360 Model 25

The IBM System/360 Model 25 was a member of the System/360 family of computers. The Model 25 was announced on January 1968 as an entry-level IBM mainframe for small and middle size businesses. The Model 25 machine type was 2025.

Characteristics

The Model 25 implemented the System/360 "universal instruction set" architecture. The commercial instruction set was standard, and the scientific or universal instruction set was an optional feature.

Standard Features

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

Optional Features

• 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

2314 High Speed Channel

CPU

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

Storage

The system could have be ordered in one of four different models, corresponding to the sizes of core memory for use as program storage: D - 16K (16,384 bytes), DC - 24K (24,576 bytes), E - 32K (32,768 bytes), and ED - 48K (49,152 bytes). The Model 25 core memory was 18-bit wide, meaning that two bytes could be read or written at a time. The Model 25 was the first IBM system to store its microcode in a reloadable memory, called the control storage. The control storage used an additional 16K (16,384) bytes of core memory. There was also a small part of the core storage that was used to store the contents of registers accessible by software, as well as data used by the microcode. Finally, the Model 25 had a 64-byte high-speed (180ns) SLT Local Storage, used by the microcode.

Microcode

The Model 25 was a microcoded system. Differently from previous System/360 models, the Model 25 microcode was not stored in a read-only memory, but in an area of core memory not accessible by programs, called control storage. Once loaded in control storage, the microcode would remain loaded even when the system was powered-off. On some occasions, such as power outages, the microcode could be corrupted and would have to be reloaded. This operation was called an Initial Microprogram Load (IMPL). The microcode could be loaded from punched cards, from a 2311 disk attached through the integrated attachment, or from a channel-attached device, such as magnetic tape. Different sets of microcode were used for the emulation features. To switch from operating in System/360 mode to 1401 compatibility mode, for instance, the user would have to load a different microcode set. The Model 25 microcode instructions (words) were 16-bit wide.

Integrated Attachments

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.

Compatibility Features

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.