[p_l]

PALcode implemented context switches for all systems, including Linux (which uses Digital Unix PALcode). Specifically, PALcode covered various internal details of the CPU presenting unified interface to OS - each CPU needed custom PALcode implementation, though IIRC there were only few mandatory PALcode calls (related to memory barriers, iirc). The kernel covered switching OS-specific structure pointers over, PALcode handled all sorts of MMU, TLB, PASID indexing of TLB, internal CPU state etc.

L4/Alpha PALcode was limited to specific set of CPUs it was implemented for, so unlike VMS, NT or Digital Unix PALcode it wasn't available outside of those few machines.