There is provided a microprocessor system that can execute a specific set
of instructions at a high speed while limiting the increase in size of
the circuitry. The microprocessor system, which executes instructions
described in a program, comprises a main processor which executes a first
set of instructions by means of hardware and executes a second set of
instructions by means of software and a co-processor which operates under
the control of the main processor to execute the second set of
instructions by means of hardware. When the co-processor encounters a
specific instructions of the second set for which data under the control
of the main processor needs to be operated, the co-processor issues a
notification of this fact to the main processor to request it to execute
the specific instruction. In this case, the co-processor updates its
stack pointer and program counter by itself by means of hardware.