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Published byAnya Whitfill Modified over 10 years ago
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startup
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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)
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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
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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
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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
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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
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MBR layout ContentOffset in MBR sector MBR program codex’0000’ MBR messagesx’008b’ Partition tablex’01be’ MBR signaturex’01fe’
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Boot manager Read partition table Create menu of bootable choices Locate selected bootable partition Load its boot record
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