RTC

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Introduction

Most OSs use the PIT for timing purposes. However, I believe the RTC works just as well. RTC stands for Real Time Clock. It is the chip that keeps your computers clock up-to-date. Within the chip is also the CMOS 64 bytes of RAM.

Capabilities

The RTC is capable of multiple frequencies. However, the only one acceptable for keeping time is 1024hz. For this reason, it is strongly NOT recommended that you change the frequency of the RTC, or your computers clock will no longer be accurate. However, if you need an interrupt frequency higher than 1024hz, it is possible to reprogram just the rate of the interrupt. The RTC can handle 15 interrupt rates between 2hz and 32767 Hz, theoretically. On most machines however, the interrupt rate can not go higher than 8khz.

Avoiding the NMI

When programming the RTC, it is important that the NMI (non-maskable-interrupt) be disabled. This is because if an NMI happens, then your programming code will be interrupted. This would usually not be a big deal, but the RTC is never initialized by BIOS. Actually, BIOS initializes itself from the CMOS. Note, just cause the computer reboots, doesn't mean the RTC does. The RTC is powered by an internal battery and is separate from the rest of the computer.

You can disable the NMI with

outportb(0xA0, inportb(0xA0) | 0x80);

You can later reenable the NMI with

outportb(0xA0, inportb(0xA0) & 0x7F);

See Also

Periodic Interrupts with the Real Time Clock

CMOS Ram Data Register

Using the 1024 HZ Timer Interrupt