write about mainframe computers...
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Mainframe computers or mainframes (colloquially referred to as "big iron")[1] are computers used primarily by large organizations for critical applications; bulk data processing, such as census, industry and consumer statistics, enterprise resource planning; and transaction processing. They are larger and have more processing power than some other classes of computers: minicomputers, servers, workstations, and personal computers.
The term originally referred to the large cabinets called "main frames" that housed the central processing unit and main memory of early computers.[2][3] Later, the term was used to distinguish high-end commercial machines from less powerful units.[4] Most large-scale computer system architectures were established in the 1960s, but continue to evolve. Mainframe computers are often used as servers.