Model-specific registers (MSRs) are control register

Control register

A control register is a processor register which changes or controls the general behavior of a CPU or other digital device. Common tasks performed by control registers include interrupt control, switching the addressing mode, paging control, and coprocessor control.-CR0:The CR0 register is 32 bits...

s provided by processor

Central processing unit

The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...

 implementations to provide system software

System software

System software is computer software designed to operate the computer hardware and to provide a platform for running application software.The most basic types of system software are:...

 with features that are provided on specific processor implementations, but not others.

MSRs are used for performance monitoring, debugging

Debugging

Debugging is a methodical process of finding and reducing the number of bugs, or defects, in a computer program or a piece of electronic hardware, thus making it behave as expected. Debugging tends to be harder when various subsystems are tightly coupled, as changes in one may cause bugs to emerge...

, testability

Testability

Testability, a property applying to an empirical hypothesis, involves two components: the logical property that is variously described as contingency, defeasibility, or falsifiability, which means that counterexamples to the hypothesis are logically possible, and the practical feasibility of...

 and program execution tracing, but also to enable and disable certain model-specific features of the processor implementation.

For example, on the AMD K6

AMD K6

The K6 microprocessor was launched by AMD in 1997. The main advantage of this particular microprocessor is that it was designed to fit into existing desktop designs for Pentium branded CPUs. It was marketed as a product which could perform as well as its Intel Pentium II equivalent but at a...

 and later and most other newer processors (including all processors implementing AMD64 architecture), the individual bit

Bit

A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...

s of the EFER (Extended Feature Enable Register) configure whether the SYSCALL and SYSRET instructions are available, whether long mode

Long mode

In the x86-64 computer architecture, long mode is the mode where a 64-bit application can access 64-bit instructions and registers...

 can be activated and is active and if the no-execute page-protection

NX bit

The NX bit, which stands for No eXecute, is a technology used in CPUs to segregate areas of memory for use by either storage of processor instructions or for storage of data, a feature normally only found in Harvard architecture processors...

 feature is enabled.

Examples for rather well-known MSRs are the Memory Type Range Registers

Memory Type Range Registers

Memory type range registers are a set of processor supplementary capabilities control registers that provide system software with control of how accesses to memory ranges by the CPU are cached...

 (MTRR) and the Address Range Registers

Address Range Registers

Address-range registers are control registers of the Cyrix 6x86, 6x86MX and MII processors that are used as a control mechanism which provides system software with control of how accesses to memory ranges by the CPU are cached, similar to what memory type range registers provide on other...

(ARR).

Documentation regarding which MSRs a certain processor implementation supports is usually found in the processor documentation of the CPU vendor.