1. Hardware Timer
A timer is a hardware device comprising a clock source and a programmable counter. The clock source is usually a crystal oscillator, which generates periodic electrical signals to drive the counter at a precise frequency. The counter is programmed with a count-down value, which decrements by 1 on each clock signal. When the count decrements to 0, the counter generates a timer interrupt to the CPU, reloads the count value into the counter and repeats the count-down again. The period of the counter is known as a timer tick, which is the basic timing unit of the system.
2. PC Timers
The Intel x86 based PC has several timers (Bovet and Cesati 2005).
(1). Real-Time Clock (RTC): The RTC is powered by a small backup battery. It runs continually even when the PC’s power is turned off. It is used to keep real time to provide time and date information. When Linux boots up, it uses the RTC to update a system time variable to keep track of the current time. In all Unix-like systems, the time variable is a long integer containing the number of seconds elapsed since the beginning of January 1 of 1970.
(2). Programmable Interval Timer (PIT) (Wang 2015]: The PIT is ahardware timer separated from the CPU. It can be programmed to provide timer ticks in resolutions of milliseconds. Among all I/O devices, the PIT interrupts at the highest priority IRQ0. PIT timer interrupts are handled by the timer interrupt handler of the Linux kernel to provide basic timing units for system operation, such as process scheduling, process interval timers and a host of other timing events.
(3). Local Timers in Multicore CPUs (Intel 1997; Wang 2015): In a multicore CPU, each core is an independent processor, which has its own local timer driven by the CPU clock.
(4). High Resolution Timers: Most PCs have a Time Stamp Counter (TSC), which is driven by the system clock. Its contents can be read via a 64-bit TSC register. Since the clock rates of different system boards may vary, the TSC is unsuited as a real-time device but it can provide timer resolutions in nanoseconds. Some high-end PCs may also be equipped with a special high speed timer to provide timer resolutions in nanoseconds range.
Source: Wang K.C. (2018), Systems Programming in Unix/Linux, Springer; 1st ed. 2018 edition.