Von Neumann Architecture

Von Neumann Architecture

• Fig. 6.2.1 shows the general structure of a Von Neumann machine (IAS computer).

• It consists of five basic units whose functions can be summarized as follows:

• The input unit transmits data and instructions from the outside world to machine. It is operated by control unit.

• The memory unit stores both, data and instructions.

• The arithmetic-logic unit (ALU) performs arithmetic and logical operations.

• The control unit fetches and interprets the instructions in memory and causes them to be executed.

• The output unit transmits final results and messages to the outside world.

• In the original IAS machine (Von Neumann machine), memory unit consists of 4096 storage locations of 40 bits each, referred to as words. These memory locations are used to store data as well as instructions.

Features of Von Neumann Model

• It uses stored program concept. The program (instructions) and data are stored in a single read-write memory.

• The contents of read-write memory are addressable by location, without regard to the type of data contained there.

• Execution of instructions occurs in a sequential manner (unless explicitly modified) from one instruction to the next.

Von Neumann Bottleneck: Because of the stored program architecture of Von-Neumann machine, the processor performance is tightly bound to the memory performance. That is, since we need to access memory at least once per cycle to read an instruction, the processor can only operate as fast as the memory. This is sometimes known as the Von Neumann bottleneck or memory wall.

Detail Structure of IAS / Von Neumann Machine

• Fig. 6.2.2 shows detail structure of IAS computer.

• It consists of various processing and control units, along with a set of high speed registers (AC, MQ, DR, IBR, PC, IR and AR). These registers are used to store instructions, memory addresses and data.

• The complete instruction cycle involves three operations: Instruction fetching, opcode decoding and instruction execution.

• The control circuits in the program control unit are responsible for fetching instructions, decoding opcodes, routing information correctly through the system and providing proper control signals for Central Processing Unit (CPU) actions.

• After decoding, the arithmetic logic circuits of the data processing unit perform actions specified by the instruction.

• An electronic clock circuit (not shown in the Fig. 6.2.2) is used to generate the basic timing signals to synchronize the operation of the different parts of the system.

• The functioning of different registers is as given below:

• PC (Program Counter): It is an address register. It is used to store the address of the next instruction to be executed and hence also referred to as instruction address register.

• AR (Address Register): It is a 12-bit address register. It is used to specify the address in memory of the word to be written into or read from the DR.

• DR (Data Register): It is a 40-bit register. It is used to store any 40-bit word. A word transfer can take place between the 40-bit data register DR of the CPU and any memory location. The DR may be used to store an operand during the execution of an instruction.

• AC (Accumulator) and MQ (Multiplier-Quotient): These are two 40-bit registers used for the temporary storage of operands and results.

• IR (Instruction Register) and IBR (Instruction Buffer Register): Program control unit fetches two instructions simultaneously from memory. The opcode of the first instruction is placed in the instruction register (IR) and the instruction that is not to be executed immediately (second instruction) is placed in the Instruction Buffer Register (IBR).

Instructions Supported by IAS Computer

The instructions of IAS computer are divided in five groups :

• Data transfer

• Unconditional branch

• Conditional branch

• Arithmetic

• Address modify

Table 6.2.1 shows the instruction set of IAS computer.

Review Questions

1. Draw and explain the Von Neumann architecture.

2. Draw Von Neumann architecture and explain function of registers in it.

3. Explain IAS (Von Neumann) architecture with the help of neat diagram and list the instructions supported by IAS computer.