EXTERNAL MICROCODE OPERATION IN A MULTI-LEVEL MICRO- PROCESSOR

摘要

The data processing system has a central processing unit (CPU) which is responsive to and executes microinstructions generated by the decoding of macroinstructions so as to provide one or more data processing operations. The system is arranged so that the microinstructions can be supplied to the CPU from microcode decoding logic in the CPU or from one or more external microcode decoding units (13). Each of the external units has a decoder (21) which identifies a macroinstruction which it is capable of decoding by a MYINS (my instruction) signal and which provides a first microinstruction to the CPU via a multiplexer (22) which is enabled by TS only in the presence of MYINS. The unit (13) further includes logic (25) for externally providing one or more successive microinstructions which result from the decoding process. If an external microcode unit and the CPU-resident decode logic are both capable of such decoding operation, the external unit overrides the CPU decoding logic by sending an acknowledge signal (ACK) to the CPU and controls the decoding operation externally. The external microcode unit includes logic for monitoring the number of microinstructions supplied to the CPU which have not yet been executed by the CPU.