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Comment by formerly_proven | Hacker News Reader

RTX 40 doesn’t have NVLink on the PCBs, though the silicon has to have it, since some sibling cards support it. I’d expect it to be fused off.

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A cursory google search suggests that it's been removed at the silicon level.

Some do: https://wccftech.com/gigabyte-geforce-rtx-4090-pcb-shows-lef...

I'm pretty sure that's just a remnant of a 3090 PCB design that was adapted into a 4090 PCB design by the vendor. None of the cards based on the AD102 chip have functional NVLink, not even the expensive A6000 Ada workstation card or the datacenter L40 accelerator, so there's no reason to think NVLink is present on the silicon anymore below the flagship GA100/GH100 chips.

How to unfuse it?

I don't know about this particular scenario, but typically fuses are small wires or resistors that are overloaded so they irreversibly break the connection. Hence the name.

Either done during manufacture or as a one-time programming[1][2].

Though sometimes reprogrammable configuration bits are sometimes also called fuse bits. The Atmega328P of Arduino fame uses flash[3] for its "fuses".

[1]: https://www.nxp.com/docs/en/application-note/AN4536.pdf

[2] https://www.intel.com/programmable/technical-pdfs/654254.pdf

[3]: https://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-7810-...

Wires, flash, and resistors can be replaced

Not at the scale we're talking about here. These structures are very thin, far thinner than bond wires which is about the largest structure size you can handle without a very, very specialized lab. And you'd need to unsolder the chip, de-cap it, hope the fuse wire you're trying to override is at the top layer, and that you can re-cap the chip afterwards and successfully solder it back on again.

This may be workable for a nation state or a billion dollar megacorp, but not for your average hobbyist hacker.

You’re absolutely right. In fact, some billion dollar megacorps use fuses as a part of hardware DRM for this reason.

These are part of the chip, thus microscopic and very inaccessible.

There are some good images here[1] of various such fuses, both pristine and blown. Here's[2] a more detailed writeup examining one type.

It's not something you fix with a soldering iron.

[1]: https://semiengineering.com/the-benefits-of-antifuse-otp/

[2]: https://www.eetimes.com/a-look-at-metal-efuses/

I miss the days when you could do things like connecting the L5 bridges on the surface of the AMD Athlon XP Palomino [0] CPU packaging with a silver trace pen to transform them into fancier SMP multi-socket capable Athlon MPs, e.g. Barton [1].

https://arstechnica.com/civis/threads/how-did-you-unlock-you...

Some folks even got this working with only a pencil, haha.

Nowadays, silicon designers have found highly effective ways to close off these hacking avenues, with techniques, such as the microscopic, nearly invisible, and as parent post mentions, totally inaccessible e-fuses.

[0] https://upload.wikimedia.org/wikipedia/commons/7/7c/KL_AMD_A...

[1] https://en.wikichip.org/w/images/a/af/Atlhon_MP_%28.13_micro...

I'm one of those folks that did it with a pencil. Haha. Maybe I was lucky? That was my first overclock and it ran pretty well.

Use a Focused Ion Beam instrument.

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