Mainframe computers, often called "big iron," are powerful systems primarily used by large organizations for critical tasks like bulk data processing, enterprise resource planning, and large-scale transaction processing. These machines are defined by their exceptional reliability, availability, and serviceability (RAS), often running for decades without interruption, a feature crucial for applications where downtime is costly. Modern mainframes, such as IBM Z and Unisys Dorado/Libra, are renowned for their high security and strict backward compatibility, ensuring older software continues to function seamlessly. Evolving from rudimentary batch processing with punched cards in the 1950s and 60s, they adopted interactive timesharing for hundreds of users by the 1970s. Despite their long history, mainframe architectures continue to evolve, making them indispensable servers today, highly valued for their virtualization capabilities and robust performance.