Mainframe & Legacy Systems in 2026 — IBM z17 (Sept 2025) / IBM i / COBOL / RPG / JCL / CICS / watsonx Code Assistant for Z Deep Dive

Prologue — They said it was dead, but it's still there

"Mainframes are dead" has been a yearly headline since the 1990s. And yet in May 2026, the moment you pull money from an ATM, swipe a credit card, book a flight seat, pay your taxes, or register a birth at a government office, the last line of that transaction will, with very high probability, run through a COBOL program on a mainframe.

• IBM Z — z17 launched September 2025, Telum II processor, on-chip AI acceleration.
• IBM i (formerly AS/400) — 130,000+ companies still operate it. ERP, warehousing, insurance.
• COBOL — an estimated 200~220B (billion) lines run in production. Every day.
• RPG / JCL / CICS / DB2 z/OS / IMS / PL/I — all still in active duty.
• Fujitsu BS2000 / NEC ACOS / Unisys ClearPath — the world of non-IBM mainframes.

And since 2023 this enormous legacy has met a new variable: AI. IBM's watsonx Code Assistant for Z translates COBOL into Java. GitHub Copilot supports COBOL completion. AWS Mainframe Modernization lifts mainframe workloads to the cloud.

This article covers mainframes as of 2026: how they didn't die, what the new hardware looks like, how AI is chipping at this enormous legacy, and — if you're a developer — why some people decide right now to learn COBOL.

1. The Mainframe in 2026 — A Giant That Refused to Die

Let's start with numbers. Estimates of the mainframe ecosystem as of 2026:

(Sources: IBM, Forrester, Gartner market estimates — exact numbers vary by source. All figures in this post are estimates based on 2024~2025 public reports.)

"Why didn't they switch" is the wrong question. The right question is "why was not switching the rational decision?" The answer compresses to three things:

1. Proven reliability — z/OS availability is often reported at 99.999% (5-nines) or higher. Around 5 minutes of downtime per year. Matching that on a new system takes years.
2. Risk of untested change > value of change — imagine rewriting a 20-million-line insurance core in Java. Two years. A hundred million dollars. And no new features in that window. CFOs don't sign.
3. Data gravity — DB2 z/OS, IMS, VSAM hold decades of accumulated data. Moving data is harder than moving code.

So the mainframe didn't die. Instead of dying, it evolves slowly. That's what this post is about.

2. IBM z17 (September 2025) — Telum II + Integrated AI

In September 2025, IBM announced z17. The previous generation (z16) launched in April 2022, so the cadence is roughly three years. The core of z17 is two things:

Telum II processor

• A roughly 5nm-class process (IBM does not generally disclose the exact node)
• 8 cores, clock in the 5 GHz range (per-core performance improved over Telum I in z16)
• On-chip AI accelerator — strengthened from Telum I's on-die AI inference. Allows ML inference (e.g., fraud detection) to be inlined into a transaction with sub-millisecond latency.
• Improved L2 cache structure, optimized NUMA topology.

In short: Telum II lets you embed AI inference into the transaction itself. The fraud-detection model runs on the same chip, at the moment the card payment happens. You don't ship data out to a GPU cluster.

Spyre accelerator (optional)

The IBM Spyre AI accelerator introduced with z16 (a separate PCIe card) is also available as an option for z17. Spyre handles larger models (generative AI inference) and can complement Telum II's on-chip acceleration.

What changed (vs z16)

z17 is deeper AI integration rather than a revolution. IBM's message — transactions plus AI inference plus security all on one chip — gets stronger.

Who buys z17

• Global banks and card networks — fraud detection inlined in transactions.
• Large insurers — claim processing plus risk scoring.
• Governments and tax agencies — massive batch plus security.
• Airlines and logistics — reservation and inventory backbones.

Prices aren't public. The usual line is "starts in the millions of dollars." And the companies writing those checks believe it's reasonable.

