Fig: Expanded structure of IAS computer Memory buffer register
(MBR):-Contains a word to be stored in memory or sent to the I/O unit, or is used to receive a word from memory or from the I/O unit.
Memory address register (MAR) :-Specifies the address of memory in the word to be written from or read into the MBR.
Instruction register (IR) :-Contains the 8 bits Opcode instruction being executed.
Instruction buffer register (IBR):-Employed to hold temporarily the right- hand instruction from a word in memory.
Program counter (PC):-Contains the address of the next instruction-pair to be fetched from memory.
Accumulator (AC) and multiplier quotient (MQ):-Employed to hold temporarily operands and results of ALU operations. For example, the result of multiplying two 40-bit numbers is an 80-bit number; the most significant 40 bits are stored in the AC and the least significant in the MQ.
Central processing unit (CPU):-Controls the operation of the computer and performs its data processing functions; often simply referred to as processor.
I/O:-Moves data between the computer and its external environment.
The IAs operates by respectively performing as instruction cycle. Each instruction cycle consists of two sub-cycles.
Fetch Cycle:-The opcode of next instruction is loaded into the IR and the address portion is loaded into the MAR. This instruction may be taken from the IBR, or it can be obtained from memory by loading a word into the MBR, and then down to the IBR, IR and MAR.
Execute CycleThe control circuitry interprets the Opcode & executes the instruction by sending out the appropriate control signals to cause data to be moved or an operation to be performed by the ALU.
Fig. IAS Format
Data transfer:-Move data between memory and ALU register or between two ALU registers.
Unconditional branch:-The control unit executes instruction in sequence from memory. There sequence can be changed by a branch instruction which facilitates repetitive operations.
Conditional branch:-Branch can be made dependent on a condition thus allowing decision part.
Arithmetic:-It comprises operation performed by ALU
Address modify:-It permits address to be computed in ALU and then inserted into instruction stored in memory.
Computer organization- Deals with all physical components of computer systems that interacts with each other to perform various functionalities.
Computer architecture- Refers as a set of attributes of a system as seen by programmer