A .sys file is a system file, primarily used in Windows operating systems as a driver. These files contain essential instructions allowing hardware devices (like printers or graphics cards) or low-level software components to communicate with the operating system core. Unlike standard executable (.exe) files run by users, .sys files are loaded automatically during the system boot process or when a specific device is connected, acting as intermediaries between the OS and hardware.

Common examples include keyboard and mouse drivers (e.g., kbdhid.sys, mouclass.sys), which enable basic input functionality. Antivirus software also often uses .sys files to implement core real-time scanning components operating at the deepest level of the system. Industries relying heavily on specialized hardware, like manufacturing or healthcare diagnostics, depend on device-specific .sys drivers for their equipment to function correctly within Windows environments.

These files offer critical functionality but require high-level system privileges. This makes them potential targets for malware seeking deep system access; corrupt or malicious .sys files can cause severe instability. To mitigate risks, Microsoft mandates driver signing, ensuring authenticity. The role of .sys files remains vital, though virtualization and standardized driver frameworks (like WDF) continuously improve their development, security, and management. Updates typically occur through Windows Update or vendor installers.