The IBM System/36 and the IBM AS/400 (Application System/400) are two of the most successful midrange computers in IBM’s history, but they represent very different eras and philosophies. The System/36 was the immediate predecessor in many markets, while the AS/400 was designed as its long-term successor. Below is a detailed side-by-side comparison of their histories, architectures, target markets, and lasting impact.
| Aspect | IBM System/36 (S/36) | IBM AS/400 (Application System/400) |
|---|---|---|
| Announcement | May 1983 | June 21, 1988 (available August 1988) |
| General Availability | July 1983 | August 1988 |
| Replacement For | System/32 and older 1130/360 models | System/36 and System/38 (dual replacement) |
| Development Project | “Cheetah” project (started late 1970s) | “Silverlake” project (started December 1985) after the failed “Fort Knox” project |
| Design Goal | Low-cost, easy-to-use system for small businesses | Unified, future-proof midrange platform with extreme backward compatibility |
| End of Production | 1994 (last models) | Original hardware discontinued ~2013; platform lives on as IBM i |
| Peak Installations | ~300,000 units by 1990 | ~500,000 units shipped by 1997; millions of applications still running today |
The System/36 was introduced to fill the gap left by the aging System/32 and to compete with smaller systems from competitors like DEC, HP, and Wang. It was a great success, but by the mid-1980s IBM realized it needed a single, long-lived platform to replace both the S/36 and the more advanced System/38. That led to the creation of the AS/400.
| Feature | System/36 (S/36) | AS/400 (and later IBM i) |
|---|---|---|
| Processor Architecture | 16-bit proprietary (SCP processor) | 48-bit CISC (IMPI) → 64-bit PowerPC/POWER family (1995 onward) |
| Operating System | SSP (System Support Program) – simple, menu-driven | OS/400 (later i5/OS, now IBM i) – object-based, integrated |
| Memory Model | Traditional segmented memory (64 KB segments) | Single-level store (64-bit virtual address space) |
| Programming Model | Procedural, RPG II/III, COBOL, BASIC, assembly | Object-oriented, RPG IV/ILE, COBOL, C, C++, Java, PASE, etc. |
| Database | Flat files, indexed sequential (S/36-style) | Integrated relational database (Db2 for i) |
| Backward Compatibility | Good within S/36 family, but limited to System/32 | Exceptional: 1988 applications still run on modern IBM i (2025) |
| Hardware Abstraction | Minimal; applications tied to hardware | Technology Independent Machine Interface (TIMI) – full abstraction |
| Security Model | Basic user profiles and passwords | Object-based authority, five security levels (up to C2) |
| Virtualization | None | LPAR (logical partitioning) since 1999, later KVM and containers |
| Networking | SNA, Token-Ring, later TCP/IP add-ons | Native TCP/IP, SNA, later Ethernet, cloud integration |
The most revolutionary aspect of the AS/400 was its Technology Independent Machine Interface (TIMI). This allowed applications compiled for the original 48-bit IMPI processors to run unchanged on 64-bit POWER processors decades later. The System/36 had no such abstraction, so applications had to be recompiled or rewritten when moving to new hardware.
| Aspect | System/36 | AS/400 (and IBM i) |
|---|---|---|
| Primary Market | Small businesses, departments, first-time computer users | Small-to-medium businesses, mid-size enterprises, large companies |
| Typical Installation | 1–30 users, often in offices with minimal IT staff | 1–hundreds of users, mission-critical applications |
| Ease of Use | Very easy; menu-driven, “turnkey” solutions | Still user-friendly but far more powerful and scalable |
| Price Range (1980s) | $10,000–$80,000 | $20,000–$500,000+ (wide range of models) |
| Typical Software | MAPICS, BPCS, custom RPG applications | MAPICS, JD Edwards, BPCS, SAP R/3, custom RPG/COBOL |
The System/36 was the entry-level system that introduced thousands of companies to IBM midrange computing. Many of those customers later migrated to the AS/400 because it could run System/36 applications with minimal or no changes (via the System/36 Environment and later the 36/38 compatibility layer).
IBM made the transition remarkably smooth:
As a result, many companies that started on a System/36 in the 1980s still run the same applications today on modern IBM Power Systems running IBM i 7.6 (2025).
| Aspect | System/36 | AS/400 / IBM i |
|---|---|---|
| Last Hardware Shipment | 1994 | Original AS/400 hardware discontinued ~2013 |
| Current Platform | None (emulation only) | IBM i on IBM Power Systems (Power10, Power11 in preview) |
| Active Installations | Very few (mostly emulated) | Tens of thousands of production systems worldwide |
| Support Status | Ended decades ago | IBM i 7.5 support until ~2030; 7.6 released 2025 |
| Modern Use Cases | Mostly legacy or hobbyist emulation | Mission-critical ERP, banking, manufacturing, cloud |
While the System/36 is now largely a historical footnote (though still fondly remembered by many midrange veterans), the AS/400’s architecture lives on as IBM i. The same core principles—object-based design, single-level store, and TIMI—continue to deliver unmatched reliability and backward compatibility.
In essence, the System/36 was the stepping stone; the AS/400 was the bridge that carried those businesses into the modern era—and it’s still carrying them today.