Introduction When we tlk of computer hrdwre the three relted terms tht require introduction re computer rchitecture computer orgniztion nd computer design
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2. The Computer System Hardware
2.1. Introduction
When we talk of computer hardware, the three related terms that require introduction are—computer architecture, computer organization and computer design. Computer architecture refers to the structure and behavior of the computer. It includes the specifications of the components, for example, instruction format, instruction set and techniques for addressing memory, and how they connect to the other components. Given the components, computer organization focuses on the organizational structure. It deals with how the hardware components operate and the way they are connected to form the computer. Given the system specifications, computer design focuses on the hardware to be used and the interconnection of parts. Different kinds of computer, such as a PC or a mainframe computer may have different organization; however, basic organization of the computer remains the same.
A computer consists of three main components—(1) Input/Output (I/O) Unit, (2) Central Processing Unit (CPU), and (3) Memory Unit. The computer user interacts with the computer via the I/O unit. The purpose of I/O unit is to provide data and instructions as input to the computer and to present relevant information as output from the computer. CPU controls the operations of the computer and processes the received input to generate the relevant output. The memory unit stores the instructions and the data during the input activity, to make instructions readily available to CPU during processing. It also stores the processed output. This chapter discusses the hardware components of the computer and the interaction between them.
2.2. Central Processing Unit
Central Processing Unit (CPU) or the processor is also often called the brain of computer. CPU (Figure 2.1) consists of Arithmetic Logic Unit (ALU) and Control Unit (CU). In addition, CPU also has a set of registers which are temporary storage areas for holding data, and instructions. ALU performs the arithmetic and logic operations on the data that is made available to it. CU is responsible for organizing the processing of data and instructions. CU controls and coordinates the activity of the other units of computer. CPU uses the registers to store the data, instructions during processing.
Figure 2.1. CPU
2.3. Memory Unit
The memory unit consists of cache memory and primary memory. Primary memory or main memory of the computer is used to store the data and instructions during execution of the instructions. Random Access Memory (RAM) and Read Only Memory (ROM) are the primary memory. In addition to the main memory, there is another kind of storage device known as the secondary memory. Secondary memory is non-volatile and is used for permanent storage of data and programs. A program or data that has to be executed is brought into the RAM from the secondary memory.
2.3.1. Cache Memory
- The data and instructions that are required during the processing of data are brought from the secondary storage devices and stored in the RAM. For processing, it is required that the data and instructions are accessed from the RAM and stored in the registers. The time taken to move the data between RAM and CPU registers is large. This affects the speed of processing of computer, and results in decreasing the performance of CPU.
- Cache memory is a very high speed memory placed in between RAM and CPU. Cache memory increases the speed of processing.
- Cache memory is a storage buffer that stores the data that is used more often, temporarily, and makes them available to CPU at a fast rate. During processing, CPU first checks cache for the required data. If data is not found in cache, then it looks in the RAM for data.
- To access the cache memory, CPU does not have to use the motherboard’s system bus for data transfer. (The data transfer speed slows to the motherboard’s capability, when data is passed through system bus. CPU can process data at a much faster rate by avoiding the system bus.)
- Cache memory is built into the processor, and may also be located next to it on a separate chip between the CPU and RAM. Cache built into the CPU is faster than separate cache, running at the speed of the microprocessor itself. However, separate cache is roughly twice as fast as RAM.
- The CPU has a built-in Level 1 (L1) cache and Level 2 (L2) cache, as shown in Figure 2.3. In addition to the built-in L1 and L2 cache, some CPUs have a separate cache chip on the motherboard. This cache on the motherboard is called Level 3 (L3) cache. Nowadays, high-end processor comes with built-in L3 cache, like in Intel core i7. The L1, L2 and L3 cache store the most recently run instructions, the next ones and the possible ones, respectively. Typically, CPUs have cache size varying from 256KB (L1), 6 MB (L2), to 12MB (L3) cache.
Figure 2.3. Illustration of cache memory
- Cache memory is very expensive, so it is smaller in size. Generally, computers have cache memory of sizes 256 KB to 2 MB.
2.4. Instruction Format
A computer program is a set of instructions that describe the steps to be performed for carrying out a computational task. The program and the data, on which the program operates, are stored in main memory, waiting to be processed by the processor. This is also called the stored program concept.
