AN/GYK-12

The AN/GYK-12 is an obsolete 32-bit minicomputer developed by Litton Industries for the United States Army. The AN/GYK-12 is a militarized version of the L-3050 computer ruggedized for use in the TACFIRE tactical fire direction system. The design dates from the 1960s.
In 1980 the Army introduced the "Nebula" instruction set architecture (MIL-STD-1862), intended as an upgrade to the AN/GYK-12. Nebula is also a 32-bit architecture with 32-bit addressing mode and instructions optimized for running programs written in Ada.
Description.
The basic system consists of three rack-mounted modules: The CPU, the IOU (Input/Output unit), and the MCMU. Modules are mounted vertically and are wide by high.
The AN/GYK-12 has a 32-bit instruction format and operates on data from one bit to a doubleword (64 bits) in size. Only fixed-point binary arithmetic is provided.
The system can have a maximum of 32768 pages—64 million words—of 2 μs memory (256 MB). Physically memory is divided into banks of 8 KW to minimize memory contention. Logically it is divided into pages of 2 KW each. Access to pages is controlled by 16 "Page Control and Address Registers" per "program level", allowing an individual program to directly address 128 KB at one time.
The AN-GYK-12 CPU features 64 hardware priority "program levels", numbered from 0 (highest) to 63 (lowest). One task can run at each level. Level 0 is reserved for "power off". Level 1 is power-on restart. Level 2 handles hardware and program errors. Level 63 is entered after completion of a boot load. Therefore the system can support a maximum of 60 user tasks. Tasks are scheduled preemptively—a task will run until an error occurs, until it gives up control to another task, or until successful completion of an input/output operation transfers control to the so-called "normal program level".
Each task also has a "privilege level", which controls the operations it is allowed to perform. Level '00'b is used for non-privileged programs. Level '01'b programs are "semi-privileged". Level '10'b programs are privileged and can execute all instructions and I/O operations. Level '11'b is unused.
The system uses Sylvania Universal High Level II Integrated Circuits (SUHL II), manufactured by Collins Radio, Honeywell, Raytheon and Motorola Semi-Conductor Division.