Computer Architecture Concepts

INTRODUCTION

Architecture (computer science), a general term referring to the structure of all or part of a computer system. The term also covers the design of system software, such as the operating system (the program that controls the computer), as well as referring to the combination of hardware and basic software that links the machines on a computer network. Computer architecture refers to an entire structure and to the details needed to make it functional. Thus, computer architecture covers computer systems, microprocessors, circuits, and system programs. Typically the term does not refer to application programs, such as spreadsheets or word processing, which are required to perform a task but not to make the system run.

DESIGN ELEMENTS

In designing a computer system, architects consider five major elements that make up the system’s hardware: the arithmetic/logic unit, control unit, memory, input, and output. The arithmetic/logic unit performs arithmetic and compares numerical values. The control unit directs the operation of the computer by taking the user instructions and transforming them into electrical signals that the computer’s circuitry can understand. The combination of the arithmetic/logic unit and the control unit is called the central processing unit (CPU). The memory stores instructions and data. The input and output sections allow the computer to receive and send data, respectively.

Different hardware architectures are required because of the specialized needs of systems and users. One user may need a system to display graphics extremely fast, while another system may have to be optimized for searching a database or conserving battery power in a laptop computer.

In addition to the hardware design, the architects must consider what software programs will operate the system. Software, such as programming languages and operating systems, makes the details of the hardware architecture invisible to the user. For example, computers that use the C programming language or a UNIX operating system may appear the same from the user’s viewpoint, although they use different hardware architectures.

PROCESSING ARCHITECTURE

When a computer carries out an instruction, it proceeds through five steps. First, the control unit retrieves the instruction from memory—for example, an instruction to add two numbers. Second, the control unit decodes the instructions into electronic signals that control the computer. Third, the control unit fetches the data (the two numbers). Fourth, the arithmetic/logic unit performs the specific operation (the addition of the two numbers). Fifth, the control unit saves the result (the sum of the two numbers).

Early computers used only simple instructions because the cost of electronics capable of carrying out complex instructions was high. As this cost decreased in the 1960s, more complicated instructions became possible. Complex instructions (single instructions that specify multiple operations) can save time because they make it unnecessary for the computer to retrieve additional instructions. For example, if seven operations are combined in one instruction, then six of the steps that fetch instructions are eliminated and the computer spends less time processing that operation. Computers that combine several instructions into a single operation are called complex instruction set computers (CISC).

However, most programs do not often use complex instructions, but consist mostly of simple instructions. When these simple instructions are run on CISC architectures they slow down processing because each instruction—whether simple or complex—takes longer to decode in a CISC design. An alternative strategy is to return to designs that use only simple, single-operation instruction sets and make the most frequently used operations faster in order to increase overall performance. Computers that follow this design are called reduced instruction set computers (RISC).

RISC designs are especially fast at the numerical computations required in science, graphics, and engineering applications. CISC designs are commonly used for nonnumerical computations because they provide special instruction sets for handling character data, such as text in a word processing program. Specialized CISC architectures, called digital signal processors, exist to accelerate processing of digitized audio and video signals.

OPEN AND CLOSED ARCHITECTURES

The CPU of a computer is connected to memory and to the outside world by means of either an open or a closed architecture. An open architecture can be expanded after the system has been built, usually by adding extra circuitry, such as a new microprocessor computer chip connected to the main system. The specifications of the circuitry are made public, allowing other companies to manufacture these expansion products.

Closed architectures are usually employed in specialized computers that will not require expansion—for example, computers that control microwave ovens. Some computer manufacturers have used closed architectures so that their customers can purchase expansion circuitry only from them. This allows the manufacturer to charge more and reduces the options for the consumer.

One problem in computer architecture is caused by the difference between the speed of the CPU and the speed at which memory supplies instructions and data. Modern CPUs can process instructions in 3 nanoseconds (3 billionths of a second). A typical memory access, however, takes 100 nanoseconds and each instruction may require multiple accesses. To compensate for this disparity, new computer chips have been designed that contain small memories, called caches, located near the CPU. Because of their proximity to the CPU and their small size, caches can supply instructions and data faster than normal memory. Cache memory stores the most frequently used instructions and data and can greatly increase efficiency.

Although a larger cache memory can hold more data, it also becomes slower. To compensate, computer architects employ designs with multiple caches. The design places the smallest and fastest cache nearest the CPU and locates a second larger and slower cache farther away. This arrangement allows the CPU to operate on the most frequently accessed instructions and data at top speed and to slow down only slightly when accessing the secondary cache. Using separate caches for instructions and data also allows the CPU to retrieve an instruction and data simultaneously.