3. IBM z16 — Still in Active Service

The arrival of z17 doesn't retire z16. Launched in April 2022, the z16 is still actively sold and installed in 2026. Mainframe lifecycles are long — once a machine is in, it stays for a decade.

z16 highlights:

• Telum I processor — IBM's first generation with on-chip AI acceleration.
• Quantum-safe cryptography — Crypto Express 8S with NIST-standardized post-quantum algorithms (a subset).
• Pentide (IBM Telum) cache structure — a quirky design that uses L2 as virtual L3/L4 to expose roughly 256 MB of virtual L4 cache.
• z/OS 2.5 / 3.1 support.

If a bank just installed z16, the case to jump to z17 is weak. Many will run z16 plus firmware updates well into the early 2030s.

4. IBM i (formerly AS/400) — 130K+ Companies

Less famous than IBM Z but equally important: IBM i, the successor to the AS/400 of 1988.

History of the name

• System/38 (1978)
• AS/400 (1988) — Application System/400. The icon of mid-market ERP in the 80s and 90s.
• eServer iSeries (2000)
• System i (2006)
• IBM i (2008~present) — both the OS name and the platform name.

The hardware runs on IBM Power servers. So IBM i in 2026 is Power Systems hardware plus the IBM i OS, with versions like IBM i 7.5 / 7.6 running on Power10.

Why 130,000 companies still use it

• Single-object model — files, programs, and DB objects live in the same system object model. Operations are simple.
• TIMI (Technology Independent Machine Interface) — hardware abstraction. New Power chips run old binaries unchanged.
• Built-in Db2 for i — no separate database to operate.
• Lower running cost — by mainframe standards. One box can run ERP, warehouse, and finance.

Who runs it

• US and European mid-market manufacturing
• Japanese food, retail, and logistics
• A number of Korean mid-market manufacturing and distribution (many specifics aren't public)
• European insurers
• Retail chains

ERP packages like JD Edwards, Infor (formerly SSA Global), and BPCS run on IBM i.

RPG — the main language of IBM i

The dominant programming language on IBM i is RPG (Report Program Generator). We'll dive in next.

5. COBOL — An Estimated 200~220B Lines in Production

COBOL was designed by a committee in 1959 that included Grace Hopper. The name means COmmon Business Oriented Language. A language older than 60 years still has 200B lines running.

Why so much

• Banking core systems — almost every large bank's core banking.
• Insurance claims and policies — auto, life, P&C.
• Government tax and welfare — the US IRS, parts of Korea's NTS, the UK's HMRC, and so on.
• Airline reservations — Sabre and Amadeus, the global distribution systems, still have COBOL in their backbone.
• Retail and manufacturing ERP — running on IBM i with RPG.

The 200B figure is cumulative. New COBOL written each year decreases, but old code rarely gets deleted. So the line count grows.

What COBOL code looks like

       IDENTIFICATION DIVISION.
       PROGRAM-ID. INTEREST-CALC.
       AUTHOR. YJ.
      *
       ENVIRONMENT DIVISION.
       CONFIGURATION SECTION.
       SOURCE-COMPUTER. IBM-Z.
       OBJECT-COMPUTER. IBM-Z.
      *
       DATA DIVISION.
       WORKING-STORAGE SECTION.
       01  WS-PRINCIPAL       PIC 9(7)V99 VALUE 1000000.00.
       01  WS-RATE            PIC 9(3)V99 VALUE 3.50.
       01  WS-YEARS           PIC 9(2) VALUE 5.
       01  WS-INTEREST        PIC 9(9)V99.
       01  WS-TOTAL           PIC 9(9)V99.
      *
       PROCEDURE DIVISION.
       CALC-INTEREST.
           COMPUTE WS-INTEREST = WS-PRINCIPAL * (WS-RATE / 100) * WS-YEARS.
           COMPUTE WS-TOTAL    = WS-PRINCIPAL + WS-INTEREST.
           DISPLAY "Principal : " WS-PRINCIPAL.
           DISPLAY "Interest  : " WS-INTEREST.
           DISPLAY "Total     : " WS-TOTAL.
           STOP RUN.

