Priority handling hardware and software for the HP 2100A computer



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Although it is general purpose and highly efficient data processor, the HP 2100A Computer lacks efficiency for fast computational process, if it is not equipped with the high speed input/output peripherals. It is designed for input/output flexibility with plug-in interfaces as the main feature. In order to have an efficient computational processing system, a high speed paper tape reader and a high speed paper tape punch were interfaced with the HP 2100A Computer. The high speed reader, which operates on photo sense principle, reads at a rate of 300 characters-per-second. The high speed punch punches data at a rate of 60 characters-per-seconds. A selecting feature was added to facilitate the full use of a high speed reader and a high speed punch, as common devices between the HP 2100A Computer and the SDS-92 Computer. In a process control environment there would be several tasks to be performed, parameters to be input and messages to be printed to the operator. It would be desirable to have the priority for these tasks which could be controlled by the operator. This function was achieved by writing a priority handler. A priority handler was written partly in FORTRAN and partly in assembly language. An optional feature was created for interrupting a task service subroutine by one of the higher priority service subroutines. A queue table was generated to keep track of all task subroutines asking for service. Also, a capacity for nesting all interrupts from different subroutines was developed. The indication for the errors were generated by the error messages. A priority table was developed for the priority of the different task service subroutines, which could be controlled by the operator by re-configuring it. The priority handler was written to accept the request for servicing task subroutines by setting bits of a word on a particular interface card. The programming problems, due to the existing HP software were also resolved.