Abstract: To debug software programs, an economical and efficient serial in-circuit emulator (ICE) according to the invention can pause the operation of a CPU to read/write current data from/to a register of the CPU or to modify the current data in a register of the CPU. Furthermore, this serial in-circuit emulator can also read/write current data from/to an external memory or other external devices, or modify these data. The CPU mentioned above has an instruction register and a debugging register which allows the serial in-circuit emulator to read/write required data. This serial in-circuit emulator, which comprises a serial in-circuit emulator control register, a serial in-circuit emulator address register and a serial data input/output terminal, can be easily integrated with the CPU without affecting the operating speed thereof as well as has all functions that conventional serial in-circuit emulators should provide.