A microprocessor includes an externally accessible port and a serial communication bus connected to the port. An execution pipeline of the processor includes a pipeline satellite circuit coupling the pipeline to the bus. The satellite enables an external agent to provide an instruction directly to the pipeline via the serial bus. A dedicated register and register satellite circuit couple the register to the communication bus. The execution pipeline can access the dedicated register during execution of the instruction. In this manner, the satellite circuits enable the external agent to access architected state. The communication bus enables access to the satellites while a system clock to the processor remains active. In one embodiment, the pipeline satellite accesses the pipeline "downstream" of the decode stage such that the set of instructions that may be "rammed" into the pipeline is not limited to the set of instructions that the decode stage can generate.