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Thursday, September 22, 2016

Down to Silicon Level Debugging

First off, I'm not a "forward" BIOS/UEFI engineer. At least not one who worked officially in a BIOS/UEFI software development company or motherboard company. I did got an official access to AMIBIOS Core8 source code and tools back then under NDA for one of my clients to customize it for a custom x86 motherboard. But, that's as far as I got into the game. This is relevant to this post as I don't know exactly the process of silicon level firmware development validation. The farthest I went was a sort of "Serial ICE" with Coreboot and also debugging via Power-On Self Test (POST) code passed over serial port in AMIBIOS Core8. That was it. I didn't even manage to do debugging via PCI POST card, but I presume it would be similar to its "redirected" cousin in the serial port. There is another way to do firmware debugging, via JTAG pins, and by using In-Circuit Emulator (ICE). Despite having been an Electrical Engineering student, I'm not yet familiar with those territories. But, AFAIK both are as good as if not more powerful firmware debugging technique compared to using Serial ICE or POST card.

Let's focus on the ICE part. There was at least one mistake I did in my BIOS book that I didn't realize due to my handicap in not having an ICE and its related skills. You can see it in the quoted errata below (it's also in the addendum part of my book over at github):

The address aliasing mentioned in Chapter 4 section 4.1.1 page 4 (the paging messed-up in the PDF) should cover both E-segment and F-Segment (E_0000h-F_FFFFh), not just the last 64-KB segment. Somebody used a sort of CPU logic analyzer to confirm this fact.
The guy who tipped me over about this was using an expensive ICE to validate the fact above. I'm not exactly sure how he tapped all of the "wires" on the chipsets and the CPU itself, but very probably similar in principle to what "Bunnie" did to the first XBox version (see: http://www.xenatera.com/bunnie/proj/anatak/xboxmod.html). IIRC, the was using one of Arium ICE products. Arium was acquired by another company, see: http://www.asset-intertech.com. However, their ICE products live on as (very probably) the ScanWorks and SourcePoint line of products from asset-intertech. These ICE products used to cost north of $20k a piece back then. I don't know about it at the moment though. With an ICE, you essentially put the CPU in a "hard" debug mode, where you can freeze it in a way ordinary debugger cannot because there is no OS or firmware required for that to happen.

Anyway, I was quite surprised to find a "low cost" version of this kind of ICE over at: http://www.loper-os.org/?p=1667. Well, I'd like to thank to whoever posted a comment about this ICE in my previous post. It's very interesting nonetheless ;-).

Anyway, for the uninitiated, a (not so) useful background is at https://en.wikipedia.org/wiki/In-circuit_emulation
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4 comments:

Stanislav Datskovskiy said...

It's low-cost "while supplies last." And I already have no idea where one might buy it, the stockpiles appear to have run dry some time ago.

Darmawan Salihun said...

Thanks for the info anyway ;)

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