An instruction is designed to perform a task and is an elementary operation that the processor can accomplish. An instruction is divided into groups called fields. The common fields of an instruction are—Operation (op) code and Operand code (Figure 2.5). The remainder of the instruction fields differs from one computer type to other. The operation code represents action that the processor must execute. It tells the processor what basic operations to perform. The operand code defines the parameters of the action and depends on the operation. It specifies the locations of the data or the operand on which the operation is to be performed. It can be data or a memory address.
2.5. Instruction Set
A processor has a set of instructions that it understands, called as instruction set. An instruction set or an instruction set architecture is a part of the computer architecture. It relates to programming, instructions, registers, addressing modes, memory architecture, etc. An Instruction Set is the set of all the basic operations that a processor can accomplish. Examples of some instructions are shown in Figure 2.7. The instructions in the instruction set are the language that a processor understands. All programs have to communicate with the processor using these instructions. An instruction in the instruction set involves a series of logical operations (may be thousands) that are performed to complete each task. The instruction set is embedded in the processor (hardwired), which determines the machine language for the processor. All programs written in a high-level language are compiled and translated into machine code before execution, which is understood by the processor for which the program has been coded.
Figure 2.7. Examples of some instructions
2.6. Instruction Cycle
The primary responsibility of a computer processor is to execute a sequential set of instructions that constitute a program. CPU executes each instruction in a series of steps, called instruction cycle (Figure 2.8).
- A instruction cycle involves four steps (Figure 2.9)—
- Fetching The processor fetches the instruction from the memory. The fetched instruction is placed in the Instruction Register. Program Counter holds the address of next instruction to be fetched and is incremented after each fetch.
- Decoding The instruction that is fetched is broken down into parts or decoded. The instruction is translated into commands so that they correspond to those in the CPU’s instruction set. The instruction set architecture of the CPU defines the way in which an instruction is decoded.
- Executing The decoded instruction or the command is executed. CPU performs the operation implied by the program instruction. For example, if it is an ADD instruction, addition is performed.
- Storing CPU writes back the results of execution, to the computer’s memory.
Figure 2.8. Instruction cycle
Figure 2.9. Steps in instruction cycle
- Instructions are of different categories. Some categories of instructions are—
- Memory access or transfer of data between registers.
- Arithmetic operations like addition and subtraction.
- Logic operations such as AND, OR and NOT.
- Control the sequence, conditional connections, etc.
2.7. Microprocessor
A processor’s instruction set is a determining factor in its architecture. On the basis of the instruction set, microprocessors are classified as—Reduced Instruction Set Computer (RISC), and Complex Instruction Set Computer (CISC). The x86 instruction set of the original Intel 8086 processor is of the CISC type. The PCs are based on the x86 instruction set.
- CISC architecture hardwires the processor with complex instructions, which are difficult to create otherwise using basic instructions. CISC combines the different instructions into one single CPU.
- CISC has a large instruction set that includes simple and fast instructions for performing basic tasks, as well as complex instructions that correspond to statements in the high level language.
- An increased number of instructions (200 to 300) results in a much more complex processor, requiring millions of transistors.
- Instructions are of variable lengths, using 8, 16 or 32 bits for storage. This results in the processor’s time being spent in calculating where each instruction begins and ends.
- With large number of application software programs being written for the processor, a new processor has to be backwards compatible to the older version of processors.
- AMD and Cyrix are based on CISC.
- RISC has simple, single-cycle instructions, which performs only basic instructions. RISC architecture does not have hardwired advanced functions. All high-level language support is done in the software.
- RISC has fewer instructions and requires fewer transistors, which results in the reduced manufacturing cost of processor.
- The instruction size is fixed (32 bits). The processor need not spend time in finding out where each instruction begins and ends.
- RISC architecture has a reduced production cost compared to CISC processors.
- The instructions, simple in nature, are executed in just one clock cycle, which speeds up the program execution when compared to CISC processors.
- RISC processors can handle multiple instructions simultaneously by processing them in parallel.
- Apple Mac G3 and PowerPC are based on RISC.
2.8. Interconnecting the Units of a Computer
CPU sends data, instructions and information to the components inside the computer as well as to the peripherals and devices attached to it. Bus is a set of electronic signal pathways that allows information and signals to travel between components inside or outside of a computer. The different components of computer, i.e., CPU, I/O unit, and memory unit are connected with each other by a bus. The data, instructions and the signals are carried between the different components via a bus. The features and functionality of a bus are as follows—
- A bus is a set of wires used for interconnection, where each wire can carry one bit of data.
