Draw the extended architecture of ISA computer and mention the function of the registers.

Figure: Expanded structure of ISA computer 

(i) Memory buffer register (MBR): Contains a word to be stored in memory or sent to the I/O unit, it is used to receive a word from memory or from the I/O unit.

(ii) Memory address Register (MAR): Specifies the address of memory in the word to be written from or read into the MBR.

(iii) Instruction register (IR): Contains the 8bit Op-code instruction being executed.

(iv) Instruction buffer register (IBR): Employed to hold temporarily the right-hand instruction from a word in memory.

(v) Program counter (PC): Contains the address of the next instruction-pair to be fetched from memory.

(vi) Accumulator(AC) and multiplier quotient(MQ): Employed to hold  temporarily operands and results of ALU operations.

For example, the result of multiplying two 40-bits numbers is an 80bit number, the most significant 40 bits are stored in the AC and the least significant in the MQ.

(vii) Central processing unit(CPU): Controls the operation of the computer and performs its data processing functions, often simply referred to as processor.

(viii) Input / Output: Moves data between the computer and its external environment.