What Is a Mainframe?

A few weeks ago, at a tech event I attended, the topic of mainframes came up. Someone pointed out that the credit card payment I had just made was almost certainly processed by one. I had heard the term before but had never stopped to understand what it actually meant. This post is that investigation: what a mainframe is, how it works, and who depends on it.

IBM describes mainframes as "data servers that are designed to process up to 1 trillion web transactions daily with the highest levels of security and reliability." That single sentence contains a lot of engineering. The rest of this post unpacks it.

According to a study from the IBM Institute of Business Value, mainframes handle approximately 70% of the world's production IT workloads. 45 of the top 50 banks, 4 of the top 5 airlines, 7 of the top 10 global retailers, and 67 of the Fortune 100 run them as their core platform. The system processing your credit card authorization, your ATM withdrawal, and your airline booking is almost certainly a mainframe.

What One Looks Like

Wipe the room-sized-machine image from your head. Early mainframe systems occupied between 2,000 and 10,000 square feet and required enormous amounts of electrical power and dedicated air conditioning. That era is over. The IBM z17, released in April 2025, is a single-frame system built on a standard 19-inch rack. It fits in a modern data center the same way any other rack-mounted server does. The nickname "Big Iron" belongs to the past. The hardware has not been big for decades.

A Brief History

Five dates that explain how we got here:

• 1937: Harvard Mark I, the first mainframe, designed at Harvard and later used by the US Navy Bureau of Ships to solve mathematical calculations during World War II.
• 1953: IBM Model 701, IBM's first commercial mainframe, 25 to 50 times faster than its predecessors and purpose-built for business use.
• 1964: IBM System/360, the first mainframe with portable software. Before it, every new machine required custom-written programs. The System/360 separated software from hardware so that code written for one machine in the line ran on all of them. This is the origin of the "software platform" concept.
• 1964: IBM CP/CMS, the first hypervisor, which created virtual machines by virtualizing the underlying hardware. Commercial virtualization started on the mainframe. VMware was founded 34 years later, in 1998.
• 1991: InfoWorld analyst Stewart Alsop predicted "the last mainframe will be unplugged on March 15, 1996." In 2025, IBM launched the z17 with an integrated AI inference processor.

How a Modern Mainframe Works

Early mainframes had a single CPU that handled every type of work. Modern mainframes have a Central Processor Complex (CPC): a collection of specialized processors, each designed for a specific type of workload.

General-purpose engines handle business logic. Dedicated cryptographic coprocessors handle encryption and decryption. Separate processors manage I/O (input/output) channel traffic. Additional engines run Linux and Java workloads. Compression offload processors handle data compression independently.

On a commodity server, all of these compete for the same CPUs. When one workload spikes, others slow down. On a mainframe, each workload type has its own processor pool. A surge in cryptographic operations does not take CPU time away from transaction processing. The isolation is architectural, not just a scheduling policy.

This design is what allows a single mainframe to process up to one trillion secure web transactions per day with the highest levels of security and reliability.

Benefits

The core value of the mainframe is captured in three letters: RAS.

• Reliability: Hardware includes extensive self-checking and error-correction circuitry. Problems are detected and corrected before they become failures.
• Availability: When a component fails, the system isolates it and continues running. Failed components are replaced without halting the machine. Continuous high availability is the baseline, not the target.
• Serviceability: The system diagnoses its own failures and identifies which component was responsible, allowing targeted replacement with minimal disruption to everything else.

Beyond RAS, three properties define the modern mainframe:

• Security: Built-in cryptographic hardware verifies and encrypts data at the hardware layer. The system manages privacy by policy and can detect cyberattacks and fraud using AI models running on the machine.
• Scalability: The architecture supports massive simultaneous transactions and high I/O throughput with built-in capacity-on-demand and shared memory for direct application communication.
• Resiliency: Redundancy exists at every component level: power supplies, cooling systems, backup batteries, CPUs, I/O channels, and cryptographic modules. The machine is designed and tested for extreme conditions.

Who Uses Mainframes

Financial institutions process millions of transactions daily: credit card authorizations, ATM withdrawals, account updates, interbank transfers. Mainframes provide the throughput and reliability these operations require. Emerging financial technologies like blockchain also depend on mainframes for the same reasons: speed, scale, and security.

Healthcare organizations use mainframes to store and process large volumes of sensitive patient data. Insurance companies managing millions of policyholder records, claims, and billing operations need the security and reliability guarantees that the mainframe's hardware-level controls provide.

Government agencies across law enforcement, national security, and public services depend on mainframes for critical operations that cannot tolerate downtime or security breaches. The combination of resilience and hardware-enforced security makes them the standard choice for systems where failure carries serious consequences.

Retail uses mainframes to handle the transaction volumes that e-commerce generates at scale: purchases, returns, inventory updates, and loyalty program activity across millions of simultaneous sessions on mobile and web platforms.

Where Cloud Fits

Cloud does not replace the mainframe. The two systems coexist in what the industry calls a hybrid cloud: each platform handles the workloads it is best suited for.

An airline runs a cloud-native mobile app for customers to check flight status and manage bookings. The app handles cloud-based reservation information directly. For data like flight arrival and departure times, the app calls an API that queries the mainframe. Neither system replaces the other. The cloud layer handles the user-facing flexibility; the mainframe holds the authoritative operational data underneath.

Most of the systems you interact with daily follow this pattern, whether you can see it or not.

Looking Ahead

With AI moving from experimentation into production infrastructure, and with Europe's push for digital sovereignty (the idea that sensitive workloads should run on infrastructure you own and control), the case for mainframe investment is getting stronger, not weaker. IBM's z17 ships with an integrated AI inference processor. The logic is straightforward: if regulation says the data cannot leave, and the model needs to run on the data, you run the model where the data already lives. I think the demand for mainframe capacity will grow over the next decade, and the engineers who understand this infrastructure will be well positioned when those conversations happen.

References

1. IBM Think, "What is a mainframe?": https://www.ibm.com/think/topics/mainframe
2. IBM z/OS Basic Skills, "Mainframe hardware concepts": https://www.ibm.com/docs/en/zos-basic-skills?topic=concepts-mainframe-hardware
3. IBM z/OS Basic Skills, "Clustering on the mainframe: Parallel Sysplex and other techniques": https://www.ibm.com/docs/en/zos-basic-skills?topic=concepts-clustering-mainframe-parallel-sysplex-other-techniques
4. Open Mainframe Project, "About Open Mainframe Project": https://openmainframeproject.org/about/
5. IBM Redbooks, "An Introduction to the New Mainframe: z/OS Basics" (SG24-6366): https://www.redbooks.ibm.com/abstracts/sg246366.html