IBM System/360 Model 44

The IBM System/360 Model 44 is a specialized member of the IBM System/360 family, with a variant of the System/360 computer architecture, designed for scientific computing, real-time computing, process control and numerical control (NC). The Model 44 was announced August 16, 1965 and withdrawn September 24, 1973. The base Model 44 lacks the storage-to-storage character and decimal instruction sets of a standard System/360, however an "extended instruction set" feature was available to provide the missing instructions. The machine features four unique instructions: Change Priority Mask (CHPM), Load PSW Special (LPSX), Read Direct Word (RDDW), and Write Direct Word (WRDW). The system comes with four memory sizes: E (32 KiB), F (64 KiB), G (128 KiB), and H (256 KiB), with an access time of 1 μs, which puts it closer to the Model 65 (.75 μs) than the Model 50 (2.0 μs). Storage protection is an optional feature. General purpose registers are normally located in a non-addressable portion of 1 μs core storage termed "bump storage". For added speed, the general purpose registers can be implemented in Solid Logic Technology (SLT) circuitry with an access time of .25 μs. A unique feature of the Model 44 is "variable-length precision floating point arithmetic". It has the same short floating-point instructions and long floating-point instructions as the other models in the System/360 line, but it also has a rotary switch on the front panel which can be used to set the precision of long floating-point numbers. The mantissa portion of long floating-point numbers can be chosen as 32, 40, 48, or 56 bits, with 56 bits being the standard value. Whatever the setting, long floating-point numbers still occupy 64 bits in memory (the first eight bits are the sign and the exponent); the setting only leads, when it was less than 56 bits, to long floating-point operations ignoring some of the least significant bits of these numbers. This provides an improvement in speed when greater precision is not needed. An optional feature provides six external interrupt lines.