Time Stamp Counter
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The Time Stamp Counter (TSC) is a 64-bit register present on all x86 processors since the Pentium. It counts the number of cycles since reset. The instruction
RDTSC returns the TSC in EDX:EAX. In x86-64 mode,
RDTSC also clears the higher 32 bits of RAX. Its opcode is
0F 31. Pentium competitors such as the Cyrix 6x86 did not always have a TSC and may consider
RDTSC an illegal instruction. Cyrix included a Time Stamp Counter in their MII.
The time stamp counter has, until recently, been an excellent high-resolution, low-overhead way of getting CPU timing information. With the advent of multi-core/hyper-threaded CPUs, systems with multiple CPUs, and hibernating operating systems, the TSC cannot be relied on to provide accurate results — unless great care is taken to correct the possible flaws: rate of tick and whether all cores (processors) have identical values in their time-keeping registers. There is no promise that the timestamp counters of multiple CPUs on a single motherboard will be synchronized. In such cases, programmers can only get reliable results by locking their code to a single CPU. Even then, the CPU speed may change due to power-saving measures taken by the OS or BIOS, or the system may be hibernated and later resumed (resetting the time stamp counter). In those latter cases, to stay relevant, the counter must be recalibrated periodically (according to the time resolution the application requires).
Reliance on the time stamp counter also reduces portability, as other processors may not have a similar feature. Recent Intel processors include a constant rate TSC (identified by the
constant_tsc flag in Linux's
/proc/cpuinfo). With these processors, the TSC reads at the processor's maximum rate regardless of the actual CPU running rate. While this makes time keeping more consistent, it can skew benchmarks, where a certain amount of spin-up time is spent at a lower clock rate before the OS switches the processor to the higher rate. This has the effect of making things seem like they require more processor cycles than they normally would.
Under Windows platforms, Microsoft strongly discourages using the TSC for high-resolution timing for exactly these reasons, providing instead the Windows APIs
QueryPerformanceFrequency. Under *nix, similar functionality is provided by reading the value of
CLOCK_MONOTONIC clock using the POSIX
Starting with the Pentium Pro, Intel processors have supported out-of-order execution, where instructions are not necessarily performed in the order they appear in the executable. This can cause
RDTSC to be executed later than expected, producing a misleading cycle count. This problem can be solved by executing a serializing instruction, such as CPUID, to force every preceding instruction to complete before allowing the program to continue, or by using the
RDTSCP instruction, which is a serializing variant of the
RDTSC instruction (starting from Core i7 and starting from AMD Athlon 64 X2 CPUs with Socket AM2 (Windsor & Brisbane)).
Implementation in various processors 
Intel processor families increment the time-stamp counter differently:
- For Pentium M processors (family [06H], models [09H, 0DH]); for Pentium 4 processors, Intel Xeon processors (family [0FH], models [00H, 01H, or 02H]); and for P6 family processors: the time-stamp counter increments with every internal processor clock cycle. The internal processor clock cycle is determined by the current core-clock to busclock ratio. Intel SpeedStep technology transitions may also impact the processor clock.
- For Pentium 4 processors, Intel Xeon processors (family [0FH], models [03H and higher]); for Intel Core Solo and Intel Core Duo processors (family [06H], model [0EH]); for the Intel Xeon processor 5100 series and Intel Core 2 Duo processors (family [06H], model [0FH]); for Intel Core 2 and Intel Xeon processors (family [06H], display_model [17H]); for Intel Atom processors (family [06H], display_model [1CH]): the time-stamp counter increments at a constant rate. That rate may be set by the maximum core-clock to bus-clock ratio of the processor or may be set by the maximum resolved frequency at which the processor is booted. The maximum resolved frequency may differ from the maximum qualified frequency of the processor.
The specific processor configuration determines the behavior. Constant TSC behavior ensures that the duration of each clock tick is uniform and supports the use of the TSC as a wall clock timer even if the processor core changes frequency. This is the architectural behavior moving forward for all Intel processors.
AMD processors up to the K8 core always incremented the time-stamp counter every clock cycle. Thus, power management features were able to change the number of increments per second, and the values could get out of sync between different cores or processors in the same system. For Windows, AMD provides a utility to periodically synchronize the counters on multiple core CPUs. Since the family 10h (Barcelona/Phenom), AMD chips feature a constant TSC, which can be driven either by the HyperTransport speed or the highest P state. A CPUID bit (
Fn8000_0007:EDX_8) advertises this.
Operating system support 
RDTSC instruction can be enabled or disabled by operating systems. For example, on some versions of the Linux kernel, seccomp sandboxing mode disables
RDTSC. It can also be disabled using the
PR_SET_TSC argument to the
Other architectures 
Other processors also have registers which count CPU clock cycles, but with different names. For instance, on the AVR32, it is called the Performance Clock Counter (PCCNT) register. SPARCv9 provides the
See also 
- High Precision Event Timer (HPET)
- Intel 64 and IA-32 Architectures Software Developer's Manual Volume 2B: Instruction Set Reference, M-Z. p. 460.
- Game Timing and Multicore Processors. pp. 251–252.
- "Using the RDTSC Instruction for Performance Monitoring".
- "x86 32-bit opcode summary, showing RDTSC and RDTSCP".
- "Volume 3A, Chapter 16". Intel 64 and IA-32 Architectures Software Developer's Manual.
- "Volume 3". AMD64 Architecture Programmer's Manual.
- "AMD Dual-Core Optimizer".
- "cr0 blog: Time-stamp counter disabling oddities in the Linux kernel". May 2009.
- Linux Programmer's Manual – System Calls –
- cycle.h - C code to read the high-resolution timer on many CPUs and compilers.
- Programming examples - Very simple C code to read the timer on an x86 machine. This reads the 64-bit value into two 32-bit integers and combines them - using just one 64-bit integer is another option.
- AMD engineer on TSC drift in AMD processors