Timer

The timer is a module that consists of two internal registers:

• the CONT register (address 10h in the I/O memory),
• and the COMP register (address 11h in the I/O memory).

When the clock ticks, the CONT register increments by one. It starts counting after the first write to the CONT register. When the CONT register matches the COMP register, an interrupt is triggered via the INT1 line of the PIC.

It is based on Intel's 8253 PIT, but with some modifications to simplify its operation.