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From: Lawrence D'Oliveiro <ldo@nz.invalid>
Newsgroups: comp.sys.raspberry-pi
Subject: Re: How to boot from SD but run from USB?
Date: Tue, 28 Jan 2025 02:20:09 -0000 (UTC)
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On Mon, 27 Jan 2025 14:04:07 +0000, The Natural Philosopher wrote:

> Yup Arm/broadcomm based Pis 'do it their way'
> 
> It goes back to the chips inception as a set top box embedded processpor

Also remember that GRUB depends on BIOS or UEFI, and the former is x86-
specific -- not sure about the latter.

Basically, every vendor’s ARM chipset came up with its own way of booting. 
In the absence of a BIOS-style interface for querying what hardware is 
available, the Linux kernel is built with a “device tree” structure that 
hard-codes this information for your specific chipset.

There is now an equivalent spec standardized for the ARM world (adaptation 
of UEFI??), but I understand this is only in use on servers with AArch64, 
and the Raspberry Pi predates it anyway.