Execution Unit (EU)

Execution Unit (EU)

The BIU sends out addresses, fetches instructions, read data from ports and memory and writes data to ports and memory. i.e the BIU handles all transfers of data and addresses on the buses required by the Execution Unit whereas the Execution Unit   tells the BIU where to fetch instructions or data from, decodes the instructions and executes the instructions. 

The BIU contains

the circuit for physical address calculations

a pre-decoding instruction byte queue (6 bytes long)

four 16-bit segment registers (ES, CS, SS, DS)

16-bit instruction pointer (IP) 

The EU contains

control circuitry, instruction decoder and ALU

16-bit flag registers

four 16-bit general purpose registers (AX, BX, CX, DX)

16-bit pointer registers (SP, BP) and 

16-bit index registers (SI, DI)