COBOL hallmarks:

• English-like syntax — COMPUTE, DISPLAY, STOP RUN. Reads like sentences.
• Fixed-column format (traditional) — 1-6 sequence, 7 indicator, 8-11 area A, 12-72 area B. Modern COBOL also supports free-format.
• PIC clauses — define a data picture. PIC 9(7)V99 means 7 integer digits and 2 decimals.
• DIVISION structure — four parts: IDENTIFICATION, ENVIRONMENT, DATA, PROCEDURE.

COBOL standards over time

• COBOL-60 → COBOL-68 → COBOL-74 → COBOL-85 (de facto standard for decades) → COBOL 2002 → COBOL 2014 → COBOL 2023.
• Modern standards add OOP, Unicode, and XML/JSON processing.

COBOL compilers

• IBM Enterprise COBOL for z/OS — the canonical compiler.
• IBM COBOL for AIX / Linux on Z
• Micro Focus Visual COBOL (now under OpenText) — Windows/Linux/mainframe-compatible.
• GnuCOBOL — open source. Transpiles to C. (More in a later chapter.)

6. RPG / JCL / CICS / DB2 z/OS / IMS / PL/I

COBOL is just the most famous mainframe language. The environment has other essentials.

RPG (Report Program Generator) — the main IBM i language

The name says "report generator" but modern RPG is a general-purpose business language. Timeline:

• RPG II (1960s+)
• RPG III (System/38)
• RPG IV / RPG/ILE (AS/400 era through today)
• Free-format RPG (modern) — not tied to column positions, similar to free-format COBOL.

Modern free-format RPG example:

**free
ctl-opt main(main);

dcl-proc main;
  dcl-s principal packed(9:2) inz(1000000);
  dcl-s rate     packed(5:2) inz(3.50);
  dcl-s years    packed(2:0) inz(5);
  dcl-s interest packed(11:2);
  dcl-s total    packed(11:2);

  interest = principal * (rate / 100) * years;
  total    = principal + interest;

  dsply ('Total: ' + %char(total));
end-proc;

RPG is very tightly coupled to the database. Embedded SQL inside RPG is the norm.

JCL (Job Control Language)

The language for defining batch jobs on z/OS. People call it "an unpleasant ancient shell script," but it is the lifeline of mainframe operations.

//PAYROLL  JOB  (ACCT1234),'PAYROLL JOB',
//             CLASS=A,MSGCLASS=X,REGION=4M
//STEP1    EXEC PGM=PAYCALC
//STEPLIB  DD   DSN=PROD.LOADLIB,DISP=SHR
//INPUT    DD   DSN=PROD.PAYROLL.MASTER,DISP=SHR
//OUTPUT   DD   DSN=PROD.PAYROLL.NEWRUN,DISP=(NEW,CATLG,DELETE),
//              SPACE=(CYL,(10,5)),
//              DCB=(LRECL=200,BLKSIZE=27800,RECFM=FB)
//SYSPRINT DD   SYSOUT=*

JCL features:

• Column-position-sensitive syntax starting with //.
• JOB → EXEC → DD card structure — job → execution step → dataset.
• DSN for dataset name, DISP for disposition (SHR/MOD/NEW), SPACE for allocation.
• STEPLIB for libraries, SYSPRINT for log output.

JCL is an operations asset. A good JCL line can save a company. A bad JCL line — like the eternal joke of using DISP=NEW,DELETE on a production master — can bankrupt one.

CICS (Customer Information Control System)

The transaction monitor on z/OS, in operation since 1969. CICS:

• Receives requests from terminals (traditionally 3270) or web/mobile clients.
• Executes business logic (COBOL/PL/I and others) as transactions.
• Returns the result.
• Handles distributed transactions (2PC), integrates with Db2 and IMS.

Modern CICS, called CICS Transaction Server, runs on z/OS and supports REST/JSON interfaces.

DB2 z/OS (IBM Db2 for z/OS)

The relational database of z/OS. It is a different codebase from Db2 in other environments (Db2 LUW, Db2 for i). As of 2026, Db2 for z/OS V13 and later run in production.

Db2 for z/OS identity:

• Transaction processing and data warehousing on a single system.
• Pentide cache plus Telum on-chip acceleration plus AI Query Optimizer — performance fused to mainframe hardware.
• Massive accumulated data — petabyte-scale operation is common.

