Re: my simple OS won't boot in QEMU

[Donald R Laster Jr]

Bill,

  The first question to consider is how did you remove /dev/sda4?  Did you use 
[c]fdisk, or an appropriate W95/DOS disk partition tool, to delete the 
/dev/sda4 partition, or was some other manual method used (i.e. a short cut 
copy process).  It is possible the loader is expecting specific information in 
the disk partition table. Also, remember the last sector on the disk often 
contains a copy of the MBR.

  I did a duckduckgo search using "disk partition boot record" and one of the 
sites found with information that might be useful is:

        https://datacadamia.com/data_storage/mbr

What you may want to do is setup the virtual machine that is using the disk 
image to effectively boot from a bootable CD/DVD device/image and from within 
the booted OS change the disk partition data  with [c]fdisk or appropriate 
W95/DOS disk partition tool.

  And since you are using what appears to be a raw image, i.e. .img, the way 
you removed /dev/sda4 may have caused the issue due to the you not using the 
expected size of the disk image.

  Don


bilsch01 wrote on 4/2/21 3:15 PM:
> This post concerns booting sda2 using QEMU.  sda2 boots just fine when run on 
> HD.
> The layout of the HD looks like this:
>
> Device     Boot     Start             End Sectors    Size       Id    Type
> /dev/sda1            63                3903794         3903732 1.9G       6    
> FAT16
> /dev/sda2   *        3903795      4032314           128520 62.8M    e     W95 
> FAT16 (LBA)
> /dev/sda3            4032315      4082714             50400 24.6M    e     W95 
> FAT16 (LBA)
> /dev/sda4            4083710   976771071     972687362 463.8G 5     Extended
>
> The command line I have been using is:  qemu-system-i386  -drive 
> file=qemu4dell.img
>
> File qemu4dell.img is an image of the first 3 partitions created using the 
> linux dd command as follows:
>
> dd if=/dev/sda of=qemu4dell.img bs=512 count=4082720 (the file has 6 extra 
> bytes).
>
> In the first sector (the MBR) of qemu4dell.img I have zeroed out line 4 of the 
> partition table because the file does not include partition 4.
>
> If the boot flag is set for sda1 in file qemu4dell.img the command line above 
> boots sda1 (MSDOS 6.22) just fine.
>
> If the boot flag is set for sda2 in file qemu4dell.img the command line above 
> results in error message:
>
> missing operating system.
>
> Careful examination of the boot program (IPL) in the first sector of 
> qemu4dell.img shows that that message occurs if the IPL can't find the hex 
> value AA55 at the end of the first sector of the partition to be booted (which 
> is sda2). I refer to the first sector of sda2 as vbs2. I have double checked in 
> qemu4dell.img and ensured that vbs2 sector is in the correct location in the 
> file and that the value AA55 is in the correct place in the sector.
>
> I can see no reason why QEMU doesn't boot sda2, because QEMU can boot sda1 ok, 
> and also sda2 boots fine on the HD itself. Perhaps someone in this QEMU group 
> knows the answer.
>
> Thanks.
>
>
> Bill Schaible