Execution unit and Bus Interface Unit

Processor is partitioned into two logical units

1. Execution unit (EU) - to execute instructions

2. Bus Interface Unit (BIU) - to deliver instructions and data to EU

Execution unit:

• Maintains CPU status and control flags
• manipulates general registers and instruction operands. (Registers and data paths are 16 bits wider)
• has no connection to "outside world". 
• obtains instructions from Instruction Q maintained by BIU.
• when an instruction requires access to memory or to a peripheral device, EU requests the BIU to obtain or store the data

Bus Interface unit:

• performs all bus operations for EU
• data transferred between CPU and momory/IO devices upon demand from EU
• during periods, when EU is busy executing instructions, the BIU "looks ahead" and fetches more instructions from memory.
• these instructions are stores in an internal RAM array called "Instruction Stream Q" - from which EU takes instructions to execute