IBM System/360 Model 50
| Company type | Mainframe computer model |
|---|---|
| Industry | Computer hardware |
| Founded | April 1964 |
| Headquarters | Armonk , United States |
| Products | Mainframe computer systems |
The IBM System/360 Model 50 belongs to the IBM System/360 series. It was introduced in April 1964 alongside other initial variants and began delivery in August 1965, with the Bank of America as its first customer.
Models
The 360/50 is available in four variants, each distinguished by the amount of core memory: F50 (65,536 bytes), G50 (131,072 bytes), H50 (262,144 bytes), and I50 (524,288 bytes). Additionally, it can connect to IBM 2361 Large Capacity Storage units, which provide up to 8,388,608 bytes of slower storage operating with an 8-microsecond cycle time, compared to the processor’s 2-microsecond memory cycle.
Relative performance
Operating with a CPU cycle time of 500 nanoseconds, the Model 50 delivers performance approximately 25% higher than the IBM System/360 Model 40, yet operates at around 40% of the speed of the IBM System/360 Model 65, which features a 200-nanosecond cycle time. Its magnetic core memory transfers data in four-byte blocks every 2 microseconds and incorporates both protected and local core storage for registers and buffers, with respective cycle times of 200 and 500 nanoseconds.
Features
The Model 50 supports the complete System/360 "universal instruction set" architecture, including built-in floating-point, decimal, and character operations. The "direct control" instruction set is available as an optional feature. Compatibility options include hardware logic, microcode, and software enabling operation of IBM 1410/7010 or IBM 7070/7074 systems.
Operator consoles can be equipped with an IBM 1052 printer/keyboard unit. Input/output setups include one channel-to-channel adapter (CTCA) and a maximum of three selector channels. All models come standard with a multiplexer channel dedicated to slower-speed peripherals. The F50 model supports up to 64 subchannels on its multiplexer channel; other versions accommodate 128 subchannels by default, with optional expansion up to 256 on the H50 and I50 models.
Microcode
Microinstructions in the Model 50 utilize a "horizontal microcode" format consisting of either 90-bit or 85-bit words, each subdivided into 15 or 25 fields respectively. The system contains a total of 2816 microcode words.
The read-only control store for microcode is based on "balanced capacitor technology" (BCROS), featuring a cycle time of 500 nanoseconds. This design was developed at IBM’s Hursley facility by Anthony Proudman and implemented by Fernando "Fred" Neves. BCROS represents each bit through a pair of capacitors.
System software
The IBM System/360 Model 50 was capable of running DOS/360, OS/360 MFT (Multi-programming with a Fixed number of Tasks), or OS/360 MVT (Multi-programming with a Variable number of Tasks), though MVT was rarely utilized.
Operating system selection largely depended on available main memory. The F50 model’s 65,536 bytes were insufficient for OS/MFT, which required at least 131,072 bytes; meanwhile, DOS/360 had a minimum requirement of just 16,384 bytes.
Systems equipped with at least 131,072 bytes could operate OS/360. While some configurations with one megabyte or more ran MVT, this practice was uncommon. An IBM engineer described running MVT on the Model 50 as "[getting] blood out of the turnip," highlighting that most users opted not to use MVT.
Factors favoring MFT over MVT for the Model 50 included:
- MVT’s minimum memory requirement of 256KB—beyond what the F50 and G50 could provide;
- CPU performance constraints—the next tier machine, System/360 Model 65, offered roughly three times the processing capability.
Market trends from the early 1970s indicate that MVT installations were predominantly on Model 65 systems rather than on Model 50 machines.
Time-sharing (CALL/OS)
IBM advanced its time-sharing technology through the introduction of CALL/360, later renamed CALL/OS, which featured customized versions of BASIC, FORTRAN IV, and PL/I designed specifically for time-sharing systems. In contrast to the typical MFT/MVT compilers, CALL/OS provided dedicated language variants that improved interactive computing on System/360 hardware.
Certain sites operating larger System/360 machines opted to run a combination of MFT and CALL/OS rather than upgrading to MVT, a requirement for the TSO. This strategy enabled these installations to maintain time-sharing capabilities without performing a comprehensive system upgrade.