Final answer:
The hardware mechanism that a device uses to notify the CPU is known as an interrupt, which allows for efficient and asynchronous handling of requests without the need for constant CPU polling.
Step-by-step explanation:
The hardware mechanism that allows a device to notify the CPU is called an interrupt. An interrupt is a signal sent to the CPU by either a hardware device or a software program indicating that it needs the CPU's attention. When this signal is received, the CPU halts its current tasks and serves the request by running a special piece of code known as the interrupt handler or interrupt service routine (ISR). This process allows the system to handle events asynchronously without the need for constant polling by the CPU. Polling is a process where the CPU repeatedly checks the status of an interface controller at regular intervals, which can be inefficient. Instead, interrupts provide a more efficient means of communication between the devices and the CPU.