Functions of Execution Unit and Bus Interface Unit in 8086 Krish Advani 18/04/2013 Engineering The 8086 Processor is divided into two parts: Execution Unit and the Bus interface unit. Both of these units perform a different operation which are essential to fetch, decode and execute and instruction in 8086. Functions of Bus Interface Unit (BIU) Major function of BIU is to fetch instruction data from memory and write data (result) back to memory. It also reads and writes data from Input/output ports. To perform memory operations it generates 20-bit physical address. Functions of Execution Unit(EU) Control system inside the execution unit controls all internal and external operations by sending control signals. This results in fetching Instructions and Data from memory. It decodes and executes instructions fetched by Bus-Interface Unit.