Time Stamp Counter
The Time Stamp Counter (TSC) is a 64-bit register present on all x86 processors since the Pentium. It counts the number of CPU cycles since its reset. The instruction codice_1 returns the TSC in EDX:EAX. In x86-64 mode, codice_1 also clears the upper 32 bits of RAX and RDX. Its opcode is codice_3. Pentium competitors such as the Cyrix 6x86 did not always have a TSC and may consider codice_1 an illegal instruction. Cyrix included a Time Stamp Counter in their MII. Use. The Time Stamp Counter was once an excellent high-resolution, low -overhead way for a program to get 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 upon 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. Therefore, a program can get reliable results only by limiting itself to run on one specific CPU. Even  then, the CPU speed may change because of power-saving measures taken by the OS or BIOS, or the system may be hibernated and later resumed, resetting the TSC. In those latter cases, to stay relevant, the program must re-calibrate the counter periodically. Relying on the TSC also reduces portability, as other processors may not have a similar feature. Recent Intel processors include a constant rate TSC (identified by the kern.timecounter.invariant_tsc sysctl on FreeBSD or by the "codice_5" flag in Linux's codice_6). With these processors, the TSC ticks at the processor's nominal frequency, regardless of the actual  CPU clock frequency due to turbo or power saving states. Hence TSC ticks are counting the passage of time, not the number of CPU clock cycles elapsed. On Windows platforms, Microsoft strongly discourages using the TSC for high-resolution timing for exactly these reasons, providing instead the Windows APIs codice_7 and codice_8 (which itself uses RDTSCP if the system has an invariant TSC, i.e. the freqency of the TSC doesn't vary according to the current core's frequency). On POSIX systems, a program can get similar function by reading the value of codice_9 clock using the codice_10 function. Starting  with the Pentium Pro, Intel processors have practiced out-of-order execution, where instructions are not necessarily performed in the order they appear in the program. This can cause the processor to execute codice_1 earlier than a simple program expects, producing a misleading cycle count. The programmer can solve this problem by inserting a serializing instruction, such as CPUID, to force every preceding instruction to complete before allowing the program to continue. The codice_12 instruction is a variant of codice_1 that features partial serialization of the instruction stream, but should not be considered as serializing. Implementation in various processors. Intel  processor families increment the time-stamp counter differently: The specific processor configuration determines the behavior. Constant TSC behavior ensures that the duration of each clock tick is uniform and makes it possible to use of the TSC as a wall clock timer even if the processor core changes frequency. This is the architectural behavior for all later 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 (codice_14) advertises this; Intel-CPUs also report their invariant TSC on that bit. Operating system use. An operating system may provide methods that both use and don't use the codice_1 instruction for time keeping, under administrator control. For example, on some versions of the Linux kernel, seccomp sandboxing mode  disables codice_1. It can also be disabled using the codice_17 argument to the codice_18 system call. Use in exploiting cache side-channel attacks. The time stamp counter can be used to time instructions accurately which can be exploited in the Meltdown and Spectre security vulnerabilities. However if this is not available other counters or timers can be used, as is the case with the ARM processors vulnerable to this type of attack. 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. SPARC V9 provides the codice_19 register. PowerPC provides the 64-bit codice_20 register. ARMv7 and ARMv8 architectures provide a generic counter which counts at a constant frequency. ARMv7 provides the Cycle Counter Register (CCNT instruction) to read and write the counter, but the instruction is privileged.