456 Part 2 ½ Regions of Computer Space Section 6 ½ Fault-Tolerant Systems
configuration description followed by queries on the various reliability parameters of interest and their behavior with respect to mission time, fault coverage, failure rates, dormancy factors, allocated spares, and partitioning. The CARE program is extensible, and it may be updated to incorporate new reliability models as they become available.
STAR Computer Software System
Early in the design of the STAR computer it became evident that the fault-tolerant architecture would impose unconventional constraints on its software. The development of the software system for the STAR computer was initiated in 1968 and closely followed the hardware development. It is partitioned into two subsystems. The programming subsystem consists of three modules: an assembler, a loader, and a functional simulator. An executive program facilitates coordinated use of these modules. The operating subsystem consists of two modules: the resident executive module and the applications programs module. The programming subsystem has been implemented on the Univac 1108 computer of the Scientific Computing Facility at JPL. The first version of a resident executive for the STAR computer is nearing completion.
SCAP (the star computer assembly program) is the first module of STAR software. Programs for the STAR computer are written in the assembly language SCAL. SCAP is a traditional two-pass assembler incorporating machine instructions, pseudo-operations, and macrofacilities. A unique feature of SCAP is the encoding of instruction and data words as required by the STAR computer. SCAP calculates the code required and generates the encoded value of the word. Another feature of SCAP is the COMPILE pseudo-operation which implements automatic compilation of simple arithmetic statements by the assembler.
The second module LOAD (the loader) reads the program into the simulated STAR computer memory. After all decks have been read, a COMMON area is allocated, relocation is completed, and external linkage is accomplished. A map and cross-reference table are printed to aid in debugging and documenting the program. The third module of STAR software is the functional simulator, which is modular in nature and follows the latest STAR hardware configuration. Two special features are incorporated in the simulator. The first is the facility to simulate hardware errors in order to test the software aspects of error recovery. The second feature provides STAR register and memory dumps. An executive program facilitates the coordinated use of the assembler, loader, and simulator.
The modules of the operating subsystem of the STAR computer software system consist of the resident executive module and the applications programs module. The STAR resident executive augments the self-testing and repairing features of the hardware in addition to its normal functions. The standard features include interrupt control, input/output processing, and job scheduling. Novel features incorporated due to the fault-tolerance architecture of the STAR computer include a "cold start" capability, reconfiguration processing, rollback assistance, and diagnosis of faulty units. The cold start capability resets the hardware and software after a disaster restart as well as prior to an initial load.