GDDR6X
A high-speed graphics memory (Micron + Nvidia) that uses PAM4 signaling to pack more data per cycle than regular GDDR6. It drives the memory bandwidth in Nvidia's RTX 30- and 40-series top cards, but runs hotter.
Mechanic: GDDR6 sends 2 levels per symbol (NRZ); GDDR6X uses PAM4 (4 voltage levels = 2 bits/symbol) → more bandwidth without doubling the clock. Result: very high memory bandwidth needed for 4K and AI workloads. Trade-off: PAM4 is more sensitive to noise and GDDR6X modules became known for high temperatures (especially backside-mounted ones on the RTX 3090), which required extra cooling. Exclusive to Nvidia (a Micron collaboration); AMD stuck with regular GDDR6. Succeeded by GDDR7 (PAM3). An example of the same signaling trick (PAM) that drives PCIe 6.0. Related to PCIe 6.0 and HBM.