IBM System/360 Model 50

There are four models of the 360/50.[3]^{:page 5} They vary by the amount of core memory with which the system is offered. The F50, or 2050F is equipped with 65,536 bytes, the G50 has 131,072 bytes, the H50 262,144 bytes, and the I50 524,288 bytes.[3] The system can also attach IBM 2361 Large Capacity Storage (LCS) modules which provide up to 8,388,608 bytes of additional storage, however with a considerably slower memory cycle time of 8 microseconds compared to the 2 microseconds of processor storage.[3]

The Model 50 implements the complete System/360 "universal instruction set" architecture, including floating-point, decimal, and character operations as standard features. The "direct control" instructions are an optional feature. Optional logic, microcode and software providing compatibility with either the IBM 1410/7010 or 7070/7074 systems is available.

An IBM 1052 printer/keyboard for use as an operator's console is optional. The I/O options include one channel-to-channel adapter (CTCA) and up to three selector channels. A multiplexer channel for attachment of slow-speed devices is standard on all models. The F50 has 64 subchannels, so it can attach up to 64 slow-speed devices on its multiplexer channel. The other models have 128 subchannels. This can optionally increase to 256 subchannels on the H50 and I50.[3]

The Model 50 uses a 90 bit (or 85 bit, depending on definition) "horizontal microcode" instruction format, with each word containing 15 (or 25) separate fields.[4] There are 2816 words of microcode storage.[5]

Read-only control storage for microcode employs "balanced capacitor technology" (BCROS) with a cycle time of 500 nanoseconds, designed by Anthony Proudman in IBM's Hursley laboratory and implemented by Fernando "Fred" Neves. This technology uses two capacitors to represent each bit.

B&W closeup of 360/50 console

It was possible to choose 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) as the Operating System of an IBM System/360 Model 50. Few chose MVT.[6]

The choice of operating system for the System/360 Model 50 was based primarily on the amount of main storage. The F50, with 65,536 bytes of main storage, can not run OS/MFT, which requires a minimum of 131,072 bytes of main storage.[7] DOS/360 has a minimum of 16,384 bytes of main storage.[8]

Systems with 131,072 or more bytes of main storage could run OS/360. Although 360/50 systems equipped with 1 MB or more[9] could and did run MVT[10][11] one IBMer described this as "[getting] blood out of the turnip", and noted that "most didn't run MVT".[6]

Reasons for a 360/50 site to run MFT[12] rather than MVT were:

• MVT's minimum memory requirements of 256KB[13][14] - the F50 and G50 models had less;
• CPU power: the next larger System/360, the Model 65, had triple the power.[15][16]