IMS (Information Management System)

Built in 1968 for the Apollo program, IMS is a hierarchical database plus transaction monitor. It predates relational databases (Db2) and a number of large banks, airlines, and manufacturers still run IMS DB and IMS TM. "Old" does not mean "unused" — old means proven and fast.

PL/I (Programming Language One)

In 1964 IBM tried to merge COBOL and Fortran into one language. It never replaced either, but parts of large systems (notably European banks and some operating-system code) still use it.

TEST: PROC OPTIONS(MAIN);
  DCL PRINCIPAL FIXED DEC(9,2) INIT(1000000);
  DCL RATE      FIXED DEC(5,2) INIT(3.50);
  DCL YEARS     FIXED BIN(15)  INIT(5);
  DCL INTEREST  FIXED DEC(11,2);
  DCL TOTAL     FIXED DEC(11,2);

  INTEREST = PRINCIPAL * (RATE / 100) * YEARS;
  TOTAL    = PRINCIPAL + INTEREST;

  PUT SKIP LIST('Total: ', TOTAL);
END TEST;

PL/I is more expressive than COBOL but has a steeper learning curve. "If it's there, it's there; nobody starts new in PL/I."

7. Fujitsu BS2000 / NEC ACOS / Unisys ClearPath — The Non-IBM Mainframes

People often equate "mainframe" with "IBM." That's inaccurate. Japan, parts of Europe, and parts of the US still run non-IBM mainframes.

Fujitsu BS2000 (Japan/Germany)

• Originally the BS2000 operating system from Germany's Siemens. Fujitsu later absorbed Siemens' computer business.
• Today it is the Fujitsu FUJITSU Server BS2000 line. The OS evolved into OSD/BC, BS2000/OSD, etc.
• Mostly operates at government agencies, banks, and insurers in Germany, Austria, and Switzerland.
• Supports COBOL, ASSEMBLER, C/C++. There are SQL DBMSes too (SESAM, UDS).
• Fujitsu also has Japan-domestic mainframe lines (such as the PRIMEFORCE series over the years — naming and lineup change over time, so refer to Fujitsu's official material).

NEC ACOS (Japan)

• NEC's ACOS-4 OS runs on its mainframes. Parts of the Japanese government and some regional banks run NEC ACOS as their core.
• Japan Post and certain regional banks are example operators.
• It has its own COBOL plus 4GLs like ADL (ACOS Data Language).

Unisys ClearPath

• Unisys' mainframe line from the US, with two OS families:
  • MCP (Master Control Program) — successor to Burroughs B5000 (1961). A fossil of OS history because it is ALGOL-based.
  • OS 2200 — successor to Sperry UNIVAC.
• ClearPath Dorado (the 2200 line) and ClearPath Libra (the MCP line) have been the brand names.
• Unisys has pivoted to ClearPath Forward, which emulates mainframe hardware on x86 servers.
• Still in operation at US airlines, government, and some banks.

Summary — what non-IBM mainframes share

8. AWS Mainframe Modernization

In 2022, AWS launched AWS Mainframe Modernization (AWS MMA) — a managed service to help move mainframe workloads onto AWS.

Two patterns

The two official migration strategies AWS pushes:

1. Replatform — leave the COBOL alone, but move the mainframe environment somewhere else (AWS containers or EC2). AWS exposes an option backed by the Micro Focus runtime (now OpenText).
2. Refactor — auto-convert COBOL into Java. AWS acquired and integrated Blu Age, a tool that translates COBOL to modern Java/Spring/Angular.

Replatform is the more common path

Most large migrations are replatform projects, because:

• Business logic is untouched — no risk of changing behavior.
• Mainframe operational costs go down — IBM license/MIPS cost replaced by AWS compute.
• Modern tool integration — CI/CD, containers, observability.

But there are traps:

• Data migration — Db2 z/OS to Aurora/RDS PostgreSQL is not easy. Triggers, stored procedures, and SQL dialect differences all surface.
• Transaction monitor parity — replicating exact CICS behavior is hard.
• MIPS price comparison — "savings" aren't always real. Matching equivalent availability and performance can land at similar cost.

