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→System bootstrapping: The existing text vaguely suggested the CPU doing two things at once, something it can't do. So I moved the words about a bit to make it clear that the CPU enters real mode, THEN starts to run the boot code from the hard disk. Although in 8088 CPUs, there are no modes but real mode! The text is somewhat valid perhaps for 286 CPUs and up, but doesn't make sense for the computer it's describing. Further work is needed. |
→System bootstrapping: Sorry, forgot a "the"! |
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== System bootstrapping ==
On [[IBM PC-compatible]] computers, the [[bootstrapping]] [[firmware]] (contained within the [[read-only memory|ROM]] [[BIOS]]) loads and executes the master boot record.<ref name="OSDev_2011_MBR"/> The [[PC XT|PC/XT (type 5160)]] used an [[Intel 8088]] [[Computer processor|microprocessor]]. In order to remain compatible, all x86 BIOS architecture systems start with the microprocessor in an [[X86#Operating modes|operating mode]] referred to as [[real mode]]. The BIOS reads the MBR from the storage device into [[physical memory]], and then it directs the microprocessor to the start of the boot code. The BIOS will switch the processor to real mode, then begin to execute the MBR program, and so the beginning of the MBR is expected to contain real-mode [[machine code]].<ref name="OSDev_2011_MBR"/>
Since the BIOS bootstrap routine loads and runs exactly one sector from the physical disk, having the partition table in the MBR with the boot code simplifies the design of the MBR program. It contains a small program that loads the [[Volume Boot Record]] (VBR) of the targeted partition. Control is then passed to this code, which is responsible for loading the actual operating system. This process is known as [[chain loading]].
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