privileged instruction

[′priv·ə·lijd in′strək·shən]

(computer science)

A class of instructions, usually including storage protection setting, interrupt handling, timer control, input/output, and special processor status-setting instructions, that can be executed only when the computer is in a special privileged mode that is generally available to an operating or executive system, but not to user programs.

privileged instruction

A machine code instruction that may only be executed when the processor is running in supervisor mode. Privileged instructions include operations such as I/O and memory management.

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Because the binary translation of privileged instructions is so cumbersome, hardware-assisted virtualization has been added to modern x86 chips from Intel (Intel VT) and AMD (AMD-V).

Calls to privileged instructions are replaced with hypercalls handled by a special privileged guest VM (called Domain0 in Xen, or the root partition in Hyper-V).

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To perform the emulation of privileged instructions, Disco additionally maintains the privileged registers and TLB contents of the virtual CPU in this structure.

Supervisor mode allows the operating system to use a protected portion of the address space (the supervisor segment), but does not give access to privileged instructions or physical memory.

To reduce this overhead, we patched the HAL of IRIX to convert these frequently used privileged instructions to use nontrapping load and store instructions to a special page of the address space that contains these registers.