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.

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.

References

OpenVMS FAQ (Hewlett-Packard)
"PALcode for Alpha Microprocessors System Design Guide" (PDF). Digital Equipment Corporation. May 1996. Retrieved November 7, 2013.
Bruce Hutton. "Some simple PAL code".

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