A few stupid questions regarding MBR and boot0

[Trond Endrestøl]

On Thu, 14 Apr 2016 14:00+0200, Trond Endrestøl wrote:

> On Thu, 14 Apr 2016 14:48+0300, Aleksander Alekseev wrote:
> 
> > Hello
> > 
> > I was trying to figure out how FreeBSD boot process works and noticed
> > something strange.
> > 
> > 1) If I understand correctly first 512 bytes of /dev/ad0
> > and /boot/boot0 file should match, but they don't:
> > 
> > http://afiskon.ru/s/31/665dc9755c_mbr.txt
> 
> The MBR consists of two parts, the bootcode and the partition table. 
> Any discrepancies is generally due to the partition table being zero 
> in /boot/boot0 and realistic data stored in block 0.

BTW, check out /boot/mbr.

> > Could someone please explain why they don't match?
> > 
> > 2) I tried to compile boot0 from source (r297956) but compilation
> > failed:
> > 
> > ```
> > $ cd sys/boot/i386/boot0
> > $ make
> > 
> > Warning: Object directory not changed from
> > original /usr/home/eax/freebsd-src/sys/boot/i386/boot0
> > 
> > /usr/local/bin/clang38 -O2 -pipe -O2 -pipe -g -march=native
> > -DVOLUME_SERIAL -DPXE -DFLAGS=0x8f  -DTICKS=0xb6  -DCOMSPEED="7 << 5 +
> > 3" -march=i386 -ffreestanding  -msoft-float -m32 -std=gnu99
> > -Qunused-arguments -m32 -c boot0.S -o boot0.o clang -cc1as: fatal
> > error: error in backend: invalid .org offset '430' (at offset '439')
> > *** Error code 1
> > 
> > Stop.
> > make: stopped in /usr/home/eax/freebsd-src/sys/boot/i386/boot0
> > ```
> > 
> > Am I doing something wrong?
> 
> I have never attempted to build the FreeBSD sources outside /usr/src. 
> The advice below might be plain wrong.
> 
> Try:
> 
>   cd sys/boot/i386/boot0
>   make clean
>   make OBJDIR=/usr/home/eax/freebsd-obj obj
>   make OBJDIR=/usr/home/eax/freebsd-obj all
> 
> If you want it installed, run
> 
>   make OBJDIR=/usr/home/eax/freebsd-obj install

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