Flow

[Mainframe]                  [AWS]
   COBOL source ────────▶    COBOL source (or Java conversion)
   JCL batch    ────────▶    AWS Batch / EventBridge Scheduler
   CICS         ────────▶    Mainframe Modernization Runtime
   Db2 z/OS     ────────▶    Aurora PostgreSQL / Db2 LUW
   VSAM         ────────▶    DynamoDB / S3 / Aurora
   IMS          ────────▶    Custom mapping (hardest)

AWS provides assessment, analysis, and cutover support as a managed service. You still need mainframe veterans paired with AWS solution architects.

9. Micro Focus → OpenText (2022) → Visual COBOL

For a long time, the center of mainframe modernization was Micro Focus — a UK-headquartered company and one of the largest suppliers of COBOL compilers and runtimes.

2022 — The OpenText acquisition

In 2022, the Canadian information-management company OpenText acquired Micro Focus for roughly six billion dollars. The Micro Focus brand is being phased into OpenText as the deal completes.

Visual COBOL — the flagship product

Micro Focus's (now OpenText's) flagship is Visual COBOL. What it does:

• Lets mainframe COBOL run on Windows/Linux/.NET/JVM.
• Visual Studio / IntelliJ / Eclipse integration — edit, debug, and refactor COBOL inside modern IDEs.
• CICS emulation — provides an equivalent transactional environment so mainframe CICS apps can be lifted as-is.
• JCL translation — runs mainframe JCL outside the mainframe environment.

Enterprise Server / Enterprise Developer

OpenText's mainframe lineup usually includes:

• Enterprise Developer — the developer IDE plus compiler.
• Enterprise Server — production runtime (CICS-compatible, IMS-compatible environments).
• AppMaster Builder, Enterprise Test Server, Modernization Workbench — supporting tools.

Some replatform options inside AWS Mainframe Modernization run on this OpenText stack.

What it means

OpenText is not in the business of killing the mainframe. They are in the business of letting mainframe code keep running elsewhere. Their model assumes COBOL is alive into the 2030s.

10. IBM watsonx Code Assistant for Z (August 2023) — AI for COBOL Modernization

In August 2023, IBM announced watsonx Code Assistant for Z, an AI tool for helping convert mainframe COBOL into Java.

What it does

• COBOL → Java conversion — automatic or semi-automatic. Moves business logic into modern Java.
• Code explanation — uses an LLM to explain COBOL written in the 1980s. Natural-language queries like "what does this module do."
• Test generation — produces test cases for existing COBOL.
• Incremental conversion — module-by-module and service-by-service rather than big-bang.

How it works

watsonx Code Assistant for Z combines an LLM (built on IBM's granite code model family) with mainframe-specific domain knowledge. Not just generic ChatGPT, but a model trained on COBOL/JCL/CICS patterns.

Limits and reality

Don't take IBM marketing at face value. In practice:

• Auto-converted Java needs review — true of all LLM-generated code.
• Subtle business logic — COBOL's decimal arithmetic (PIC 9V99), EBCDIC, indicator semantics — these can be lossy.
• Data migration is a separate problem — moving code isn't the end of the project.

Still, as a starting point, it has value. Rather than starting from zero, humans refine an AI-generated first pass.

Trends in 2024~2025

• IBM generalized the watsonx Code Assistant family — Code Assistant for Java, for Ansible, and others.
• The IBM Z variant is being applied to larger cases — one insurer scoped 8 million lines of COBOL incrementally.
• Competitors: Amazon Q Developer (formerly CodeWhisperer) with mainframe support, GitHub Copilot's COBOL coverage.

11. GitHub Copilot COBOL Support

GitHub Copilot went GA in 2021, initially known for JavaScript, Python, and TypeScript. Language coverage has expanded over time, and COBOL completion is supported.

What it feels like

How Copilot helps in COBOL:

• PIC clause completion — type a variable name and Copilot suggests a PICTURE clause like PIC 9(7)V99.
• Paragraph names and flow suggestions — predicts the next PERFORM, MOVE, COMPUTE.
• JCL generation — a natural-language comment yields a JCL snippet.
• Db2 embedded SQL — autocompletes EXEC SQL ... END-EXEC blocks.

