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><H1
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><A
NAME="BIOS-TIPS"
>BIOS Tips</A
></H1
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="AEN3168"
>Booting from CD-ROMs</A
></H2
><P
>A bootable CD-ROM has a special layout that is detected by the
BIOS boot loader code, and executed if it conforms the specifications.
This layout is called "El Torito Bootable CD-ROM Format Specification"
and has been published by Phoenix and IBM. A copy of this spec is on
<A
HREF="http://bochs.sourceforge.net/techdata.html"
TARGET="_top"
>Bochs tech specs page</A
>.
</P
><P
>El Torito specifies 3 ways to have a bootable CD:
<P
></P
><UL
><LI
><P
>floppy emulation boot: A standard floppy image is burnt on the CD.
In this case the BIOS has to redirect all first floppy accesses to this
image and the real floppy drive becomes the second one.</P
></LI
><LI
><P
>a "no emulation" boot: In this case the BIOS is instructed to load an
arbitrary number of sectors straight into memory, and execute it.</P
></LI
><LI
><P
>hard disk emulation: A hard disk image is burnt on the CD. The
BIOS has to redirect all hard disk accesses to that image. The real hard disks
are still available, with BIOS numbers 81h and up.</P
></LI
></UL
>
In Bochs 2.0, hard disk emulation is not implemented in the BIOS.
There are also subtilities about multiple boot-images CD-ROMs, that are
not handled by Bochs.
</P
><P
>However, our BIOS may be more strict than real PC BIOSes, I don't know.
But I would definitely be interested to know of any CD that can boot
on real hardware, but does not in Bochs.
</P
><P
>When failing to boot from CD-ROM, the BIOS outputs
the reason of the failure as
an error code, in the log file, and on the screen.
</P
><P
>Here is a summary of what can happen when booting from the CD.
</P
><DIV
CLASS="TABLE"
><A
NAME="AEN3183"
></A
><P
><B
>Table 1. CD Boot error codes</B
></P
><TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
CELLSPACING="0"
CELLPADDING="4"
CLASS="CALSTABLE"
><THEAD
><TR
><TH
>Error code</TH
><TH
>Reason</TH
></TR
></THEAD
><TBODY
><TR
><TD
> 0x01 </TD
><TD
> no atapi device found </TD
></TR
><TR
><TD
> 0x02 </TD
><TD
> no atapi cdrom found </TD
></TR
><TR
><TD
> 0x03 </TD
><TD
> can not read cd - BRVD </TD
></TR
><TR
><TD
> 0x04 </TD
><TD
> cd is not eltorito (BRVD) </TD
></TR
><TR
><TD
> 0x05 </TD
><TD
> cd is not eltorito (ISO TAG) </TD
></TR
><TR
><TD
> 0x06 </TD
><TD
> cd is not eltorito (ELTORITO TAG) </TD
></TR
><TR
><TD
> 0x07 </TD
><TD
> can not read cd - boot catalog </TD
></TR
><TR
><TD
> 0x08 </TD
><TD
> boot catalog : bad header </TD
></TR
><TR
><TD
> 0x09 </TD
><TD
> boot catalog : bad platform </TD
></TR
><TR
><TD
> 0x0A </TD
><TD
> boot catalog : bad signature </TD
></TR
><TR
><TD
> 0x0B </TD
><TD
> boot catalog : bootable flag not set </TD
></TR
><TR
><TD
> 0x0C </TD
><TD
> can not read cd - boot image </TD
></TR
></TBODY
></TABLE
></DIV
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
>0x01 no atapi device found
0x02 no atapi cdrom found</PRE
></TD
></TR
></TABLE
>
For the first two errors, an ata-*: type=cdrom is probably missing
from the configuration file. This is what you get if no cdrom has
been defined in Bochs conf file.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
>0x03 can not read cd - BRVD</PRE
></TD
></TR
></TABLE
>
For this error, the cdrom support has not been compiled in Bochs,
or Bochs could not open the file or device. This is what you get if
Bochs is not able to read the cd.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
>0x04 cd is not eltorito (BRVD)
0x05 cd is not eltorito (ISO TAG)
0x06 cd is not eltorito (ELTORITO TAG)</PRE
></TD
></TR
></TABLE
>
For these errors, the data has been read from the cd, but
the cd does not conform to the El Torito specification. This
is what you get if the cd is not bootable.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
>0x08 boot catalog : bad header
0x09 boot catalog : bad platform
0x0A boot catalog : bad signature
0x0B boot catalog : bootable flag not set</PRE
></TD
></TR
></TABLE
>
now the cd is eltorito, but the boot catalog is corrupted, or
the cd was made to boot on a ppc system. This should not happen
for a x86 bootable cd.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
>0x07 can not read cd - boot catalog
0x0C can not read cd - boot image</PRE
></TD
></TR
></TABLE
>
here, specific part of the cd could not be read. This should
definitely not happen.</P
></DIV
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="BIOS-DISK-TRANSLATION"
>Disk translation</A
></H2
><P
>Since the beginning of the PC era, disks have grown in size by a factor of 10000. Due to
differences between the ATA specification and BIOSes implementations, when disks reached
critical sizes, it
became necessary to translate the CHS geometry (cylinders, heads, sectors per track)
between the BIOS (int 13h) and the ATA interface. Please refer to the
<A
HREF="http://burks.brighton.ac.uk/burks/pcinfo/hardware/atafaq/atafq.htm"
TARGET="_top"
>ATA-FAQ</A
>
and
<A
HREF="http://www.ata-atapi.com/hiwchs.htm"
TARGET="_top"
>Hale Landis' document</A
>
for a complete discussion of the problem.