- A bus width is defined by the number of wires in the bus.
- A computer bus can be divided into two types—Internal Bus and External Bus.
- The Internal Bus connects components inside the motherboard like, CPU and system memory. It is also called the System Bus. Figure 2.10 shows interaction between processor and memory.
Figure 2.10. Interaction between CPU and memory
- The External Bus connects the different external devices, peripherals, expansion slots, I/O ports and drive connections to the rest of computer. The external bus allows various devices to be attached to the computer. It allows for the expansion of computer’s capabilities. It is generally slower than the system bus. It is also referred to as the Expansion Bus.
- A system bus or expansion bus comprise of three kinds of buses - data bus, address bus and control bus.
- The interaction of CPU with memory and I/O devices involves all the three buses.
- The command to access the memory or the I/O device is carried by the control bus.
- The address of I/O device or memory is carried by the address bus.
- The data to be transferred is carried by the data bus.
2.9. Performance of a Computer
There are a number of factors involved that are related to the CPU and have an effect on the overall speed and performance of the computer. Some of the factors that affect the performance of the computer include—
- Registers The size of the register (word size) indicates the amount of data with which the computer can work at any given time. The bigger the size, the more quickly it can process data. A 32-bit CPU is one in which each register is 32 bits wide.
- RAM It is used to store data and instructions during execution of the instructions. Anything you do on your computer requires RAM. When the computer is switched on, the operating system, device drivers, the active files and running programs are loaded into RAM. If RAM is less, then the CPU waits each time the new information is swapped into memory from the slower devices. Larger the RAM size, the better it is. PCs nowadays usually have 1 GB to 4 GB of RAM.
- System Clock The clock speed of a CPU is defined as the frequency with which a processor executes instructions or the data is processed. Higher clock frequencies mean more clock ticks per second. The computer’s operating speed is linked to the speed of the system clock. The clock frequency is measured in millions of cycles per second or megahertz (MHz) or gigahertz (GHz) which is billions of cycles per second. A CPU’s performance is measured by the number of instructions it executes in a second, i.e., MIPS or BIPS. PCs nowadays come with a clock speed of more than 1 GHz. In Windows OS, you can select the System Properties dialog box to see the processor name and clock frequency.
- Bus Data bus is used for transfering data between CPU and memory. The data bus width affects the speed of computer. In a 16-bit processor, 16-bit wire bus can carry 16 bits of data. The bus speed is measured in MHz. Higher the bus speed the better it is. Address bus connects CPU and RAM with a set of wires similar to data bus. The address bus width determines the maximum number of memory locations the computer can address. Pentium Pro, II, III, IV have 36-bit address bus that can address 236 bytes or 64 GB of memory. PCs nowadays have a bus speed varying from 100 MHz to 400 MHz.
- Cache Memory Two of the main factors that affect a cache’s performance are its size (amount of cache memory) and level L1, L2 and L3. Larger the size of cache, the better it is. PCs nowadays have a L1 cache of 256KB and L2 cache of 1MB.
2.10. Inside a Computer Cabinet
The computer cabinet encloses the components that are required for the running of the computer. The components inside a computer cabinet include the power supply, motherboard, memory chips, expansion slots, ports and interface, processor, cables and storage devices.
2.10.1. Motherboard
The computer is built up around a motherboard. The motherboard is the most important component in the PC. It is a large Printed Circuit Board (PCB), having many chips, connectors and other electronics mounted on it. The motherboard is the hub, which is used to connect all the essential components of a computer. The RAM, hard drive, disk drives and optical drives are all plugged into interfaces on the motherboard. The motherboard contains the processor, memory chips, interfaces and sockets, etc.
Summary
- Different computers may have different organization, but the basic organization of the computer remains the same.
- I/O Unit, CPU and Memory Unit are the main components of the computer.
- CPU or microprocessor is called the brain of the computer. It processes the data and the instructions. It also supervises the operations of other parts of the computer.
- Registers, Arithmetic Logic Unit and Control Unit are the parts of CPU.
- Cache memory, primary memory and secondary memory constitute the memory unit. Primary memory consists of RAM and ROM.
- Registers are low-storage capacity, high-speed storage areas within the CPU. The data, instructions, addresses and intermediate results of processing are stored in the registers by the CPU.