Limits

• 80-column format handling — traditional column-based COBOL is harder for LLMs. Free-format works better.
• Company coding standards — Copilot knows generic patterns. It does not know your internal naming conventions or shop standards.
• Test data semantics — it doesn't know what your business data means.

Copilot vs watsonx Code Assistant for Z

• Copilot — generic coding assistant. Supports COBOL but is not specialized.
• watsonx CA for Z — mainframe-specialized. Tuned for code conversion and explanation.

These are less competitors than different layers. Use Copilot for everyday coding and watsonx for large conversion programs.

12. zCX (z/OS Containers) + Linux on Z + LinuxONE

Another stream pulling the mainframe into the cloud era is containers and Linux.

zCX (z/OS Container Extensions)

A z/OS feature introduced in 2019. It runs Docker containers inside z/OS by spinning up a Linux on Z environment as a built-in virtual machine and running containers inside that.

Why this matters:

• Low-latency access to mainframe data (Db2 z/OS, VSAM).
• Pull modern stacks — Kafka, Elastic, Grafana — next to the mainframe.
• Add modern interfaces without touching the transaction backbone.

Linux on Z

Linux running natively on z hardware. Coexists with z/OS via LPAR partitioning, so a single mainframe can host both environments.

• Major distributions: RHEL on Z, SUSE on Z, Ubuntu on Z.
• Mainframe reliability extended to Linux workloads.
• A single system can host hundreds or thousands of Linux VMs and containers.

IBM LinuxONE

A Linux-only mainframe. Same hardware family as IBM Z, but no z/OS. So: "mainframe reliability for Linux." Use cases:

• Large-scale payment processing (including blockchain nodes).
• Environments where data protection matters (combined with IBM Cloud Hyper Protect).
• Green data centers (less power for the same workload compared with x86).

IBM Cloud Hyper Protect

IBM Cloud's confidential computing offering. Brings mainframe security technology (Crypto Express, Secure Service Container) to the cloud as services — key management, trusted virtual servers, databases hosted at mainframe-grade security.

13. Hercules + GnuCOBOL — Open-Source Emulation

Can you try a mainframe at home? Yes.

Hercules — a System/370 / ESA/390 / z/Architecture emulator

An open-source mainframe emulator that mimics z/Architecture on regular PCs/Macs/Linux. Caveats:

• Commercial z/OS requires a license — so legally running z/OS at home is hard.
• MVS 3.8j and similar old IBM OSes are public domain — you can experience 1970s~80s mainframes.
• Linux on Z — free and legal. The s390x builds of RHEL/SUSE/Ubuntu run on Hercules.

GnuCOBOL

An open-source COBOL compiler. It transpiles COBOL to C, then builds with a normal C compiler.

# Install (e.g., macOS Homebrew)
brew install gnu-cobol

# Compile
cobc -x -o interest interest.cob

# Run
./interest

• Covers a large subset of COBOL 85 / 2002 / 2014.
• Does not support IBM-OS-specific features (CICS, Db2 z/OS, etc.).
• Suitable for learning, education, and small-scale operation.

Learning path

• If you're starting COBOL: GnuCOBOL plus VS Code (or Eclipse).
• For a mainframe experience: Hercules plus MVS 3.8j, plus IBM's Z trial or IBM Z learning programs.
• IBM has long operated free learning programs for students and beginners — IBM Z Xplore, Master the Mainframe, etc. (Names and content change over time; check IBM's official pages.)

14. Modernization Patterns — Strangler Fig and Anti-Corruption Layer

Replacing legacy systems is not unique to mainframes. The two most famous and practical patterns:

The Strangler Fig pattern (Martin Fowler)

Named after the Australian strangler fig — a vine that grows around another tree and eventually replaces it. Martin Fowler used the metaphor for system renewal in a 2004 article.

Strategy:

1. Put a proxy/facade in front of the legacy system.
2. Route some traffic to a new system.
3. Gradually route more traffic to the new system.
4. One day, legacy traffic is at 0%. Only then remove it.

