Mainframe computer

Mainframes (often colloquially referred to as big iron) are large and "expensive" computers used mainly by government institutions and large companies for mission critical applications, typically bulk data processing such as censuses, industry/consumer statistics, ERP, and financial transaction processing.

Mainframes vs. supercomputers

The distinction between supercomputers and mainframes is not a hard and fast one, but generally one can say that supercomputers focus on problems which are limited by calculation speed while mainframes focus on problems which are limited by Input/Output and reliability. As a consequence:

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• Because of the parallelism visible to the programmer, supercomputers are often quite complicated to program and require specialized, task-specific software. In contrast, mainframes hide parallelism from the programmer. (One side effect is that even older software can benefit from adding mainframe CPs.)
• Supercomputers are optimized for complicated computations that take place largely in memory, while mainframes are optimized for simple computations involving huge amounts of external data accessed from databases ("mixed workload").
• Supercomputers tend to cater to science and the military, while mainframes tend to target business and civilian government applications. Weather modeling, protein folding analysis, and digital film rendering are all tasks well suited to supercomputers. Credit card processing, bank account management, market trading, and social insurance processing are tasks well suited to mainframes. (Exception: Certain military applications require high security, a mainframe strength.)
• Supercomputers often run tasks that can tolerate interruption (for example, global warming forecasts and academic research). Mainframes tend to run those functions that must run reliably, even for years of continuous service (for example, airline bookings or credit-card processing).
• Supercomputers are often purpose-built for one or a very few specific institutional tasks. Mainframes typically handle a wide variety of important, everyday tasks.
• Mainframes assiduously and thoroughly support older software (dating back to applications written in the mid-1960s, in IBM's case) alongside new software. Supercomputers tend not to have backward compatibility as a central design feature.
• Mainframes tend to have numerous ancillary service processors assisting their main central processors (for cryptographic support, I/O handling, monitoring, memory handling, etc.) so that the actual "processor count" is much higher than would otherwise be obvious. Supercomputer design tends not to include as many service processors since they don't appreciably add to raw number crunching power.

There's also some blurring of the term "mainframe" with high-end PC and UNIX servers. (Some PC and UNIX server vendors occasionally refer to their systems as "mainframes" or "mainframe-like.") That blurring of the term is not widely accepted, with the market in general agreement that true mainframes (particularly IBM zSeries) are genuinely and demonstrably different.

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