PALcode (Privileged Architecture Library code)[1][2] is the name used by DEC in the Alpha instruction set architecture (ISA) for a set of functions in the System Reference Manual (SRM) or AlphaBIOS firmware, providing a hardware abstraction layer for system software, covering features such as cache management, translation lookaside buffer (TLB) miss handling, interrupt handling, and exception handling.
It evolved from a feature of the DEC PRISM architecture named Epicode.
[3] PALcode is Alpha machine code, running in a special mode[4] that also allows access to internal registers specific to the particular Alpha processor implementation.
PALcode is operating system-specific,[5] so different versions of PALcode are required by OpenVMS, Tru64 UNIX, and Windows NT.
Tru64 UNIX PALcode is also used by NetBSD, FreeBSD, OpenBSD and Linux.