[Client] ──▶ [Facade / Proxy] ──▶ [Legacy mainframe]
                  │
                  └──────────────▶ [New microservice]
                                   (gradually more routes)

Pros:

• No big-bang migration risk — you don't switch 100% at once.
• Rollbackable — if the new system breaks, reverse the routing.
• Incremental learning — you grow operational know-how as traffic grows.

Cons:

• Takes years — not a fast answer.
• Double operating cost — you run both during the transition.
• Data synchronization — both systems share data, so you pay sync cost during transition.

Anti-Corruption Layer (Eric Evans, DDD)

A pattern from Domain-Driven Design. Place a translation layer between the legacy and the new system.

[New domain model] ──▶ [Anti-corruption layer] ──▶ [Legacy model]
   (clean model)        (translate/map/insulate)    (old model)

Goals:

• Prevent legacy's odd concepts (60-year-old business assumptions) from leaking into the new system.
• The new system gets a clean model of its own.
• Translation cost is concentrated in one place.

Strangler Fig and Anti-Corruption Layer go together. Strangler is the routing pattern, Anti-Corruption is the domain-model insulation behind that routing.

In practice — where to start

1. Draw boxes — group modules running on the mainframe by domain.
2. Prioritize by change frequency and risk — split the most-changed and risky parts first.
3. Start with reads — moving query traffic to a new (read-only replica) database is the easiest.
4. Slowly add writes — dual-write (legacy plus new) for a period, then end on the new system only.
5. Monitor and observe — shadow traffic, compare responses across both systems.
6. Rollback plan — always.

15. Korean Financial Sector — KB / Shinhan / Woori / Hana / IBK / NongHyup Mainframes

Korean financial-sector mainframe operations are partly public, partly estimated. Based on 2024~2025 public information and industry estimates:

(The text below is a general summary based on publicly known patterns. Many bank-specific system details are non-public. Some content is estimated.)

The big picture

• The five major commercial banks plus NongHyup and IBK — most have operated IBM Z (or the prior generation of mainframe) alongside next-generation core systems.
• Next-generation core projects — large next-gen projects were run in the late 2000s and 2010s. The trend of reducing mainframe dependency coexists with the trend of preserving it.
• 2020s trend — cloud transition under review, partial downsizing of some systems, integration with AI initiatives.

General per-bank flow (based on public reports and industry knowledge)

(Note: The table above is a general summary of publicly known patterns. Exact system configurations are largely non-public, and the contents include estimates.)

Korean mainframe talent market

• Many veterans in their 60s and 70s carry the core operations.
• New entrants are scarce — almost no university teaches mainframe.
• Hence salaries rise — anyone who knows COBOL/RPG/JCL and speaks Korean can essentially set the price.
• IBM Korea's Master the Mainframe / Z Xplore programs in Korea helped seed new entrants.

Korean non-financial

• Public sector and tax — some systems operate on mainframe.
• Airlines — parts of Korean Air, Asiana, and so on.
• Telecom — some backbones.
• Manufacturing and distribution — many IBM i sites reportedly operate.

16. Japan — MUFG / SMBC / Mizuho / NTT Data / Fujitsu BS2000

Japan is another stronghold of mainframes. Unlike Korea, Japan has its own mainframe industry (Fujitsu, NEC, Hitachi). That uniqueness shapes the story.

Three megabanks — general flow

(General summary based on public sources and industry knowledge. Exact system configurations are largely non-public.)

NTT Data

The hub of Japan's SI market. Partner for building and operating public-sector and financial core systems. NTT Data executes mainframe migrations and outsourced operations at scale.

Fujitsu — its own mainframe

• FUJITSU BS2000 — mainframe originating in Germany. Operates in parts of Europe and Japan.
• Japan-domestic mainframe lineups (names and lineups have shifted over the years; check Fujitsu's official material for the latest).
• In the 2020s Fujitsu announced partial wind-down of mainframe business — including reported decisions to stop new mainframe development and shift to x86-based GS / PRIMERGY lines (exact schedule and scope per Fujitsu's official announcements).
• This trend has notable impact on the Japanese mainframe market.

