Linux startup Major phases q BIOS q MBR

Linux startup

Major phases q BIOS q MBR program q Boot loader (Partition boot program ) q OS’s kernel Loader q CPU always starts running at x’FFFF 0’=1, 048, 560 (built-in into BIOS and CPU)

BIOS q BIOS �Perform hw-specific tasks �Locate a bootable device �Locate cyl 0, head 0, sector 1 (the MBR) �Put 1 st byte of partition table into DL register Always x’ 80’ �Use int x’ 13’ to load into RAM @ 0000: 7 c 00 (why not 0000: 0000? ) Int x’ 13’ expects boot drive address in the DL reg. First drive IS x’ 80’ �Jump to this MBR program

MBR boot program q Locate an “active” partition q Locate boot sector q Load boot program into 0000: 7 c 00 �Overlays MBR program q Jump to 0000: 7 c 00 (this is the boot loader) q Move self to 0000: 0600

Boot loader q Locate �the OS’s kernel loader program �or in some cases the kernel itself �or perhaps a boot manager program Allows selecting which system to actually boot q Load it into RAM at x’ 7 c 00’ q Jump to it

Kernel loader or kernel q q Initializes reserved RAM Loads rest of kernel � Essential hardware setup � Basic memory manager � Start_kernel( ) Interrupts, Remaining memory management Device initialization Drivers Start kernel processes: Idle Scheduler Init (a user-space process!!) executes scripts Start non-o/S services, Allow user environment creation Presents login screen q Scheduler takes over