IBM System/360 Model 25
| Company type | Mainframe computer model |
|---|---|
| Industry | Computer hardware |
| Predecessor | IBM 1400 series (compatibility option) |
| Founded | January 3, 1968 |
| Defunct | October 7, 1977 |
| Headquarters | Armonk , United States |
| Products | Mainframe computer |
The IBM System/360 Model 25 represents an entry-level system within the IBM System/360 series. Introduced on January 3, 1968, it was released three years prior to the IBM System/360 Model 22 and functioned as a transitional solution linking IBM's legacy and contemporary computing platforms.
History
Facing pressure from more affordable competitors, including service bureaus—32 of which supported the US National Institutes of Health (NIH) by 1968—and older models like the Honeywell 200, IBM positioned the Model 25 as an interim offering. It delivered full compatibility with the System/360 architecture but at reduced speed and cost, targeting customers likely to switch due to price concerns.
Both the Model 25 and Model 22, which were promoted as budget-friendly options, were withdrawn from the market on October 7, 1977.
Models
Configurations for the Model 25 included options for 16K, 24K, 32K, or 48K of core memory.
Characteristics
The standard Model 25 supports the System/360 base and commercial instruction sets, while scientific or universal instruction sets could be added as optional enhancements.
Its circuitry is built from IBM SLT modules mounted on removable cards fixed to hinged metal frames, interconnected via flat cables.
Significantly, the Model 25 was IBM’s pioneer in utilizing rewritable control storage for its microcode, employing an extra 16 K bytes of core memory. A portion of this core memory holds registers accessible to both software and microcode. Additionally, it incorporates a rapid-access (180 ns) SLT Local Storage of 64 bytes dedicated to microcode operations.
Standard Features
- Commercial instruction set
- Connection for 1052 Printer-Keyboard Model 7
Optional Features
- Scientific or universal instruction sets
- Either a byte multiplexer or selector channel (limited to one)
- Memory protection capabilities
- Direct control with external interrupt or external interrupt only
- Timer based on line-frequency
- Compatibility modes for IBM 1401, IBM 1460, IBM 1440, and Model 20
- Built-in CPU attachments supporting:
- One 1403 Printer Models 2, 7, or N1
- Up to four 2311 Disk Storage Drives Model 1 (including file scan functionality)
- One 2540 Card Read Punch Model 1 (featuring column binary and punch feed read control)
- One 2560 Multi-function Card Machine Model A1 (for Model 20 mode; System/360 mode read/punch available with added 2540 emulation)
- CPU-integrated communications interface
- One 2314 High Speed Channel (offered via Request Price Quotation; absent from initial announcement)
Integrated Attachments
The Model 25 comes standard with an attachment for the 1052 Model 7 console printer-keyboard, which operates as the system’s operator console. It can optionally be equipped with a System/360 I/O channel, either byte-multiplexor or selector type. Frequently used peripherals such as the 2540 or 2560 card reader-punch units, the 1403 printer, and disk drives like the 2311 or 2314 connect through optional internal attachments that are recognized by the operating system as channel-attached devices.
A notable feature of the Model 25 is its integrated attachment design, reminiscent of the single disk storage drive integration found in the IBM System/360 Model 44.
Compatibility Options
To facilitate the transition from IBM 1400 series machines, the Model 25 provides an optional Compatibility Option. This includes supplementary hardware along with DOS/360 support software, enabling IBM 1401/1440/1460 object code to execute in an emulated environment with minimal code modification.
System Setup
While standard System/360 peripherals were offered, by this stage many clients preferred IBM-compatible third-party devices.
A representative Model 25 configuration typically consisted of:
- IBM 2025 central processing unit
- IBM 1052 console printer-keyboard
- IBM 2540 card reader and punch
- IBM 1403 line printer
- One or more IBM 2311 or 2314 disk storage units
- Magnetic tape drives (either IBM 2415 or IBM 2401 model 1) connected through a selector channel
Software
The Model 25 primarily ran DOS/360, which was loaded from disk storage. Other operating systems included TOS/360, which loaded from tape, and the less frequently used BOS/360, loaded from cards. Programming languages were oriented toward business applications, featuring COBOL, RPG, and Assembler; Fortran IV catered to scientific programming needs; PL/I was also offered.