What is the function of the "D" key pressed during startup on a Mac?

Pressing the "D" key during startup on a Mac initiates the Apple Hardware Test (AHT) or Apple Diagnostics, depending on the model and operating system version. This function allows users to check their hardware for potential issues, enabling the identification of problems with components such as the memory, logic board, and other internal parts.

The Apple Hardware Test is particularly useful when troubleshooting hardware issues, as it runs a series of automated tests and provides a report on any detected problems. This helps technicians and users diagnose issues before considering more extensive repairs or replacements.

Understanding the role of the "D" key reinforces the importance of hardware diagnostics in maintenance and support, highlighting the proactive approach needed in managing a Mac's performance and reliability.