Real Time Clock

All PCs include a clock called Real Time Clock (RTC ), which is independent of the CPU and all other chips.

The RTC continues to tick even when the PC is switched off, since it is energized by a small battery or accumulator. The CMOS RAM and RTC are integrated in a single chip (the Motorola 146818 or an equivalent).

The RTC is capable of issuing periodic interrupts on IRQ 8 at frequencies ranging between 2 Hz and 8,192 Hz. It can also be programmed to activate the IRQ 8 line when the RTC reaches a specific value, thus working as an alarm clock.

Linux uses the RTC only to derive the time and date; however, it allows processes to program the RTC by acting on the /dev/rtc device file (see Chapter 13). The kernel accesses the RTC through the 0x7 0 and 0x71 I/O ports. The system administrator can set up the clock by executing the clock Unix system program that acts directly on these two I/O ports.

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