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@REPLYADDR Computer Nerd Kev
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Modern CPUs have a backstage cast
by Hugo Landau, 2023-05-30
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https://www.devever.net/~hl/backstage-cast
"If you take someone with intermediate knowledge of computing in the
right areas, and ask them how an x86 machine boots, they`ll
probably start telling you about how the CPU first comes up in real
mode and starts executing code from the 8086 reset vector of
FFFF:FFF0. This understanding of how an x86 machine boots has
remained remarkably persistent, as far as I can tell because this
basic narrative about the boot process has been handed down from
website to website, generation to generation, largely unchanged.
It`s also a pack of lies and hasn`t reflected the true nature of
the boot process for some time. It`s true the 8086 reset vector is
still used, but only because it`s a standard "ABI" for the CPU to
transfer control to the BIOS (whether legacy PC BIOS or UEFI BIOS).
In reality an awful lot happens before this reset vector starts
executing.1 Aside from people having vaguely heard about the Intel
Management Engine, this modern reality of the boot process remains
largely unknown. It doesn`t help that neither Intel nor AMD have
really gone out of their way to actually document what the modern
boot process looks like, and large parts of this process are
handled by vendor-supplied mystery firmware blobs, which may as
well be boxes with "???" written in them. Mainly we have the
substantial assistance of assorted reverse engineers and security
researchers to thank for the fact that we even have a decent
picture of what the modoern x86 boot process actually looks like
for both Intel and AMD. I could write a whole article about that
process -- but instead, I`d like to focus on something else.
Basically any multiprocessor CPU chip of reasonable sophistication
produced today -- i.e., desktop and server CPUs, and probably
smartphone SoCs for that matter, also has an entire "backstage
cast" of ancillary support cores keeping things running correctly.
Some of these mainly handle the boot process, and we`re more likely
to know about these (e.g. Intel ME/AMD PSP), but even less known
are "backstage" cores which don`t participate in the boot process
but which are instead involved in things like power and thermal
regulation." ...
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