- Cache memory is a very high-speed memory placed in between RAM and CPU, to increase the processing speed. Cache memory is available in three levels - L1, L2 and L3.
- RAM provides temporary storage, has a limited storage capacity and is volatile memory. The access speed of RAM is faster than access speed of the storage devices like hard disk. The data and the instructions stored in the hard disk are brought into the RAM so that the CPU can access the data and the instructions and process it.
- CU organizes the processing of data and instructions. It acts as a supervisor and controls and coordinates the activity of other units of computer.
- ALU performs arithmetic operations and logic operations on the data.
- An instruction is an elementary operation that the processor can accomplish. The instructions in the instruction set are the language that a processor understands. The instruction set is embedded in the processor which determines the machine language for the processor.
- A CPU instruction cycle involves four steps—(1) Fetching the instructions from the memory, (2) Decoding instructions so that they correspond to those in the CPU’s instruction set, (3) Executing the decoded instructions, and (4) Storing the result to the computer memory.
- RISC and CISC are the two kinds of microprocessors classified on the basis of the instruction set. CISC has a large and complex instruction set. RISC has fewer instructions.
- The different components of computer are connected with each other by a bus. A computer bus is of two types—system bus and expansion bus. A system bus or expansion bus comprise of three kinds of buses—data bus, address bus and control bus.
- The System Bus connects the CPU, system memory, and all other components on the motherboard.
- The Expansion Bus connects the different external devices, peripherals, expansion slots, I/O ports and drive connections to the rest of the computer.
- The performance of the computer is affected by the size of registers, size of RAM, speed of system clock, width of bus, and size of cache memory.
- Inside a computer cabinet, there is a motherboard, ports and interfaces, expansion slots, ribbon cables, RAM memory chips, high storage disk drives, and, processor.
- The motherboard is characterized by the form factor, chipset and type of processor socket. Form factor is the motherboard’s geometry, dimensions, arrangement and electrical requirements. Chipset controls the majority of resources of the computer.
- BIOS and CMOS are present on the motherboard. BIOS is stored in ROM and is used as an interface between the operating system and the motherboard. The time, the system date, and essential system settings are saved in CMOS memory chip present on the motherboard. When the computer power is switched off, CMOS chip remains alive powered by a button battery.
- Ports and interfaces are located on the sides of the computer case to which the external devices can be connected. Some of the ports and interfaces are—Serial port, Parallel port, USB port, Firewire, RJ45 connector, VGA connector, Audio plugs, PS/2 port, and SCSI port.
Keywords
Accelerated Graphics Port (AGP) 29
Address bus 28
Arithmetic Logic Unit (ALU) 20
Audio plugs 33
Basic Input Output System (BIOS) 31
Billion Instructions Per Second (BIPS) 27
Brain of computer 20
Bus 27
Cache memory 31
Central Processing Unit (CPU) 20
Chipset 31
Complementary Metal-Oxide Semiconductor (CMOS) 31
Complex Instruction Set Computer (CISC) 27
Computer architecture 19
Computer design 19
Computer organization 19
Control bus 29
Control Unit (CU) 20
CPU performance 27
Data bus 28
Decoding 25
Executing 25
Expansion bus 28
Expansion card 29
Expansion slot 29
Extended Industry Standard Architecture (EISA) 29
Fetching 25
Firewire 33
Form factor 31
Input/Output (I/O) unit 12
Instruction format 24
Instruction set 25
Instruction cycle 25
Memory Unit 22
Microprocessor 20
Million Instructions Per Second (MIPS) 27
Motherboard 20
Parallel port 32
Peripheral Component Interconnect (PCI) 29
Power On Self Test (POST) 32
Primary memory 22
PS/2 Port 33
Random Access Memory (RAM) 22
Read Only Memory (ROM) 22
Reduced Instruction Set Computer (RISC) 27
Registers 30
Ribbon cables 33
SCSI port 33
Serial port 29
Speed of computer 28
Stored program 20
Storing 25
System bus 28
System clock 30
Universal Serial Bus (USB) 29
Word size 21
Questions
Section 2.1
1. Define computer architecture, computer organization and computer design.
2. Give a brief description of the working of the computer.
Section 2.2
3. CPU is also often called the _________of computer.
4. Define a microprocessor.
5. Define a motherboard.
6. The different parts of the CPU are ______, ________ and _________.
7. _______ and ______ are the main memory.
8. What is the purpose of the main memory?
9. List the main functions of the CPU.
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