HBM is designed for the wire lengths that you're talking about. The core protocol isn't so terribly different from good old DDR4. The die-to-die links, on the

HBM is designed for the wire lengths that you're talking about. The core protocol isn't so terribly different from good old DDR4. The die-to-die links, on the other hand, have extremely short wire length limits (less than a centimeter iirc for UCIe). At the physical layer, you just waste a bunch of power and area driving a high voltage to make the signal go further. Do that enough and you basically wind up with a PCIe PHY.