4 ms·

Hi Ken, love pretty much everything you write. Was wondering, is it correct to assume that all of the 8086 would have been laid out entirely by hand? Certainly

Hi Ken, love pretty much everything you write. Was wondering, is it correct to assume that all of the 8086 would have been laid out entirely by hand? Certainly special blocks like these charge pumps must have been, but was there any sort of design automation for things like the recently discussed register file?

pjc50 6y ago
Followup question: when did we start describing these things with GDSII files (was there a GDSI?) rather than giant sheets of acetate?
kens 6y ago
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.
- userbinator 6y ago
  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.
  - dboreham 6y ago
    Electrons fly off as they try to take the 90 bend.
    - amelius 6y ago
      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.
    - userbinator 6y ago
      There is a bit of truth to that --- sharp corners are more prone to electromigration: https://en.wikipedia.org/wiki/Electromigration#Via_arrangements_and_corner_bends https://en.wikipedia.org/wiki/Electromigration#Via_arrangeme...
  - kens 6y ago
    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.
    - egsmi 6y ago
      In the suspected auto layout sections does it follow a modern standard cell convention?
      - kens 6y ago
        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.
        egsmi 6y ago
        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.
- tails4e 6y ago
  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.

pjc50 6y ago

Followup question: when did we start describing these things with GDSII files (was there a GDSI?) rather than giant sheets of acetate?

kens 6y ago

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.

userbinator 6y ago
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.
- dboreham 6y ago
  Electrons fly off as they try to take the 90 bend.
  - amelius 6y ago
    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.
  - userbinator 6y ago
    There is a bit of truth to that --- sharp corners are more prone to electromigration: https://en.wikipedia.org/wiki/Electromigration#Via_arrangements_and_corner_bends https://en.wikipedia.org/wiki/Electromigration#Via_arrangeme...
- kens 6y ago
  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.
  - egsmi 6y ago
    In the suspected auto layout sections does it follow a modern standard cell convention?
    - kens 6y ago
      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.
      - egsmi 6y ago
        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.
tails4e 6y ago
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.