Another strategy to increase speed and efficiency is the use of multiple arithmetic/logic units for simultaneous operations, called superscalar execution. In this design, instructions are acquired in groups. The control unit examines each group to see if it contains instructions that can be performed together. Some designs execute as many as six operations simultaneously. It is rare, however, to have this many instructions run together, so on average the CPU does not achieve a six-fold increase in performance.

Multiple computers are sometimes combined into single systems called parallel processors. When a machine has more than one thousand arithmetic/logic units, it is said to be massively parallel. Such machines are used primarily for numerically intensive scientific and engineering computation. Parallel machines containing as many as sixteen thousand computers have been constructed.

The Central Processing Unit

Information from an input device or from the computer’s memory is communicated via the bus to the central processing unit (CPU), which is the part of the computer that translates commands and runs programs. The CPU is a microprocessor chip—that is, a single piece of silicon containing millions of tiny, microscopically wired electrical components. Information is stored in a CPU memory location called a register. Registers can be thought of as the CPU’s tiny scratchpad, temporarily storing instructions or data. When a program is running, one special register called the program counter keeps track of which program instruction comes next by maintaining the memory location of the next program instruction to be executed. The CPU’s control unit coordinates and times the CPU’s functions, and it uses the program counter to locate and retrieve the next instruction from memory.

In a typical sequence, the CPU locates the next instruction in the appropriate memory device. The instruction then travels along the bus from the computer’s memory to the CPU, where it is stored in a special instruction register. Meanwhile, the program counter changes—usually increasing a small amount—so that it contains the location of the instruction that will be executed next. The current instruction is analyzed by a decoder, which determines what the instruction will do. Any data the instruction needs are retrieved via the bus and placed in the CPU’s registers. The CPU executes the instruction, and the results are stored in another register or copied to specific memory locations via a bus. This entire sequence of steps is called an instruction cycle. Frequently, several instructions may be in process simultaneously, each at a different stage in its instruction cycle. This is called pipeline processing.

CPU REGISTERS

Registers are 8,16, or 32 bit high speed storage locations directly inside the CPU, designed to be accessed are much higher speed than conventional memory.

Intel 16-bit registers

• General Purpose registers
• Segment Registers
• Status and Control Registers
• Flags

General Purpose Registers

• AX(Accumulator)
• BX(Base)
• CX(Counter)
• DX(Data)

Segment Registers

• CS(Code segment)
• DS(Data segment)
• SS(Stack segment)
• ES(Extra segment)

Index Registers

• BP(Base pointer)
• SP(Stack pointer)
• SI(Source index)
• DI(Destination index)
• Status and Control Registers
• IP(Instruction pointer)

Flag Registers

• Control Flags(CF)
• The Direction flag(DF)
• The Interrupt flag(IF)
• The Trap flag(TF)
• Status Flags
• The Carry flag(CF)
• The Overflow flag(OF)
• The Sign flag(SF)
• The Zero flag(ZF)
• The Auxiliary Carry
• The Parity flag

Computer Memory

INTRODUCTION

Computer Memory, a mechanism that stores data for use by a computer. In a computer all data consist of numbers. A computer stores a number into a specific location in memory and later fetches the value. Most memories represent data with the binary number system. In the binary number system, numbers are represented by sequences of the two binary digits 0 and 1, which are called bits (see Number Systems). In a computer, the two possible values of a bit correspond to the on and off states of the computer’s electronic circuitry.

In memory, bits are grouped together so they can represent larger values. A group of eight bits is called a byte and can represent decimal numbers ranging from 0 to 255. The particular sequence of bits in the byte encodes a unit of information, such as a keyboard character. One byte typically represents a single character such as a number, letter, or symbol. Most computers operate by manipulating groups of 2, 4, or 8 bytes called words.

Memory capacity is usually quantified in terms of kilobytes, megabytes, and gigabytes. Although the prefixes kilo-, mega-, and giga-, are taken from the metric system, they have a slightly different meaning when applied to computer memories. In the metric system, kilo- means 1 thousand; mega-, 1 million; and giga-, 1 billion. When applied to computer memory, however, the prefixes are measured as powers of two, with kilo- meaning 2 raised to the 10th power, or 1,024; mega- meaning 2 raised to the 20th power, or 1,048,576; and giga- meaning 2 raised to the 30th power, or 1,073,741,824. Thus, a kilobyte is 1,024 bytes and a megabyte is 1,048,576 bytes. It is easier to remember that a kilobyte is approximately 1,000 bytes, a megabyte is approximately 1 million bytes, and a gigabyte is approximately 1 billion bytes.

HOW MEMORY WORKS

Computer memory may be divided into two broad categories known as internal memory and external memory. Internal memory operates at the highest speed and can be accessed directly by the central processing unit (CPU)—the main electronic circuitry within a computer that processes information. Internal memory is contained on computer chips and uses electronic circuits to store information (see Microprocessor). External memory consists of storage on peripheral devices that are slower than internal memories but offer lower cost and the ability to hold data after the computer’s power has been turned off. External memory uses inexpensive mass-storage devices such as magnetic hard drives. See also Information Storage and Retrieval.

