MIL-STD-1750A
MIL-STD-1750A or 1750A is the formal definition of a 16-bit computer instruction set architecture (ISA), including both required and optional components, as described by the military standard document MIL-STD-1750A (1980). Since August 1996, it has been inactive for new designs. In addition to the core ISA, the definition defines optional instructions, such as a FPU and MMU. Importantly, the standard does not define the implementation details of a 1750A processor. Internals. The 1750A supports 2 16-bit words of memory for the core standard. The standard defines an optional memory management unit that allows 2  16-bit words of memory using 512 page mapping registers (in the I/O space), defining separate instruction and data spaces, and keyed memory access control. Most instructions are 16 bits, although some have a 16-bit extension. The standard computer has 16 general purpose 16-bit registers (0 through 15). Registers 1 through 15 can be used as index registers. Registers 12 through 15 can be used as base registers. Any of the 16 registers can be used as a stack pointer for the SJS and URS instructions ("stack jump subroutine" and "unstack return subroutine"), but only register 15  is used as the stack pointer for the PSHM and POPM instructions ("push multiple" and "pop multiple"). The computer has instructions for 16, and 32-bit binary arithmetic, as well as 32 and 48 bit floating point. I/O is generally via the I/O instructions (XIO and VIO), which have a separate 2 16-bit word address space and may have a specialized bus. Implementations. Because MIL-STD-1750A does not define implementation details, 1750A products are available from a wide variety of companies in the form of component, board, and system-level offerings implemented in myriad technologies, often the  most advanced and exotic of their respective periods (e.g. GaAs, ECL, SoS). 1750A systems often offer high levels of protection from radiation and other hazardous environments, making them particularly suited for military, aviation and space applications. Examples of MIL-STD-1750A implementations include: Programming. Processors based on MIL-STD-1750A are often programmed in JOVIAL, a high-level programming language defined by the United States Department of Defense which is derived from ALGOL 58. Later, the Ada was heavily used. There are also C compilers, for example Cleanscape XTC-1750A. Older versions of GNU GCC contain support for MIL-STD-1750A; it was declared  obsolete in version 3.1, and removed in subsequent versions. In addition, DDC-I provides its SCORE Integrated Development Environment (IDE) with both Ada95 and C compilers, and TADS (Tartan Ada Development System) Ada83 development environment, both targeting processors based on MIL-STD-1750A. Deployments. The U.S. Air Force defined the standard in order to have a common computing architecture and thereby reduce the costs of software and computer systems for all military computing needs. This includes embedded tasks such as aircraft and missile control systems as well as more mundane general military computing needs. The advantages of this concept are recognized  outside of the USAF, and the 1750A has been adopted by other organizations such as the European Space Agency, NASA, Israeli Aircraft Industries, and many projects in academia. Examples of military aircraft using the 1750A include: Use in space. Fully space-rated implementations make the 1750A one of the few types of computers that are applicable for use in deep space applications. Example spacecraft that use the 1750A are: 1750B. The MIL-STD-1750B was to be a successor architecture to the MIL-STD-1750A, with added and expanded features, some of which were optional. By the mid-1980s there was a published  draft of the 1750B available, and some vendors began implementations for it. However, no finalized specification was ever issued for the 1750B, as military and industry attention shifted to 32-bit architectural alternatives such as the MIPS R3000.