Central processing unit (CPU)

The central processing unit (CPU) is the “brain” of the computer and is responsible for accepting data from input devices, processing the data into information, and transferring the information to memory and output devices. The CPU is organized into the following three major sections:

1. Arithmetic logic unit (ALU) 

2. Control unit 

3. Registers 

       The function of the arithmetic logic unit (ALU) is to perform arithmetic operations such as addition, subtraction, division, and multiplication and logic operations such as AND, OR, and NOT. 

The function of the control unit is to control input/output devices, generate control signals to the other components of the computer such as read and write signals, and perform instruction execution. Information is moved from memory to the registers; the registers then pass the information to the ALU for logic and arithmetic operations.

Register:- Register is the fastest memory in a computer which holds information.

CPU Buses:- When more than one wire carries the same type of information, it is called a bus. The most common buses inside a microcomputer are the address bus, the data bus, and the control bus.

Address Bus:- The address bus defines the number of addressable locations in a memory IC by using the 2n formula, where n represents the number of address lines.

If the address bus is made up of three lines, then there are 23 ¼ 8 addressable memory locations. The size of the address bus directly determines the maximum numbers of memory locations that can be accessed by the CPU. For example, a CPU with 32 address bus can have 232 addressable memory locations.

Data Bus :- The data bus is used to carry data to and from the memory. each location can hold only four bits. The size of a memory IC is represented by 2n m where n is the number of address lines and m is the size of each location.

The size of data bus plays important factor on CPU performance, current CPU’s data bus is 32 bits or 64 bits, and a CPU with 32-bit data bus means it can read or write 32 bits of data in and from memory.

Early generation of CPU contains 8-bit data bus, and each memory location holds one byte. The CPU requires to access memory four times.

By increasing data bus from 8 bits to 32 bits, then CPU can access memory and read entire word “book.” Most CPUs offer instruction to read 1 byte, 2 bytes, or 4 bytes from memory.

Control Bus:- The control bus carries control signals from the control unit to the computer components in order to control the operation of each component. In addition, the control unit receives control signals from computer components. 

Some of the control signals are as follows:

Read signal:   The read line is set to high to read from memory location or input/output (I/O) devices.

Write signal:  The write line is used to write data into the memory.

Interrupt:  Indicates an interrupt request.

Bus request:  The device is requesting to use the computer bus.

Bus Grant:  Gives permission to the requesting device to use the computer bus.

I/O Read and Write:  I/O read and write are used to read from or write to I/O devices.

32-Bit Versus 64-Bit CPU:-  The size of register plays an important role in the performance of CPU. A 32-bit processor means it can perform operations on 32-bit data; therefore, the size of registers is 32 bits and ALU also performs 32-bit operations. A 64-bit CPU performs operation in 64-bit data; therefore, it contains 64-bit register and 64-bit ALU.

Most desktop and server computers are using AMD and Intel processors; they might use 32 bits or 64 bits. Intel and AMD processor use the same architecture; this means a program in computer with Intel processor can run on a computer with AMD processor.

CPU Technology:-  There are two types of technology used for designing CPU and they are called CISC and RIS.

CISC (Complex Instruction Set Computer) In 1978, Intel developed the 8086 microprocessor chip. The 8086 was designed to process a 16-bit data word; it had no instruction for floating point operations. At the present time, the Pentium processes 32-bit and 64-bit words, and it can process floating point instructions.

Intel designed the Pentium processor in such a way that it can execute programs written for earlier 80*86 processors.

     The characteristics of 80*86 are called complex instruction set computers (CISC), which include instructions for earlier Intel processors. Another CISC processor is VAX 11/780, which can execute programs for the PDP-11 computer. The CISC processor contains many instructions with different addressing modes, for example, the VAX 11/780 has more than 300 instructions with 16 different address modes.

The major characteristics of CISC processor are as follows:

1. A large number of instructions.

2. Many addressing modes.

3. Variable length of instructions.

4. Most instruction can manipulate operands in the memory.

5. Control unit is microprogrammed.

RISC:- Until the mid-1990s, computer manufacturers were designing complex CPUs with large sets of instructions. At that time, a number of computer manufacturers decided to design CPUs capable of executing only a very limited set of instructions.

One advantage of reduced-instruction set computer is that they can execute their instructions very fast because the instructions are simple. In addition, the RISC chip requires fewer transistors than the CISC chip. Some of the RISC processors are the PowerPC, MIPS processor, IBM RISC System/6000, ARM, and SPARC.

The major characteristics of RISC processors are as follows:

Require few instructions.

All instructions are the same length (they can be easily decoded).

Most instructions are executed in one machine clock cycle.

Control unit is hardwired.

 Few address modes.

 A large number of registers.

RISC processor uses hardware and CISC processor microprogram for control unit, and the control unit with hardware uses less space in a CPU; therefore, the designer of CPU can add more registers to RISC processor compared with CISC.

The advantage of CISC processor is that designer can add new instruction without changing the architecture of the processor. Table 6.1 shows the comparison of CISC and RISC.

CPU Architecture:- There are two types of CPU architecture and they are von Neumann and Harvard architecture.

Von Neumann Architecture:- It is a program consists of code (instructions) and data. Von Neumann uses the data bus to transfer data and instructions from the memory to the CPU.

Harvard Architecture:- Harvard architecture uses separate buses for instructions and data. The instruction address bus and instruction bus are used for reading instructions from memory. The address bus and data bus are used for writing and reading data to and from memory.