Internal memory is also known as random access memory (RAM) or read-only memory (ROM). Information stored in RAM can be accessed in any order, and may be erased or written over. Information stored in ROM may also be random-access, in that it may be accessed in any order, but the information recorded on ROM is usually permanent and cannot be erased or written over.

Internal RAM

Random access memory is also called main memory because it is the primary memory that the CPU uses when processing information. The electronic circuits used to construct this main internal RAM can be classified as dynamic RAM (DRAM), synchronized dynamic RAM (SDRAM), or static RAM (SRAM). DRAM, SDRAM, and SRAM all involve different ways of using transistors and capacitors to store data. In DRAM or SDRAM, the circuit for each bit consists of a transistor, which acts as a switch, and a capacitor, a device that can store a charge. To store the binary value 1 in a bit, DRAM places an electric charge on the capacitor. To store the binary value 0, DRAM removes all electric charge from the capacitor. The transistor is used to switch the charge onto the capacitor. When it is turned on, the transistor acts like a closed switch that allows electric current to flow into the capacitor and build up a charge. The transistor is then turned off, meaning that it acts like an open switch, leaving the charge on the capacitor. To store a 0, the charge is drained from the capacitor while the transistor is on, and then the transistor is turned off, leaving the capacitor uncharged. To read a value in a DRAM bit location, a detector circuit determines whether a charge is present or absent on the relevant capacitor.

DRAM is called dynamic because it is continually refreshed. The memory chips themselves cannot hold values over long periods of time. Because capacitors are imperfect, the charge slowly leaks out of them, which results in loss of the stored data. Thus, a DRAM memory system contains additional circuitry that periodically reads and rewrites each data value. This replaces the charge on the capacitors, a process known as refreshing memory. The major difference between SDRAM and DRAM arises from the way in which refresh circuitry is created. DRAM contains separate, independent circuitry to refresh memory. The refresh circuitry in SDRAM is synchronized to use the same hardware clock as the CPU. The hardware clock sends a constant stream of pulses through the CPU’s circuitry. Synchronizing the refresh circuitry with the hardware clock results in less duplication of electronics and better access coordination between the CPU and the refresh circuits.

In SRAM, the circuit for a bit consists of multiple transistors that hold the stored value without the need for refresh. The chief advantage of SRAM lies in its speed. A computer can access data in SRAM more quickly than it can access data in DRAM or SDRAM. However, the SRAM circuitry draws more power and generates more heat than DRAM or SDRAM. The circuitry for a SRAM bit is also larger, which means that a SRAM memory chip holds fewer bits than a DRAM chip of the same size. Therefore, SRAM is used when access speed is more important than large memory capacity or low power consumption.

The time it takes the CPU to transfer data to or from memory is particularly important because it determines the overall performance of the computer. The time required to read or write one bit is known as the memory access time. Current DRAM and SDRAM access times are between 30 and 80 nanoseconds (billionths of a second). SRAM access times are typically four times faster than DRAM.

The internal RAM on a computer is divided into locations, each of which has a unique numerical address associated with it. In some computers a memory address refers directly to a single byte in memory, while in others, an address specifies a group of four bytes called a word. Computers also exist in which a word consists of two or eight bytes, or in which a byte consists of six or ten bits.

When a computer performs an arithmetic operation, such as addition or multiplication, the numbers used in the operation can be found in memory. The instruction code that tells the computer which operation to perform also specifies which memory address or addresses to access. An address is sent from the CPU to the main memory (RAM) over a set of wires called an address bus. Control circuits in the memory use the address to select the bits at the specified location in RAM and send a copy of the data back to the CPU over another set of wires called a data bus. Inside the CPU, the data passes through circuits called the data path to the circuits that perform the arithmetic operation. The exact details depend on the model of the CPU. For example, some CPUs use an intermediate step in which the data is first loaded into a high-speed memory device within the CPU called a register.

Internal ROM

Read-only memory is the other type of internal memory. ROM memory is used to store items that the computer needs to execute when it is first turned on. For example, the ROM memory on a PC contains a basic set of instructions, called the basic input-output system (BIOS). The PC uses BIOS to start up the operating system. BIOS is stored on computer chips in a way that causes the information to remain even when power is turned off.

I/O Module

I/O module is an entity within the computer responsible for the control of one or more external devices and for the exchange of data between those devices and main memory and /or CPU registers. The major functions of an I/O module fall into the following categories:

• Control and timing
• CPU communication
• Device communication
• Data buffering
• Error detection

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