[syslinux] A bug in syslinux boot code

[Gene Cumm]

On Sun, Feb 6, 2011 at 08:50,  <j1borries at online.de> wrote:
> I used syslinux 4.03 for my brand new usb-stick (16GB FAT32, no Partition Table). It works, but on some computers i get "boot error".

Part of this may be that the BIOS of some of your computers want to
treat this as a superfloppy and not an unpartitioned hard disk.
2,105,671,680 bytes (256*255*63*512) is the maximum size that should
be treated as a superfloppy.  Every single UFD (USB flash drive) I
have came partitioned from the factory just to force it to be treated
as a hard disk.  I believe anything over ~8GB requires EDD calls to
work.  What's the type specified in the banner ("CHS" or "EDD") on
machines that do work and machines that don't?  Here's an example with
CHS:

SYSLINUX 4.03 2010-10-22 CHS Copyright (C) 1994-2010 H. Peter Anvin et al

> My investigation reveals three bugs in the boot code.
>
> Int 13 function 8 destroys the contents of register ES. Ah, now it is corrected in version 4.04 pre 5. Well done. I am happy now.
> But there are still other bugs.
>
> Look on version 4.04 pre 5, core/bootdisc.inc, line 224: Int 13 function 8 ist called with DL = 0 (in my case), not with DL = Drive. This is because at label "harddisk" a segment address was loaded into DX. So int 13 returns wrong values.

Yes, DL is garbage at this point however it calls xint13 which sets DL
prior to issuing INT13h.  You'll need to look there for proper values.
 The comments in core/diskboot.inc should be updated to reflect this.

> Now look on version 4.04 pre 5, core/bootdisc.inc, line 165 ("mov [di-76+8],cl"):
> I think the intention was "mov [di-12+4],cl" This is the place of the sector count in the disk drive parameter table.
>
> Juergen

I'm still trying to look at this one.  I know there are some places
where data is loaded multiple times.

-- 
-Gene