2015/05/22

Boot Loader and OS Kernel Startup

This is a homebrew x86 bootloader and "do nothing" kernel. Developed mostly as a learning exercise.

The key source files are:

boot.S

• Initial stage 1 boot loader installed as the disks MBR
• responsible to load the kernel into memory
• There is no file system so it just loads disk sectors

bootstrap.S

• stage 2 of the boot sequence
• initialization code to config and startup the C-level kernel code
• turns on A20 address
• switch to Protected Mode -has the GDT table included as data table
• remaps the PIC IRQs so the do not conflict with CPU exceptions
• calls C-level kernel "main"
• contains the ISR handlers (isr.S) for software exceptions and hardware interrupts -these service routines set up the stack and call C-level handlers

kernel.c

• do nothing kernel, prints message to screen and loops forever
• uses VGA console routines from vgaconsole.c
• contains the ISR handlers for both software exceptions and hardware interrupts

Source Code on github
https://github.com/lockettnb/boot

OS Development Notes

On-line Tutorials

• http://www.osdever.net/bkerndev/Docs/title.htm
• http://www.osdever.net/bkerndev/Docs/irqs.htm
• http://www.osdever.net/tutorials/

QEMU/KVM

I used qemu extensively for testing this software. It worked really well for this kind of development. The ability to connect to the emulation with GDB was especially useful.

• kvm -hda program.bin --> start from harddisk
• kvm -s -S -hda program.bin --> start "stop" and waiting for gdb
• kvm -d cpu_reset -hda program.bin --> CPU registers before each reset
• kvm -d in_asm -hda program.bin --> show assemble code flow
• kvm -d ? --> see other debug options

GDB NOTES

• gdb -x gdb_init_file --> start gdb and exec commands in file
• set disassembly-flavor intel --> use intel assembler format
• layout asm --> show window with assembler
• break -start --> break at start of code
• break *0x8000 --> break at address
• info registers (i r) --> show CPU registers
• print $register --> print specific register
• disassemble -start,+count --> diss at
• x/20i $eip --> disassemble 20 instruction
• x/20i cs * 16 + eip --> disassemble 20 instruction
• x/20i 0x802a --> disassemble 20 instruction
• x/30b 0x092d --> examine 30 bytes
• x/30x 0x092d --> examine 30 bytes
• target remote localhost:1234 --> connect to running qemu emulation
• set arch i8086 --> set disassemble for 16 bit (real mode)
• set arch i386 --> set disassemble for 32bit (protected mode)
• define ..... end --> define user command (short cuts)

Create Raw Disk Image

This sequence create a fake disk with the boot program to the first sector and the kernel program to the second sector. Essentially this is a raw disk with no filesystem. This fake disk is used to boot the qemu emulation for testing.

    dd if=/dev/zero of=drive.bin bs=512 count=16
    dd if=boot.bin of=drive.bin bs=512 conv=notrunc
    dd if=os.bin of=drive.bin bs=512 seek=1 conv=notrunc

This sequence creates a disk image with a ext filesystem. The bootloader installed on the first sector. Once this disk is mounted you could write files onto the image. (I never got this far)

dd if=/dev/zero of=disk.img bs=1024 count=1024
mkfs.ext disk.img
dd if=boot.bin of=disk.img
mount -o loop disk.img /mnt/virtual

Virtual Box Disk Swap

Generally I find this a royal pain and would recommend using qemu/kvm or another virtual system.

Assuming the VM already has an attached hard disk.

Remove the old disk file:

rm -rf drive.vdi

Convert the new binary disk file to VDI format

VBoxManage convertfromraw drive.bin drive.vdi --format VDI

Change the VM to have no disk file attached to the drive

VBoxManage storageattach boot --storagectl "IDE Controller" --port 0 --device 0 --type hdd --medium none

Have to be in the VMs directory so VBoxManage can find the disk file. We "close" it to remove it from VBoxs registry

cd /home/john/VirtualBox\ VMs/boot
VBoxManage closemedium disk /home/john/VirtualBox\ VMs/boot/drive.vdi

Move the new disk file into the VMs directory and attach it to the hard drive

cp /home/john/src/pc_boot/boot5/drive.vdi /home/john/VirtualBox\ VMs/boot/drive.vdi
VBoxManage storageattach boot --storagectl "IDE Controller" --port 0 --device 0 --type hdd --medium drive.vdi

Creating the Cross Compiler

Downloaded the source for gcc 4.9.2 and binutils 2.2.5 https://gnu.org/software/gcc/ https://gnu.org/software/binutils/

Installed dependencies

libgmp3-dev
libmpfr-dev
libisl-dev
libcloog-isl-dev
libmpc-dev
texinfo

Prepare environment

export PREFIX="$HOME/opt/cross"
export TARGET=i686-elf
export PATH="$PREFIX/bin:$PATH"

Build binutils

cd $HOME/src
mkdir build-binutils
cd build-binutils
../binutils-x.y.z/configure --target=$TARGET --prefix="$PREFIX" --with-sysroot --disable-nls --disable-werror
make
make install

Build GCC

cd $HOME/src
mkdir build-gcc
cd build-gcc
../gcc-x.y.z/configure --target=$TARGET --prefix="$PREFIX" --disable-nls --enable-languages=c,c++ --without-headers
make all-gcc
make all-target-libgcc
make install-gcc
make install-target-libgcc