IBM System/360 Model 25

From Wikipedia, the free encyclopedia
Jump to navigation Jump to search
IBM 360 Model 25.JPG
IBM System/360 Model 25
ManufacturerInternational Business Machines Corporation (IBM)
Product familySystem/360
Release dateJanuary 4, 1968 (1968-01-04)
Introductory price$253,000 ($5,330/month)
(typical configuration), including at least twice the memory of the 360/30
DiscontinuedOctober 7, 1977
Memory16 - 48K Core
WebsiteOfficial website IBM Archives

The IBM System/360 Model 25 was announced on January 3, 1968, 3 years before the IBM System/360 Model 22,[1] as a "bridge between its old and new"[2] members of the System/360 family of computers.


At a time when lower priced[3] alternatives[4][5] were available, this model provided a stop-gap measure.[3]

Both the Model 25 and the Model 22, which had been marketed as entry level systems,[6] were withdrawn on the same day, October 7, 1977.[7]


The Model 25[8] was offered with four models configured with 16K, 24K, 32K, or 48K of core memory.[2]


The Model 25 implements the System/360 "universal instruction set" architecture. The commercial instruction set is standard, and the scientific or universal instruction set is an optional feature.[9]

The Model 25 logic is built on IBM SLT modules mounted on pluggable cards, which are plugged into printed circuit boards. The boards are mounted on hinged metal gates, and are interconnected by flat cables.

The Model 25 was the first IBM system to store its microcode in a rewritable memory, called the control storage. The control storage uses an additional 16 K (16,384) bytes of core memory. There is also a small part of the core storage that is used to store the contents of registers accessible by software, as well as data used by the microcode.

The Model 25 also has a 64-byte high-speed (180 ns) SLT Local Storage, used by the microcode.

Standard Features[edit]

  • Commercial instruction set
  • Attachment for 1052 Printer-Keyboard Model 7

Optional Features[edit]

  • Scientific or universal instruction set
  • Byte multiplexer channel or selector channel (one of either)
  • Store protection
  • Direct control (with external interrupt) or external interrupt (alone)
  • Timer (line-frequency type)
  • IBM 1401/1460 compatibility
  • 1401/1440/1460 DOS compatibility
  • 1440 compatibility
  • System/360 Model 20 mode
  • CPU-integrated attachments for:
One 1403 Prnter Model 2, 7, or N1 (with multiple character set adapter)
As many as four 2311 Disk Storage Drives Model l (with file scan feature)
One 2540 Card Read Punch Model 1 (with column binary feature and punch feed read control)
One 2560 Multi-function Card Machine Model Al (for Model 20 mode; also for System/,360 mode [read/punch only] if the 2540 emulation feature was added)
CPU-integrated communications attachment
One 2314 High Speed Channel - this feature was a RPQ, and was not part of the original announcement

Integrated Attachments[edit]

The Model 25 had a standard attachment for connection of a 1052 Model 7 console printer-keyboard, which was used as the operator console. Optionally, the Model 25 could have a standard System/360 I/O channel, that could be either a byte-multiplexor or a selector channel. For the most commonly used peripherals, such as a 2540 or 2560 card reader-punch, a 1403 printer, or 2311 or 2314 disk drives, optional internal attachments were provided. These attachments connected directly to their respective peripherals, and would appear to the operating system as if the peripherals were channel-attached.

Most unusual about the 360/25 was a feature somewhat analogous to the IBM System/360 Model 44's integrated single disk storage drive, namely an integrated attachment.[8]

Compatibility Features[edit]

The Model 25 provided an optional compatibility feature that helped users of IBM 1400 series computers migrate to System/360. With the additional Compatibility Feature hardware and Compatibility Support software under DOS/360, the IBM 1401/1440/1460 object programs could be run in the emulation mode, with little or no reprogramming.

System configuration[edit]

The standard set of peripherals[8] was available, although by this time customers had found their way to IBM-compatible alternatives.[10][11]

A typical System/360 Model 25 configuration consisted of:

  • IBM 2025 CPU
  • IBM 1052 console printer-keyboard
  • IBM 2540 card reader-punch
  • IBM 1403 printer
  • One or more IBM 2311 or IBM 2314 disk drives
  • One or more IBM 2401 or IBM 2415 tape drives, attached to the selector channel (the 2401 required a 2803 control unit)


The typical operating system for the Model 25 was DOS/360, which was loaded from disk. Less common was TOS/360, which was loaded from tape, and, rarely, BOS/360, which was loaded from punched cards. Programming was mostly in the COBOL, RPG and Assembler languages for the commercial applications that were the predominant uses of this computer, but Fortran could also be used for the scientific and engineering applications.

See also[edit]



  1. ^ "IBM Archives: System/360 Model 22". IBM.
  2. ^ a b "IBM Archives: System/360 Model 25". IBM.
  3. ^ a b Molly Upton (September 5, 1977). "IBM Feared Competition to 360/30 CPU - IBM Planned to Counter 360/30 Competition". Computerworld. p. 40. If substantial inroads were made on the 30 base, a model 25 which had full compatibility, not just subset compatibility, with lower performance and lower price "would retain for IBM many installations exposed by price alone rather than by price/performance."
  4. ^ Service Bureaus: By 1968, there were 32 such service bureaus serving the US National Institutes of Health (NIH) alone - "Information Technology Corporate Histories Collection". Computer History Museum.
  5. ^ Prior generation systems: Honeywell 200, a competitor to IBM's own IBM 1401.
  6. ^ Pugh, Emerson W.; Johnson, Lyle R.; Palmer, John H. (1991). IBM's 360 and Early 370 Systems. MIT Press. ISBN 0-262-16123-0.
  7. ^ "System/360 dates and characteristics".
  8. ^ a b c "IBM System/360 Model 25 Functional Characteristics" (PDF). IBM. January 1968. A24-3S10-0.
  9. ^ Field Engineering Theory of Operation, 2025 Processing Unit (PDF) (First ed.). IBM. October 1968. Y24-3527-0.
  10. ^ "STC Holds Lion's Share". Computerworld. August 18, 1980. p. 7.
  11. ^ Marcia Blumenthal (July 2, 1979). "The Maverick Mainframer - Philosophical Underpinnings Distinguish CDC From Its Mainframe Competitors". Computerworld. p. 5.

External links[edit]