8042

The 8042, usually referred to as the i8042 keyboard/mouse controller, is an Intel microcontroller used in IBM PC compatible computers to manage the keyboard and, in many implementations, the PS/2 mouse. It originated in the early era of PC design as part of the MCS-48 family and became a standard component on many PC motherboards, especially during the IBM PC/AT generation and its successors.

Functionally, the i8042 sits on the system motherboard and interfaces with the keyboard and mouse through PS/2 connectors. Its primary role is to scan the keyboard matrix, translate key presses into scan codes, and pass those codes to the main CPU. It can also handle PS/2 mouse input. The CPU communicates with the controller via two I/O ports: a data port (commonly referenced as 0x60) for data and a status/command port (commonly 0x64) for commands and status checks. When data or a device event is available, the controller can generate interrupts (typically IRQ1 for keyboard and IRQ12 for the mouse on traditional PC architectures).

Operation and behavior are defined by a set of commands the host can issue to the 8042 and by the scan code sets it can produce (such as Set 1, Set 2, or Set 3). The controller also buffers input and output data to coordinate timing between devices and the CPU.

Legacy and modern usage: with the rise of USB, the direct role of the 8042 has diminished, but many systems retain PS/2 ports and provide firmware or firmware-assisted emulation of the i8042 for legacy keyboard and mouse support. In contemporary PCs, the 8042 interface is often still referenced in BIOS/UEFI and low-level hardware documentation as part of historical compatibility.