1. startup

2. Major phases  BIOS  MBR program  Boot loader (Partition boot program )  OS’s kernel Loader  CPU always starts running at x’FFFF0’=1,048,560 (built-in into BIOS and CPU)

3. BIOS  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 @ 0000:7c00 (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

4. MBR boot program  Locate an “active” partition  Locate boot sector  Load boot program into 0000:7c00 Overlays MBR program  Jump to 0000:7c00 (this is the boot loader)  Move self to 0000:0600

5. Boot loader  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  Load it into RAM at x’7c00’  Jump to it

6. Kernel loader or kernel  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  Scheduler takes over

7. MBR layout ContentOffset in MBR sector MBR program codex’0000’ MBR messagesx’008b’ Partition tablex’01be’ MBR signaturex’01fe’

8. Boot manager  Read partition table  Create menu of bootable choices  Locate selected bootable partition  Load its boot record