4.10 The 68HC11 MPU - A Simplified Version

• Buses
• Three external buses and two internal buses
• Three different buses type: 
• address (16 bit, uni-direction)
• data (8 bit, bi-direction)
• control (groupings of all timing and control signals)
• Registers - dedicated memory location inside MPU
• Program counter  - 16-bit counter that controls the sequence in which the instructions are fetched from the memory (hold instruction addresses)
• Data Address Register (DAR) - provides operand address to memory (hold data/operand address)
• Instruction Register - holds the op code while the instruction decoder decodes it and signals the timing and control logic (TCL) to generate proper sequence of control signals (micro-program) to complete the execution of the indicated instruction
• Accumulator and Data Register - hold the operands/data that the ALU operates on during the execution phase of an instruction
• Timing and Control Logic 
• Micro-program is built into MPU's TCL and control how MPU executes a machine language instruction; it cannot be modified by user.