IBM System/36
| Company type | Computer system |
|---|---|
| Industry | Computer hardware |
| Predecessor | IBM System/34 |
| Founded | May 16, 1983 |
| Defunct | 2000 |
| Headquarters | United States |
| Products | Midrange computer systems |
The IBM System/36 (S/36) was a midrange computer offered by IBM between 1983 and 2000, designed as a multitasking, multi-user evolution of the IBM System/34. It was mainly programmed using RPG II and, in the 5360 variant, provided optional offline storage via "magazines" containing 8-inch floppy disks. The System/36 incorporated features reminiscent of mainframe systems, such as programmable job queues and priority-based scheduling.
IBM grouped the System/32, System/34, and System/36 under the "small systems" category, which later evolved into the midrange computer class alongside the IBM System/38 and its successor, the IBM AS/400 series. Notably, the AS/400 and IBM Power Systems running IBM i are capable of running System/36 applications within the System/36 Environment after recompilation on IBM i.
Overview of the IBM System/36
Introduced on May 16, 1983, the System/36 became a widely adopted small business computing platform with a production run lasting 17 years. The initial unit released was the 5360 model.
During the 1970s, an antitrust suit against IBM affected its product development plans. Initially intending to consolidate its product lines (System/370, System/32, System/34, System/38) under a unified architecture and technology base, IBM instead divided its offerings into two distinct families: the System/38 aimed at large enterprises, and the System/36 targeting smaller companies transitioning from older systems like the System/32 and System/34. Following resolution of the lawsuit in the late 1980s, IBM combined these lines into the AS/400 platform, which superseded both.
While maintaining several technologies from the System/34—including RPG II, Screen Design Aid, and Operational Control Language—the System/36 was not object-code compatible with its predecessor. Hardware included an 8-inch floppy drive; storage options ranged from one to four hard drives between 30 MB and 716 MB; memory capacity spanned from 128 KB up to 7 MB. Backup solutions featured tape drives such as the 6157 QIC and reel-to-reel 8809 (~60 MB), as well as the Advanced/36 9402 tape drive offering up to 2.5 GB of capacity. User interaction primarily occurred through IBM 5250 terminals.
Processors
The S/36 architecture incorporated two distinct 16-bit processors: the Control Storage Processor (CSP) and the Main Storage Processor (MSP). The MSP was responsible for executing program instructions, whereas the CSP handled system management tasks and supported utilities such as $CNFIG for configuration purposes. Floating-point calculations were managed by software routines executed by the CSP.
The CSP operated at a speed more than four times faster than that of the MSP to ensure it remained fully engaged; early versions ran at 4 MHz for the CSP and 1 MHz for the MSP. The CSP preemptively loaded instructions and data ahead of MSP processing. Four processor variants were available (5360-A through 5360-D), with the "D" model offering approximately a 60% performance improvement over the "A" model.
Front panel
Models including the 5360, 5362, and 5363 featured a front panel equipped with four hexadecimal LEDs. Inputting the sequence F-F-0-0 prior to performing an Initial Program Load (IPL) bypassed numerous diagnostics routines, significantly reducing startup duration from roughly ten minutes to about one minute. However, if proper shutdown procedures—such as keysorting indexed files—were not followed, IPL times could extend up to an hour depending on file quantity.
Memory and Storage
Entry-level System/36 units originally came equipped with 128K of RAM and a 30 MB hard disk. At the higher end, configurations could be expanded to as much as 7MB of memory and nearly 1.5GB of disk capacity, with costs exceeding US$200,000 during the early 1980s. The disk drives incorporated an "extra cylinder" mechanism designed to remap defective sectors, thereby enabling more usable storage than the nominal addressable space.
Disk partitioning was constrained by addressing limits, restricting active partitions to roughly 2GB. Nevertheless, the Advanced/36 Large Package introduced support for a 4GB drive and could host up to three emulated System/36 environments simultaneously. Additionally, Advanced/36 models allowed memory expansion beyond the standard SSP cap—ranging from 32MB up to 96MB—to enhance disk caching performance.
Disk storage was segmented into blocks of 2,560 bytes each, with high-end units shipping containing approximately 550,000 such blocks. Internally, system objects utilized either block or record allocation methods based on these blocks. The System/36 also supported memory paging functionality, commonly referred to as "swapping."
Software
The System Support Program (SSP) served as the exclusive operating system for the System/36, providing capabilities such as multiprogramming, multiprocessing support, management of up to 80 devices, job and print queue handling, security features, and indexed file support. A full SSP installation occupied around 10MB of storage. The Advanced/36 expanded device connectivity to accommodate up to 160 workstations and printers, while selected OS/400 Guest/36 environments increased this limit further to 216 devices.
Application programs on the System/36 were limited to a maximum size of 64 kB, which constrained the complexity of screen-oriented software. The Advanced/36 version of SSP introduced features enabling programs to internally call other programs, effectively allowing larger composite applications. This functionality had previously been available only through third-party solutions but was integrated into SSP releases 7.1 and 7.5 for the Advanced/36 platform.
