3 ms·

I've always thought a CPU was an almost pure digital device: Maybe some analog things at the border to interface with the world, but I assumed the internals co

I've always thought a CPU was an almost pure digital device: Maybe some analog things at the border to interface with the world, but I assumed the internals could be represented as pure digital gates with not much extras. This reverse engineering shows a surprising amount of exceptions. Here again, the crossover mux has 'relatively large transistors' and the bootstrap drivers come up again. Is this still the case today? AFAIK, an FPGA design behaves like a pure digital device, apart from timing.

kens 4y ago
As Vonada said†, "Digital circuits are made from analog parts." On the one hand, you can simulate just about everything in the 8086 as digital.†† On the other hand, the transistors in the 8086 (and other processors) are carefully sized to produce the necessary current, taking into account capacitance and resistance and timing. For instance, the ALU has larger transistors because it was speed-critical. †See "Vonada's Engineering Maxims": https://twitter.com/kenshirriff/status/1633154548318732289 https://twitter.com/kenshirriff/status/1633154548318732289 ††The only "really" analog part of the 8086 is the charge pump to produce a negative voltage on the substrate.
- Espressosaurus 4y ago
  Yeah. If you've ever had to deal with metastability and eyes in digital signals, you'd know that "digital" is frequently less 0 and 1 than we normally think about. There's a reason you can't just take a System Verilog FPGA and make an ASIC out of it that will work on the first time, and that there are entire teams and libraries to do it. And they'll still get things wrong that need to be fixed after they're discovered. edit: the only reason we can blissfully ignore the underlying analog world is through the unceasing effort of asic designers, analog engineers, verification teams, firmware teams, and as a last line of defense, some poor bastard writing the driver to work around something that can't be fixed with any other method.
  - Taniwha 4y ago
    > Yeah. If you've ever had to deal with metastability ..... I believe that's Vonada's maxim #10 .....
    - Espressosaurus 4y ago
      Indeed. I didn't click through the image, so didn't see it. #12 hurts me so, so badly. #5 is way too true as well. Especially when it comes to high voltage signals running next to low voltage ones. And #4 has burned me so very many times.

kens 4y ago

As Vonada said†, "Digital circuits are made from analog parts." On the one hand, you can simulate just about everything in the 8086 as digital.†† On the other hand, the transistors in the 8086 (and other processors) are carefully sized to produce the necessary current, taking into account capacitance and resistance and timing. For instance, the ALU has larger transistors because it was speed-critical. †See "Vonada's Engineering Maxims": https://twitter.com/kenshirriff/status/1633154548318732289 https://twitter.com/kenshirriff/status/1633154548318732289 ††The only "really" analog part of the 8086 is the charge pump to produce a negative voltage on the substrate.

Espressosaurus 4y ago
Yeah. If you've ever had to deal with metastability and eyes in digital signals, you'd know that "digital" is frequently less 0 and 1 than we normally think about. There's a reason you can't just take a System Verilog FPGA and make an ASIC out of it that will work on the first time, and that there are entire teams and libraries to do it. And they'll still get things wrong that need to be fixed after they're discovered. edit: the only reason we can blissfully ignore the underlying analog world is through the unceasing effort of asic designers, analog engineers, verification teams, firmware teams, and as a last line of defense, some poor bastard writing the driver to work around something that can't be fixed with any other method.
- Taniwha 4y ago
  > Yeah. If you've ever had to deal with metastability ..... I believe that's Vonada's maxim #10 .....
  - Espressosaurus 4y ago
    Indeed. I didn't click through the image, so didn't see it. #12 hurts me so, so badly. #5 is way too true as well. Especially when it comes to high voltage signals running next to low voltage ones. And #4 has burned me so very many times.