Whether a separate clock chip or integrated into the motherboard, there are typically 3 timers available plus the real time clock. These are first 3 timers are referred to as Counter 0, Counter 1, and Counter 2.

• Counter 0 is connected to the first interrupt line (IRQ 0) of the programmable interrupt controller.

  It is used as the system timer.

• Counter 1 is used for memory refresh.

• Counter 2 generates frequency needed by the system speaker (but isn't connected to the interrupt controller).

• In addition, there is a real time clock connected to the first interrupt line on the secondary control (so IRQ 8).