# Mainframe

> A big, ultra-reliable central computer that banks and airlines use for their core work.

- Level: Intermediate
- Topics: Hardware
- URL: https://tidbit.legios.com.ar/mainframe/
- In Spanish: [Mainframe](https://tidbit.legios.com.ar/es/mainframe/)

A **mainframe** is a large central computer built to process enormous numbers of small transactions, reliably, year after year. When you pay with a card, withdraw cash or book a flight, there is a good chance that somewhere along the way a mainframe recorded it. Banks, insurers, airlines, retailers and governments still run their most critical systems on them.

Mainframes date back to the 1950s and 60s, when a computer filled a room and a whole company shared one. Today's models, mostly IBM's Z line, fit in a few refrigerator-sized cabinets, but the idea is the same: one machine with huge input and output capacity, spare parts for everything, and the ability to swap components or update software while it keeps running. What they optimize is not raw speed on one task but never stopping and never losing a transaction.

A lot of the software on them is decades old and written in COBOL, a programming language from 1959. It works, it is woven into everything, and rewriting it is risky and expensive, which is why many migrations to the cloud take years or stall. Meanwhile the people who know these systems are retiring. So mainframes are less a museum piece than a quiet backbone that the rest of the tech world still plugs into.

## An analogy

Like the central vault and ledger room of an old bank. It is not flashy and not new, but every branch, ATM and teller depends on it, so it is built to never close and never make a mistake.

## Keep in mind

- Central computers built for huge volumes of transactions and near-zero downtime.
- Still run core systems at banks, airlines, insurers and governments.
- IBM's Z line dominates the market today.
- Much of their software is in COBOL, which makes migrating off them slow and risky.

## Go deeper

- [Mainframe computer on Wikipedia](https://en.wikipedia.org/wiki/Mainframe_computer)
- [What is a mainframe? (IBM)](https://www.ibm.com/think/topics/mainframe)

## Related terms

- [IBM](https://tidbit.legios.com.ar/ibm/): A century-old computer company that sells software, consulting and mainframes.
- [Programming language](https://tidbit.legios.com.ar/programming-language/): A precise, written language people use to give instructions to a computer.
- [Cloud computing](https://tidbit.legios.com.ar/cloud-computing/): Renting computers over the internet instead of buying your own.