

To the grub team!

If one boots with lba mode enabled, than the routine copy_buffer
in stage2/start.S alters the cx register. But the ecx register
holds the total number of sectors on disc and the geometry check
fails! A bugfix to solve this problem is appended below.

Note that failure this may also explain the behavior you solved with
the NO_BUGGY_BIOS_IN_THE_WORLD flag. I think therefore, that the change
from 2000-09-05 missed the point and this flag should go away again!

Why have nobody before spotted the problem? Well, the copy_buffer
routine sets the cx register to 0, hence the total number of sectors
is the real sector count modulus 65536. Therefore this is only a
problem when the stage2 is located at the end of the disc.

Kind regards,
Herbert Nachtnebel.
DI Herbert Nachtnebel                address@hidden
Institute of Industrial Electronics           Tel: +43-1-58801-36663 
and Material Sience (CAD-Division)            Fax: +43-1-58801-36699
Gusshausstrasse 27-29/E366-2, A-1040 Wien, AUSTRIA

PS.: Patch against cvs checkout from Oct 9 appended:
diff -u -r grub-orig/ChangeLog grub-fixed/ChangeLog
--- grub-orig/ChangeLog Sun Oct  8 05:42:36 2000
+++ grub-fixed/ChangeLog        Mon Oct  9 11:53:58 2000
@@ -1,3 +1,10 @@
+2000-10-09  NACHTNEBEL Herbert <address@hidden>
+       * stage2/start.S: Bugfix. Altered ecx register in copy_buffer
+       routine but this register contains the total number of sectors
+       in lba mode: ouch. I think entry from 2000-09-05 missed the point
+       and NO_BUGGY_BIOS_IN_THE_WORLD option should go away!
 2000-10-08  OKUJI Yoshinori  <address@hidden>
        * util/grub-install.in (grub_prefix): New variable. The default
diff -u -r grub-orig/stage2/start.S grub-fixed/stage2/start.S
--- grub-orig/stage2/start.S    Sun Sep 10 10:46:52 2000
+++ grub-fixed/stage2/start.S   Mon Oct  9 11:48:09 2000
@@ -103,7 +103,9 @@
        jge     geometry_error
-       /* the maximum is limited to 0x7f because of Phoenix EDD */
+       /* the maximum is limited to 0x7f because of Phoenix EDD,
+          and even more important: we are in 16 bit mode and need
+       a buffer copy => stay away from 64K border! */
        xorl    %eax, %eax
        movb    $0x7f, %al
@@ -279,15 +281,16 @@
        addw    %ax, 6(%di)     /* add the corrected value to the destination
                                   address for next time */
-       /* get the copy length */
-       shlw    $4, %ax
-       movw    %ax, %cx
-       /* save addressing regs */
+       /* save regs used for rep movs */
        pushw   %si
        pushw   %di
+       pushw   %cx             /* %ecx contains length of disk in LBA mode */
        pushw   %ds
+       /* get the copy length */
+       shlw    $4, %ax
+       movw    %ax, %cx
        xorw    %di, %di        /* zero offset of destination addresses */
        xorw    %si, %si        /* zero offset of source addresses */
        movw    %bx, %ds        /* restore the source segment */
@@ -302,6 +305,7 @@
           (MSG modifies SI, which is saved, and unused AX and BX) */
        popw    %ds
+       popw    %cx
        popw    %di
        popw    %si

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