Computer System Organization

A digital computer consists of an interconnected system of processors, memories, and input/output devices. Processors, memories, and input/output are key concepts and will be present at every level, so we will start to study computer architecture by looking at all three in turn.

PROCESSORS

The organization of a simple bus-oriented computer is shown in the picture. The CPU (Central Processing Unit) is the ‘‘brain’’ of the computer.

A central processing unit (CPU) is the electronic circuitry within a computer that carries out the instructions of a computer program by performing the basic arithmetic, logical, control, and input/output (I/O) operations specified by the instructions. The computer industry has used the term “central processing unit” at least since the early 1960s.[1] Traditionally, the term “CPU” refers to a processor, more specifically to its processing unit and control unit (CU), distinguishing these core elements of a computer from external components such as main memory and I/O circuitry.

The CPU is composed of several distinct parts. The control unit is responsible for fetching instructions from the main memory and determining their type. The arithmetic logic unit performs operations such as addition and Boolean AND needed to carry out the instructions.

The CPU also contains a small, high-speed memory used to store temporary results and certain control information. This memory is made up of a number of registers, each of which has a certain size and function. Usually, all the registers have the same size. Each register can hold one number, up to some maximum determined by the size of the register. Registers can be read and written at high speed since they are internal to the CPU. The most important register is the Program Counter (PC), which points to the next instruction to be fetched for execution. ( The name ‘‘program counter’’ is somewhat misleading because it has nothing to do with counting anything, but the term is universally used. Also important is the Instruction Register (IR), which holds the instruction currently being executed. ( Most computers have numerous other registers as well, some of the general-purpose as well as some for specific purposes.

What is the,,bus,,?

In computer architecture, a bus is a communication system that transfers data between components inside a computer, or between computers. This expression covers all related hardware components (wire, optical fiber, etc.) and software, including communication protocols.

Early computer buses were parallel electrical wires with multiple hardware connections, but the term is now used for any physical arrangement that provides the same logical function as a parallel electrical bus. Modern computer buses can use both parallel and bit-serial connections and can be wired in either a multidrop (electrical parallel) or daisy chain topology, or connected by switched hubs, as in the case of USB.

CPU Organization

• A system bus is a link that connects every segment of a system to the central storage and carries out the data transfer in them.
• It is a pathway composed of cables and connectors which is used to carry data between a computer microprocessor and the main memory.
• It provides a communication path for the data and control signals moving between the major components of the computer system.

The types of system buses are

1. Data

• These are the pieces of information that are to be transferred.
• The data is transferred between peripherals, memory, and the CPU. The data bus can be a very busy pathway.

2. Address

• It stores information about where the data is to be transferred.
• The components pass memory addresses to one another over the address bus.

3. Control

• These are the set of instructions regarding what to do with the data.
• It is used to send out signals to coordinate and manage the activities of the motherboard components.

Characteristics of a System Bus

1. Bus Width

• The size of a bus is also known as its width.
• It determines how much data can be transferred at a time.
• This refers to the amount of information that can be transferred at once.

2. Bus Speed

• This refers to the no. of bits or bytes the bus can send per unit of time.
• Its frequency also defines it. Frequency means the number of data packets sent or received per second. Each time that data is sent or received is called a cycle.

The system bus combines the functions of the three main buses, namely Control Bus, Address Bus, Data Bus. The control bus carries the control, timing and coordination signals to manage the various functions across the system. The address bus is used to specify memory locations for the data being transferred.

The data bus, which is a bidirectional path. It carries the actual data between the processor (CPU), the memory and the peripherals (Input and Output). The system bus architecture varies from system to system and can be specific to a particular computer design. The other common characteristics of system buses are based on the primary role, connecting devices internally or externally, etc.

Internal Bus

• It is also known as an internal data bus, a memory bus, a system bus or Front-Side-Bus.
• It connects all the internal components of a computer, such as CPU and memory, to the motherboard.
• Internal data buses are also referred to as a local bus because they are intended to connect to local devices.
• This bus is quick and independent of the rest of the computer operations.

External Bus

• It is also known as an expansion bus.
• It is made up of the electronic pathways that connect the different external devices, such as a printer, etc.

Cache Function

A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost (time or energy) to access data from the main memory. A cache is a smaller, faster memory, closer to a processor core, which stores copies of the data from frequently used main memory locations. Most CPUs have different independent caches, including instruction and data caches, where the data cache is usually organized as a hierarchy of more cache levels (L1, L2, etc.).

All modern (fast) CPUs (with few specialized exceptions[2]) have multiple levels of CPU caches. The first CPUs that used a cache had only one level of cache; unlike later level 1 caches, it was not split into L1d (for data) and L1i (for instructions). Almost all current CPUs with caches have a split L1 cache. They also have L2 caches and, for larger processors, L3 caches as well. The L2 cache is usually not split and acts as a common repository for the already split L1 cache. Every core of a multi-core processor has a dedicated L2 cache and is usually not shared between the cores. The L3 cache, and higher-level caches, are shared between the cores and are not split. An L4 cache is currently uncommon and is generally on dynamic random-access memory (DRAM), rather than on static random-access memory (SRAM), on a separate die or chip. That was also the case historically with L1, while bigger chips have allowed integration of it and generally all cache levels, with the possible exception of the last level. Each extra level of cache tends to be bigger and be optimized differently.

Other types of caches exist (that are not counted towards the “cache size” of the most important caches mentioned above), such as the translation lookaside buffer (TLB) that is part of the memory management unit (MMU) that most CPUs have.

Caches are generally sized in powers of two: 4, 8, 16 etc. KiB or MiB (for larger non-L1) sizes, although the IBM z13 has a 96 KiB L1 instruction cache.

Instruction Execution Cycle

• This is a process of getting the instruction from the memory, decoding it to the machine language, and executing it. So, the three basic steps of the cycle are:

Fetch the instruction.
Decode it.
Execute.

• The whole process of fetching the instructions from the memory, decoding it to the machine language and executing it, is termed as an instruction cycle.

RISC versus CISC

PRIMARY MEMORY

Memory is the part of the computer where programs and data are stored. Some computer scientists (especially British ones) use the term store or storage rather than memory, although more and more, the term ‘‘storage’’ is used to refer to disk storage. Without a memory from which the processors can read and write information, there would be no stored-program digital computers.

Bits

The basic unit of memory is the binary digit, called a bit. A bit may contain a 0 or a 1. It is the simplest possible unit. (A device capable of storing only zeros could hardly form the basis of a memory system; at least two values are needed.) People often say that computers use binary arithmetic because it is ‘‘efficient.’’ What they mean (although they rarely realize it) is that digital information can be stored by distinguishing between different values of some continuous physical quantity, such as voltage or current. The more values that must be distinguished, the less separation between adjacent values, and the less reliable the memory. The binary number system requires only two values to be distinguished. Consequently, it is the most reliable method for encoding digital information.

Memory Addresses

In computing, a memory address is a reference to a specific memory location used at various levels by software and hardware. Memory addresses are fixed-length sequences of digits conventionally displayed and manipulated as unsigned integers. The such numerical semantic base itself upon features of the CPU (such as the instruction pointer and incremental address registers), as well upon the use of the memory like an array endorsed by various programming languages.