NEC ACOS

• Operated by parts of the Japanese government, certain prefectures, and some banks.
• NEC also pursues a gradual transition of its mainframe business.

Hitachi

• Once one of the pillars of Japan's mainframe market. Today the focus is more on systems integration and operations services than on the mainframe itself.

Japan's talent market

• Same pattern as Korea — veterans plus scarce new entrants.
• Japan's structure absorbs talent into large SIs (NTT Data, Fujitsu, NEC, Hitachi, IBM Japan), so the talent pool rotates at the company level.

17. Who Should Learn COBOL — Finance / Insurance / Government / Academia

If you're picking up a new language in 2026, COBOL probably isn't number one. But for specific people, it can be number one. Who and why.

Good candidates

• People aiming to work in finance — bank IT and insurance IT reward knowing mainframe operations. There aren't replacements.
• Public-sector / tax / welfare IT — Korea, Japan, and the US all have COBOL inside some government systems.
• Information-systems consulting — migration projects are a 10~20-year market.
• Academic curiosity — how a 60-year-old language survives is itself a CS case study.
• People who optimize for compensation — the per-hour rate can be very high (because the supply is tiny).

Less good candidates

• People choosing their first programming language in 2026 — Python/JavaScript/Go are better.
• Founders — you don't build a new product in COBOL.
• AI/ML engineers — Python owns that space.

A practical learning path

1. Install GnuCOBOL — compile and run locally.
2. COBOL fundamentals — IDENTIFICATION / ENVIRONMENT / DATA / PROCEDURE DIVISION. PIC clauses. PERFORM.
3. File handling — sequential, indexed, relative files.
4. DB integration — embedded SQL.
5. JCL — how mainframes actually run work.
6. CICS basics — transaction processing.
7. Mainframe environment experience — IBM Z Xplore / Master the Mainframe (names and offerings change over time — check IBM's official pages).
8. Modernization tools — Visual COBOL, evaluation/demo of watsonx Code Assistant for Z.

Time investment

• Basic COBOL — 100~200 hours.
• Real mainframe operations (JCL/CICS/Db2 z/OS) — 1~2 years of on-the-job exposure.

The language itself isn't hard. The environment is. That's the barrier and, simultaneously, the opportunity.

Epilogue — The Giant Moves Slowly, Very Slowly

The mainframe isn't dying. Not until at least the mid-2030s.

The big picture

• Hardware evolves — Telum II strengthens on-chip AI. z17 is a natural successor to z16.
• Software is preserved — 200B lines of COBOL don't disappear. They get moved.
• AI is the accelerator — watsonx, Copilot, and Amazon Q make modernization faster than human-only eras.
• Cloud is optional — AWS MMA, IBM Cloud Hyper Protect, Linux on Z, LinuxONE.
• Operations are still people — a great mainframe operator saves the company.

Next post ideas

Candidates:

• JCL in practice — from the first line to SYSPRINT
• A COBOL → Java conversion walkthrough — watsonx Code Assistant for Z demo
• Db2 z/OS vs PostgreSQL — why migration is hard

"Old code is alive somewhere. We don't kill it — we plant a new tree beside it and let it slowly take over." — In the spirit of the metaphor that gave us the Strangler Fig.

— Mainframes 2026, fin.

References

• IBM Z official
• IBM z17 announcement (IBM Newsroom)
• IBM z16 — Telum processor
• IBM i official
• IBM Db2 for z/OS
• IBM CICS Transaction Server
• IBM IMS
• IBM Enterprise COBOL for z/OS
• IBM watsonx Code Assistant for Z
• IBM LinuxONE
• IBM Cloud Hyper Protect
• Linux on Z (IBM)
• AWS Mainframe Modernization
• OpenText (acquired Micro Focus)
• Visual COBOL — OpenText
• Fujitsu BS2000
• NEC ACOS
• Unisys ClearPath
• GitHub Copilot
• Hercules emulator
• GnuCOBOL
• Martin Fowler — StranglerFigApplication
• Eric Evans — Domain-Driven Design (Anti-corruption Layer)
• COBOL standard (ISO/IEC 1989)
• IBM Z Xplore / Master the Mainframe (learning programs)