An enhanced realtime multitasking BASIC interpreter

This paper describes the implementation of an enhanced BASIC interpreter which provides the necessary features to support program execution in a realtime multitasking environment. The interpreter supports many of the features found in commercial realtime executives (RTXs) including process scheduling, message-based interprocess communication, interprocess synchronization and control, dynamic process creation/destruction, memory management, external event (interrupt) synchronization, and basic timing services. The kernel employs a preemptive scheduling policy based on process priority and allotted time-slice. To retain compatibility, the interpreter operates in a default single process mode if the multitasking extensions are not utilized. The interpreter is completely reentrant and may perform a context switch at any point during the execution of the current process. This allows a fast response to high priority processes made ready to execute by the occurrence of an external interrupt. The interpreter is mainly written in C and is portable to different microprocessor families. It is currently implemented for the i8088/86/186/286 family and has been ported to the IBM PC compatible systems.