</P
><P
>Unfortunately, there has never been any standard on the translation algorithms.
</P
><P
>Bochs implements 4 well-known algorithms, selectable in the configuration file
in the "<B
CLASS="COMMAND"
>ataX-xxxx: ..., translation='algorithm'</B
>" section.
</P
><DIV
CLASS="TABLE"
><A
NAME="AEN3245"
></A
><P
><B
>Table 2. Disk translation algorithms</B
></P
><TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
CELLSPACING="0"
CELLPADDING="4"
CLASS="CALSTABLE"
><THEAD
><TR
><TH
>Algorithm</TH
><TH
>Maximum disk size</TH
><TH
>Maximum logical and physical geometry (CHS)</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
>none</TD
><TD
>528MB (1032192 sectors)</TD
><TD
> LCHS:1024/16/63
PCHS:1024/16/63
</TD
><TD
> no translation is done. The CHS received at the int13h interface
is sent as is to the ATA interface.
</TD
></TR
><TR
><TD
>large</TD
><TD
>4.2GB (8257536 sectors)</TD
><TD
> LCHS:1024/128/63
PCHS:8192/16/63
</TD
><TD
> a standard bitshift algorithm (named Extended-CHS)
is used to translate the CHS between
the int13h interface
and the ATA interface. The translation is achieved by
multiplying/dividing the cylinder/head count by a power of 2
(2, 4 or 8).
(a factor of 16 could not be used because the
head count would become 256, and MS-DOS thought this was 0)
Note that the number of sectors per track is not changed, so
a lower spt value will lead to a lower maximum disk size.
</TD
></TR
><TR
><TD
>echs</TD
><TD
> </TD
><TD
> </TD
><TD
>synonym for large</TD
></TR
><TR
><TD
>rechs</TD
><TD
>7.9GB (15482880 sectors)</TD
><TD
> LCHS:1024/240/63
PCHS:15360/16/63
</TD
><TD
> a revised bitshift algorithm (called Revised Extended-CHS)
is used to translate the CHS between
the int13h interface
and the ATA interface. First the number of physical heads is forced to
15, and the number of cylinders is adjusted accordingly.
Then, as in the simple extended CHS algorithm, the translation
is achieved by
multiplying/dividing the cylinder/head count by a power of 2
(2, 4, 8 or 16).
The head count being forced to 15, it can safely be multiplied by 16
without crashing dos.
Note that the number of sectors per track is not changed, so
a lower spt value will lead to a lower maximum disk size.
</TD
></TR
><TR
><TD
>lba</TD
><TD
>8.4GB (16450560 sectors)</TD
><TD
> LCHS:1024/255/63
PCHS:16320/16/63
</TD
><TD
> a LBA-assisted algorithm
is used to translate the CHS between
the int13h interface
and the ATA interface. The translation is achieved by
first computing the physical size of the disk (LBA=C*H*S).
Then the sectors per track is forced to 63, and the head count
to 255. Then the cylinder count is computed (C=LBA/(63*255))
Note that the number of sectors per track is forced to 63
in the logical geometry, regardless of the actual geometry
reported by the disk.
Also note that the LBA-assisted algorithm has nothing to do with
LBA access at the ATA interface.
</TD
></TR
><TR
><TD
>auto</TD
><TD
> </TD
><TD
> </TD
><TD
>the best suited algorithm between none, large and lba is used</TD
></TR
></TBODY
></TABLE
></DIV
><P
>Setting a specific CHS translation should be done if you use a disk dump
of an actual disk, or use a real disk as a block device. You need to
know which geometry was used to format the disk, and which translation
was used. You must not set the translation to 'auto'.
</P
><DIV
CLASS="NOTE"
><P
></P
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="./stylesheet-images/note.gif"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>rechs translation should only be useful for Compaq users who wants to
use a disk as a block device. Please report if you know any other
system that use such translation.
</P
></TD
></TR
></TABLE
></DIV
><P
>If you plan to create a new disk image (for example with bximage),
format it and install an OS on it, select the "auto" translation
for an automatic selection
of the best algorithm based on the disk image size. Be warned that an image created
with the "auto" translation might not be readable with previous versions of Bochs.
Upward compatibility will be maintained.
</P
><DIV
CLASS="NOTE"
><P
></P
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
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><IMG
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><TD
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><P
>This translation applies only to int13h BIOS disk accesses. Older OSes (e.g. MS-DOS)
tend to use them a lot. On modern OSes, disk accesses through BIOS int13h are
limited to boot loaders.
The usual rules and tricks of the installed OS still apply (ie 1024 cylinders boot limit).</P
></TD
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