Gregory's ServerOn the 82586 chip, exponential backoff is implemented with three circuits: a 10-bit counter to generate a pseudorandom number, a 10-bit mask register to select an (exponentially scaled) part of the counter, and a delay counter to loop that many times before trying again. 5/14
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 Adding a few more transistors makes a circuit that can be toggled, flipping its value. Ten of these toggle latches makes the counter. But each bit needs a circuit to decide when to toggle. 7/14 To count, a bit is toggled when all the lower bits are 1 (just like a carry in decimal addition). So the counter has a bunch of logic gates to generate the ten toggle signals. 8/14 Here's how the 10-bit counter looks on the die. I removed the chip's metal layer so you can see the transistors underneath, constructed from silicon and polysilicon. 9/14 The 10-bit mask register grows exponentially: binary 1, 11, 111, 1111, etc. It can be initialized with 1-8 bits set, or shift left to grow by one bit. 10/14 Here's the circuitry for the mask register. Three input lines select how many bits for the mask, decoded by a bunch of gates. 11/14 So that's how exponential backoff was implemented on the Intel 82586 Ethernet chip. Also my previous tweets on the Intel 82586: A bonus photo of the 82586 die with the metal layer removed, showing the underlying circuitry. I stopped dissolving the oxide layer a bit too soon, so it's a bit ugly. There are some random colored areas due to thin-film interference. The bond wires are around the edges. 14/14  Have you come across any errata where the 82586 ring handling has a bug and the part goes deaf until you reset it? I know it had a bug like that but now I cant find the errata for the part. @bitsavers Robert Garner is trying to track down the errata sheet for the 82586; he might know more. |
Each bit is implemented with a "dynamic latch". When the clock is high, the pass transistor turns on, connecting the two transistors in a stable loop, holding the bit. When the clock is low, the transistor capacitance holds the value (like DRAM)A or a new value can be input. 6/14