How does BIOS flash work without CPU?

SkyTerran

SkyTerran

SkyTerran

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A lot, if not most motherboards made in the last few years have a feature where you can flash a BIOS using a USB flash, without installing a CPU and memory. Wonder how does that work and can't find any information online. Does the motherboard have a cheap ARM SOC for this or does it work in a different way?
 
Some motherboards can even update the BIOS when there’s no CPU in the socket at all. Such motherboards feature special hardware to enable USB BIOS Flashback, and every manufacturer has a unique procedure to execute USB BIOS Flashback
thats from amd site
though they dont explain what the special hardware is
i guess most people dont care how it works though
just that it does
 
Might be of interest to you:


"Built-in USB flashback support
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One other handy, common-sense addition to the I/O die is built-in USB flashback support—the ability to flash your BIOS to a newer version even if the current version doesn't support the CPU you have installed. Not having this feature could have been a huge headache when new AM4 processors launched; if you wanted to buy a modern CPU with a slightly older motherboard to save some money, you could never assume that the motherboard would actually ship with a BIOS that supported that CPU, even if the motherboard maker had released an updated BIOS already.
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... AMD tells us it expects some manufacturers to stick with their own USB flashback implementations, and it isn't requiring motherboards to actually enable the built-in version either; motherboard makers will need to implement some kind of physical switch or jumper to enable the feature, and AMD isn't making anyone do it. We may still see some budget-y motherboards ship with no version of USB flashback, AMD-enabled or otherwise. But the company hopes that having the capability built in will end up pushing most boards to include it."
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arstechnica.com

Everything you need to know about Zen 4, socket AM5, and AMD’s newest chipsets

Ryzen 7000 CPUs use a new platform—with plenty of changes.
arstechnica.com arstechnica.com
 
I don't actually know but I assume there is some kind of MCU involved, either ARM Cortex M0 or something like an ATtiny series with side band access to the ROM (or possibly there is fail over functionality of an MCU embedded in hardware in the chipset).
 
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Just did this yesterday with a z690 board & 13700k, to make sure it would definitely support the CPU (& save a bit of money vs a near-identical Z790 board).
You connect only the 24 Pin motherboard power & the 8-Pin CPU power, nothing else at all. No CPU or RAM installed at all.
So the board must have some kind of rudimentary processor & memory on board, because it's functioning as a very basic computer on it's own.
Not sure what type of processor, or if it's perhaps integrated into the south bridge ?
 
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I always thought it would be great if you could access the BIOS without a CPU. With the price of todays motherboards having some kind of very basic single core processor onboard or something doesn’t seem too extravagant frankly…!
 
GaryTheSnail said:
It will probably bypass the cpu
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Yup

Looking at a couple of motherboards and they have the ability to flash from a specific USB slot, my guess is it's similar to the days of floppy disc bios recovery discs, there is probably a very basic processor in the motherboard chipset that takes the input from the "Bios flash" button as an interrupt and immediately looks for a bios file on the special USB port.
Modern basic processors are so cheap these days that some USB cables have processors that are faster than old personal PC's so I can see it being relatively simple/very cheap to do it as part of the motherboards chipset.
 
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