Hardware Models
| System/36 Series Model Overview | ||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Main Line | Compatible (IBM AS/400-based) | |||||||||||||||||||
| Position | Form Factor | 1984 | 1985 | 1986 | 1987 | 1988 | 1989 | 1990 | 1991 | 1992 | 1993 | 1994 | 1995 | 1996 | 1997 | 1998 | 1999 | 2000 | ||
| Entry-Level | Desktop | 5364 | ||||||||||||||||||
| 5363 | AS/Entry (P03; 10S) | AS/Entry (150) | ||||||||||||||||||
| Desk-side | 5362 | |||||||||||||||||||
| Advanced/36 (236, 436) | Advanced/36 (170) | |||||||||||||||||||
| Full-Size | 5360 | |||||||||||||||||||
System/36 Model 5360
The initial System/36 model, designated as the 5360, weighed approximately 700 pounds (318 kilograms) and carried a price tag near $140,000. It was equipped with dual processors operating at roughly 2 MHz and 8 MHz frequencies and required either 208 or 240 volts AC power input.
This system featured five diagnostic LEDs indicating power status, processor activity, program operation, console status, and temperature monitoring. Except for the console indicator, any illuminated light signified a fault condition necessitating a system restart.
The 5360 series included C and D variants that offered improved processing speeds and supported an optional additional frame housing two extra disk drives.
System/36 Model 5362
Launched in 1984 as an entry-level alternative, the 5362, also called the "Compact 36," presented a deskside tower design tailored for standard office settings with minimal specialized demands. Its card cage was smaller, restricting peripheral expansion compared to the 5360 model.
Storage options included a built-in 8-inch floppy disk drive and up to two fixed 14-inch disk drives, each offering either 30 or 60 MB capacity. To increase storage, external channel-attached desktop DASD units (models 9332-200, -400, -600) could be connected, expanding total capacity to as much as 720 MB. Memory configurations ranged from 128 KB up to 512 KB. The system weighed approximately 150 pounds (68 kilograms) and carried a price tag of $20,000.
System/36 Model 5364
Released in June 1985, the 5364—referred to as the "System/36 PC" or "Desktop 36" and colloquially known as the "Baby/36"—represented IBM's effort to condense System/36 capabilities into hardware similar in size to a personal computer. It housed IBM microchips within a chassis comparable to an IBM PC/AT. The system featured a unique 1.2 MB 5.25-inch floppy disk drive incompatible with both PCs and other System/36 models. Its control panel or system console was provided by an IBM Personal Computer equipped with at least 256 KB of RAM, connected through an expansion card.
System/36 Model 5363
Introduced in October 1987 as the successor to the 5364, the model 5363 utilized a deskside tower enclosure approximately one-third smaller than that of the 5362. This model incorporated updated components including newer hard drives with smaller platters, a standard 5¼-inch floppy disk drive, and a redesigned SSP distribution that improved both system reliability and performance.
AS/400-based backports
The System/36 Environment available within IBM i (formerly known as OS/400) offers support for SSP utilities along with RPG II and OCL programming languages. Instead of providing direct binary compatibility, it facilitates porting of System/36 applications by recompiling source code to utilize native IBM i APIs.
Between versions V3R6 and V4R4, OS/400 allowed running up to three instances of SSP inside virtual machines called Advanced 36 Machines. These machines emulated the MSP through the OS/400 SLIC layer, enabling binary compatibility with existing SSP software.
AS/Entry (9401)
The AS/Entry system, based on the AS/400 model 9401-P03 and operating SSP Release 6, was designed for System/36 users seeking a gradual migration path to AS/400 without significant upfront expenditures. Introduced around 1991, it failed to gain commercial traction due to its costly architecture and lower reliability relative to the older model 5363. This product line was subsequently upgraded using AS/400 9401-150 hardware.
Advanced/36 (9402, 9406)
In 1994, IBM launched the AS/400 Advanced/36 series with two models—the 9402-236 and the 9402-436—priced starting at $7,995. This offering allowed System/36 customers access to faster and more modern hardware while continuing to operate SSP, the original System/36 operating system. Built upon standard AS/400 platforms, the Advanced/36 could run SSP either natively or within AS/400’s OS/400 as a virtual machine, permitting users to upgrade to full AS/400 capabilities through additional licensing. The system featured a black case slightly larger than an average PC enclosure.
The Advanced/36 extended the lifecycle of System/36 and SSP products by roughly five years. However, by the late 1990s, market dynamics shifted considerably: IBM terminals and printers were supplanted by PCs or third-party displays paired with PC-compatible printers; twinaxial cabling was replaced by adapters utilizing common telephone wiring such as Cat5 cables. Ultimately, System/36 systems were phased out in favor of AS/400 solutions at higher tiers and personal computers at lower tiers.
Later iterations of the Advanced series transitioned to AS/400 model 9406-170 hardware. By the year 2000, IBM discontinued marketing of the Advanced/36 line.