Inside the 8086 processor, tiny charge pumps create a negative voltage

Re:Inside the 8086 processor, tiny charge pumps create a neg... by kens on Saturday July 25, 2020 @04:07PM (#23951997)

I think it was a mixture of hand layout and automated layout. There are a lot of places that do strange things to squeeze out every last bit of space, and I think this optimization was done by machine.

For instance, long signal traces have a lot of twists and turns to be as close together as possible while avoiding obstacles and satisfying the design rules. Sometimes these signals do strange things like switching from poly wires to metal wires and back just so they can be a bit closer. These micro-optimizations don't seem like ones a human would make.

Another thing I've noticed is between two revisions of the chip that have identical layout except for a few traces. On one chip a trace will have a jog that's two 90-degree turns. On the other chip, the same trace will have 45 degree bends. So the two traces are identical except for this short segment. I assume that the automated layout algorithm changed slightly, resulting in this change.

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Re:Inside the 8086 processor, tiny charge pumps create a neg... by userbinator on Saturday July 25, 2020 @05:22PM (#23952533)

On one chip a trace will have a jog that's two 90-degree turns. On the other chip, the same trace will have 45 degree bends.

If the 45 degree one is the newer version, that might be a DFM optimisation - there is common folklore around PCB design that 90 or acute angles can cause various problems, with some hint of truth to it, but I suspect there are similar constraints in photolithography. If the sharp corner was causing yield losses, a newer revision would definitely address that.

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Re:Inside the 8086 processor, tiny charge pumps create a neg... by kens on Saturday July 25, 2020 @05:37PM (#23952619)

Both chips have a mixture of 45 and 90 degree bends, so it's nothing that obvious. About 98% of the paths are the same and then there are these changes with no discernible pattern.

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Re:Inside the 8086 processor, tiny charge pumps create a neg... by egsmi on Saturday July 25, 2020 @06:30PM (#23952895)

In the suspected auto layout sections does it follow a modern standard cell convention?

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Re:Inside the 8086 processor, tiny charge pumps create a neg... by kens on Saturday July 25, 2020 @06:50PM (#23952998)

It's about as far as you can get from standard cell. Every logic gate is different with the transistors shaped to fit in their surroundings.

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Re:Inside the 8086 processor, tiny charge pumps create a neg... by egsmi on Saturday July 25, 2020 @07:08PM (#23953091)

That’s crazy. It seems like it would make the software so much harder but I guess it would get the density up. I wonder how they closed timing.

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Re:Inside the 8086 processor, tiny charge pumps create a neg... by dboreham on Saturday July 25, 2020 @05:27PM (#23952565)

Electrons fly off as they try to take the 90 bend.

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Re:Inside the 8086 processor, tiny charge pumps create a neg... by userbinator on Saturday July 25, 2020 @06:20PM (#23952841)

There is a bit of truth to that --- sharp corners are more prone to electromigration:

https://en.wikipedia.org/wiki/Electromigration#Via_arrangeme...

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Re:Inside the 8086 processor, tiny charge pumps create a neg... by amelius on Saturday July 25, 2020 @06:13PM (#23952810)

Electrons in a typical circuit have a drift velocity that is very low (order of millimeters per hour). Of course, the electrons individually are very fast, but they are also almost as fast when there is no voltage applied.

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Re:Inside the 8086 processor, tiny charge pumps create a neg... by tails4e on Saturday July 25, 2020 @08:03PM (#23953314)

Modern tools pale in comparison to hand laid out designs, so if something is very smart to save area, it's unlikely it was machine created.

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