CXL (Compute Express Link)
Beginner
A standard that uses the same wires as PCIe to give the processor extra memory on a plug-in card.
Novice
An open standard that runs over the PCIe physical wires but adds two new protocols: one lets a device cache the CPU’s memory, the other lets the CPU use a device’s memory as ordinary memory.
Expert
Three dynamically multiplexed protocols on the PCIe PHY: CXL.io (PCIe semantics), CXL.cache (device caches host memory coherently) and CXL.mem (host load/store to device memory). Device Types 1, 2 and 3 use different subsets. 1.1 is single-host; 2.0 adds a switch level and pooling; 3.0 moves to 64 GT/s and fabrics.
Explained in Board and server (Systems).
See also: CXL Type 3 device (memory expander), PCIe (PCI Express).