1. A task control circuit connected via a signal line to a processor which executes a task, comprising: a state register that stores state data representing state of a task; and a task switching circuit that controls state of a task, wherein the task switching circuit, when a system call signal is received from the processor, reads out a parameter from an internal register of the processor by itself, performs a system call process based on the parameter, updates the state data, and writes back a return value resulting from the system call process in the internal register by itself.