PALcode

In computing, in the Alpha instruction set architecture, PALcode (Privileged Architecture Library code) is the name used by DEC for a set of functions in the 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.
PALcode is Alpha machine code, running in a special mode that also allows access to internal registers specific to the particular Alpha processor implementation. It is thus somewhere between the role of microcode and of a hardware emulator.
PALcode is operating system-specific; 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.