Hi again,
I think my particular problem is related to this "virtual" geometry returned by
SeaBIOS, and which apparently cannot be set directly from command line. My
image file is quite small (under 4GB), this size doesn't justify the use of 255
heads. Nevertheless, this value (255 heads) is written in the partition table
and also in the BPB (BIOS parameter block) of the FAT32 partition with OS
installed on it.
So I suppose SeaBIOS decides to use a geometry which doesn't match the one
expected by OS bootloader. Is there a way to verify which geometry is used
and/or force it to particular values? This is something I would normally do via
BIOS setup utility.
Sincerely,
Dmitry
On 10/04/2015 19:11, Dmitry Grigoryev
wrote:
Hello,
I have a disk image which won't run with QEMU. Its CHS formula is
510,255,63 isn't recognized automatically, and whenever I specify
-drive file=image.raw,cyls=510,heads=255,secs=63 QEMU exits saying
"invalid physical heads number".
Is there a reason why head count above 16 isn't supported? Is
there a workaround to make my image usable by QEMU without losing
the data?
PC historic fact: The ATA hard disk interface
(electrical/register programming) is limited to max 16384 cyls, 16
heads, 63 sectors in CHS mode, 16Mi (16 binary millions) sectors
in LBA24 mode and 256Ti (16 binary Terra) sectors in LBA48 mode.
The historic BIOS INT 13h Hard disk API is limited to 1024 cyls,
255 heads, 63 sectors in CHS mode, and a lot (I guess 2**64) of
sectors in LBA mode. The historic BIOS INT 13h Floppy disk API
was limited to 256 cyls, 255 heads, 255 sectors but only supported
devices with 1 or 2 heads.
So whenever a PC BIOS (such as the SeaBios used in qemu PC-style
virtual machines) is accessing an ATA disk > 1024 cylinders, it
converts the number of cylinders and heads to a different
multiplum such as 510 * 255 * 63 rather than the 8129 * 16 * 63
ATA CHS geometry or 8193150 ATA LBA24 sectors presented by the
signals on the ATA cable.
Because qemu settings specify the virtual hardware, not the
virtual BIOS, the CHS settings passed to qemu should be the
"hardware" 8129 * 16 * 63 or just let qemu use the default for the
number of LBA sectors in your image file. Then the SeaBios inside
the virtual machine will return that as 510 * 255 * 63 to the boot
loader etc. for the few moments before those programs start
talking to the "hardware" directly.
Enjoy
Jakob
--
Jakob Bohm, CIO, Partner, WiseMo A/S. https://www.wisemo.com
Transformervej 29, 2860 Søborg, Denmark. Direct +45 31 13 16 10
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