Ask HN: Where can I find a primer on how computers boot?

- the MIT Operating Systems Engineering OCW course [^1]

- specifically, Lecture #2 [^2] which describes the bootloader for 'xv6', a reimplementation of Unix v6, which does a great job showing what a solid OS with all the basics look like

[^1]: https://ocw.mit.edu/courses/6-828-operating-system-engineeri...

[^2]: https://ocw.mit.edu/courses/6-828-operating-system-engineeri...

https://thestarman.pcministry.com/asm/mbr/STDMBR.htm

Note that EFI/UEFI -- occurred much later in time than MBR...

The MBR boot process is also called "Legacy Boot" -- and is emulated on (U)EFI -- although it may not show as an option on some (U)EFI BIOS'es if the option is turned off...

Related: https://en.wikipedia.org/wiki/Master_boot_record

You might also wish to check out some emulators, most notably Bochs (https://bochs.sourceforge.io/) and QEMU (https://www.qemu.org/) because they simulate the boot process, and if you're in their debuggers, you should be able to inspect that process step by step -- but also more generally emulators for other machines/platforms/architectures (https://en.wikipedia.org/wiki/List_of_computer_system_emulat...) because in general, most of those emulators should realistically simulate the given machine/platform/architecture's boot process...

The basic theory of booting is that when a system starts, it contains a little bit of persistent memory (BIOS ROM, EPROM?) that contains a little bit of code, which is just enough code to load the data of the first block/sector of the hard disk (or other persistent storage boot device) to a specific address in memory as code -- and jump to it.

This data on the first block/sector -- is a small bit of machine code -- which although fairly stupid -- knows enough about the system to load the next N blocks/sectors of the hard disk (or other storage device) -- again into memory at a specific address -- and then jump (transfer control) to it.

This pattern may repeat several times, for example, GRUB's first bootsector then loads an intermediate length program (a "chainloader") from the next N contiguous blocks/sectors of the storage device, this chainloader knows more about the hardware and filesystems than the initial boot block did, and then it proceeds to do a yet longer/more complex load of the main Operating System into memory.

During the final load, the main Operating System might be on discontiguous blocks/sectors and those blocks/sectors may be part of a filesystem. But that doesn't have to be the case.

But whatever the case, the final load (or perhaps "bootstrap phase") is usually more complex than reading continuous blocks/sectors into memory and subsequently jumping to its start address in memory -- but not always...

Another way to think about it is that a 512 byte program (the bootsector) is loaded that then loads a 32K (let's say) program (the chainloader on N contiguous blocks/sectors) which then loads the multi-megabyte (or multi-gigabyte!) OS from multiple files from multiple file systems from multiple discontinuous blocks/sectors on the physical device (let's say, for example...)

Simple Program (loads) -> More Complex Program (loads) -> Most Complex Program (OS)...

Anyway, there's your basic theory...

Good luck!

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