Operating systems kernel

An operating system consists of several layers

• We have looked at many applications: editors, servers, shells, utilities, compilers, ...

• The O/O services layer is often called the kernel

• An O/S strictly only needs a kernel - the rest is just application layers on top

• Linux kernels have version numbers e.g 2.6.29 (try uname -a)

• Kernel source code is available from the Linux Kernel Archives at www.kernel.org

Supervisor mode

• Most CPUs can run in several modes

• A CPU in supervisor/kernel mode can access all h/w

• A CPU in user-space mode has less access rights

• The kernel usually runs in kernel mode, applications in user mode

• When an application needs an O/S service, it must aks the CPU to switch modes

Monolithic kernel versus microkernel

From Wikimedia Commons:

• The Linux kernel is a monolithic kernel

• Less stable - any buggy device driver can crash it

• Fewer context switches, may be faster

DLLs

• An application can be compiled statically so that all needed code is compiled into a (big) binary

• This wastes space, especially if lots of applications all use the same libraries

• Instead, an application can be compiled with "stubs" to dynamic link libraries

• The DLL is linked in at runtime when needed

• Uses less space, but may crash at runtime if the DLL is not available

Linux DLLs

• See what dlls an application needs by running ldd on the binary

  $ldd /bin/ls
  	linux-gate.so.1 =>  (0xb7fe6000)
  	librt.so.1 => /lib/tls/i686/cmov/librt.so.1 (0xb7fca000)
  	libselinux.so.1 => /lib/libselinux.so.1 (0xb7fb0000)
  	libacl.so.1 => /lib/libacl.so.1 (0xb7fa7000)
  	libc.so.6 => /lib/tls/i686/cmov/libc.so.6 (0xb7e44000)
  
  	    

• Linux dlls are usually of the form libXXX.so.N where N is the major version number

• Typically, libXXX.so.N is a symbolic link to libXXX.so.N.M e.g.

  $ls -l /lib/libc.so.6
  lrwxrwxrwx 1 root root 11 2009-04-24 21:33 /lib/libc.so.6 -> libc-2.9.so
  	    

• By default, dlls are stored in /lib and /usr/lib. Other directories may be added by setting the environment variable LD_LIBRARY_PATH or by configuring ldconfig

Kernel modules

• Kernel modules are dlls for the kernel

• They usually have the code for device drivers

• They end in .ko and exist in /lib/modules/....

• By using modules, the kernel can stay small, with modules loaded only as needed

Device drivers

• Every piece of hardware needs a software device driver

• Windows drivers are supplied by the vendor; Linux drivers are usually written by volunteers

• There are drivers for: video cards, usb devices, SCSI devices, TV tuner cards, network drivers, character devices, sound cards, file systems, ...

• Most of these can be compiled statically into the kernel or as loadable modules

Configuring Linux

• Get the source files as a .tar.xz file and unpack it by tar Jxf ...

• Change to the toplevel directory and type make xconfig

• Choose the options you want and quit and save

Make and install

• Run make

• Run make install

• You may also need to run make modules_install

• This will install a copy of the kernel in /boot and make an entry in /boot/grub/menu.lst

• Next time you boot, you get the option of the new kernel

Managing modules

• Find the list of loaded modules by lsmod

• Load a new module by modprobe module-name

• Unload a module by rmmod module-name