GDDR6X
Direct Answer
GDDR6X is not a JEDEC standard -- it's a proprietary memory technology Micron developed specifically in collaboration with Nvidia, first shipping in 2020 with the RTX 3080 and RTX 3090. It uses PAM4 signaling, the same two-bit-per-symbol approach PCIe 6.0 later adopted, to reach 19-21 Gbit/s per pin.
Source: Micron (developed with Nvidia)
Specs
| Standard | Proprietary (Micron, developed with Nvidia) -- not JEDEC |
| Signaling | PAM4 (2 bits per symbol, vs. GDDR5/6's 1-bit NRZ) |
| Data Rate | 19 - 21 Gbit/s per pin |
| Introduced | 2020 (Nvidia RTX 3080 and RTX 3090) |
A Proprietary Jump Using PAM4
Unlike GDDR5 and GDDR6, which are open JEDEC standards any manufacturer can implement, GDDR6X is a proprietary technology Micron developed specifically in close collaboration with Nvidia -- it has never been standardized by JEDEC. Its key technical advance is PAM4 (4-level pulse-amplitude modulation) signaling, encoding two bits per symbol instead of the single bit per symbol that NRZ signaling in GDDR5 and standard GDDR6 uses -- the same underlying signaling concept PCIe 6.0 later adopted for a similar reason. That approach lets GDDR6X reach 19-21 Gbit/s per pin, meaningfully faster than standard GDDR6.
GDDR6X debuted in 2020 alongside Nvidia's RTX 3080 and RTX 3090 graphics cards, both launching with it simultaneously, and has remained limited to a smaller number of flagship products given its proprietary, single-supplier nature compared to GDDR6's broader industry availability.