Computer Structure Architecture

Computer structure architecture – A computer has already been defined as a device that can store, process, and retrieve data as and when required.

The simplest possible computer structure architecture is shown in the image below. The computer structure is divided into three basic subsystems and their interconnection components.

Computer Structure Architecture

1. Processor
2. Memory
3. Input/Output devices
4. System interconnections (Bus)

Processor

The processor is also known as the Central Processing Unit (CPU). This block of the computer is responsible for controlling the operations of the computer and performing its data processing functions.

The processor executes the instructions and performs operations according to the meaning of the respective instruction. To perform various operations, the processor is made up of several building blocks. These can broadly be categorized into three segments as shown in the figure below.

1. Execution unit (includes ALU)
2. Register unit
3. Control unit

Execution unit (includes ALU)

This segment of the processor contains the hardware that executes instructions. The execution unit, including ALU (Arithmetic Logic Unit), performs arithmetic operations (addition, subtraction, etc.) and logical operations (AND, OR, EX-OR, etc.).

Many processors contain separate execution units for integer and floating-point computations because very different hardware is required to handle these two data types.

Note: ALU is the unit that decides the capability of any processor.

Register Unit

The register unit (also named as register array, register set, or register file) is a small storage area for data that the processor is using. Accessing data stored in a register unit is faster than accessing the data stored in a memory system.

This feature makes the importance of the register unit. A register unit generally supports multiple simultaneous accesses. However, various processors have different types of register units, but virtually all have a register unit of some sort.

Control Unit

This unit generates the control and timing signals to control various operations of the processor and, hence, the computer. In other words, the control unit determines when instructions can be executed and what sequence of operations is required to execute each instruction.

It also controls the flow of data between the processor and other devices, like memory and input/output peripherals. In early processors, the control unit was a very small part of the processor, but the part of the control unit has grown dramatically as processors have become more complex.

Memory

The memory system acts as a storage for the program code and data used by the computer. Semiconductor memories are used for storing information in a computer.

Memory Hierarchical Structure

The diagram below shows the hierarchical structure of memory. Though a total of six levels are shown in the figure, in many systems, there may be only three levels of memory, namely, register, primary memory, and mass storage.

From the point of view of speed of operation, the memory types listed towards the top of the pyramid are faster than those listed towards the lower end.

For example, the time required to read a word of information from a fast primary memory may be as low as 100 n sec as compared to 10 m sec required to read a word from a hard disk.

Register – Registers are a part of the processor. This is shown at the highest level of the memory pyramid in the above figure. The amount of register storage is limited from a few hundred bits to a few thousand bits. The number and types of registers vary from one processor to another. Registers are generally used for the temporary storage of data.

Cache Memory – Cache is a faster memory. To increase the performance of the system, some designers use the cache memory at the second level. Its size is typically in a few kilobytes. In some advanced processors, the cache memory is located on the processor chip itself.

However, as the cache memory consists of faster memory chips, it is expensive too. Cost, therefore, becomes one of the limiting factors of cache.

Primary Memory (Faster & Slower) – Registers require space on the processor, and hence, only a limited number of them can be provided. This memory is generally not sufficient in most of the systems, hence the primary memory is used. This is the third level from the top.

The size of primary memory may vary from a few kilobytes in small systems to several megabytes in large systems. In some systems, there is an extra level of primary memory. This extra level consists of a memory of a much larger size, through an order of magnitude slower in speed than the higher level primary memory.

Mass Storage – Several programs and data are needed to be resident within the system so that they can be loaded for execution into the primary memory without much delay. Holding these programs and data may require several megabytes of memory.

These programs and data may not be accessed very frequently. Thus, one or more mass storage devices are used for storing this information. Hard disk, floppy disk, and optical disks are some of the devices used for mass storage.

This type of mass storage is also called online storage, as all the information is accessible to the processor, though at a comparatively slower rate.

Off-line Backup – The offline backup is a type of removable storage device. When all the mass storage available in a system gets used up, then offline backup plays an important role. A removable hard disk and a pen drive are examples of offline backup.

Once such a device is available, the user can perform periodic backup operations. It is offline because the information on these devices can not be accessed by the processor just as easily as it would access information from primary or other online memories.

Memory Classification

There are two main types of semiconductor memories used by the computer.

1. RAM (Random Access Memory)
2. ROM (Read-only Memory)

RAM – The random access memory can be both read and written, and is used to hold the programs, operating system, and data required by the computer. RAM is generally volatile, meaning that it does not retain the data stored in it when the computer’s power is turned off. Any data that needs to be stored while the computer is off must be written to a permanent storage device, such as a hard disk.

There are two types of RAM used in computers. These are (i) static RAM (SRAM) and (ii) dynamic RAM (DRAM). A static RAM is made up of flip-flops. It stores the bit as a voltage. The flip-flop retains the stored information until it is overwritten or electrical power is taken off the chip.

A dynamic RAM has a much smaller cell than a static RAM. One bit of information is stored as a charge on a capacitor. The advantages of DRAM are that it has high density, low power consumption, and is cheaper than SRAM. The disadvantage is that the charge leads; therefore, to retain the information on DRAM, it should be refreshed every few milliseconds.

This refresh needs extra circuitry and makes the interfacing of DRAM to the processor more complex than the interfacing of SRAM. Generally, systems that require large memory capacity use DRAM to lower the memory cost.

ROM – The read-only memory is a nonvolatile memory. It is a preprogrammed chip and can only be read by the processor. This memory is used for programs and data that do not need to be altered. Thus, once the information has been recorded in the ROM, the chip can either be used with whatever it contains or has to be discarded.

In general, the ROM is used to hold a program that is executed automatically by the computer every time it is turned on or reset. This program is called the booting program.

A programmable ROM, also known as PROM, can be programmed by the user just once. After being programmed, the PROM behaves just like the ROM.

Another type of ROM is Erasable Programmable Read-Only Memory (EPROM). It can be erased by ultraviolet rays and then reprogrammed. In order to erase the EPROM, it has to be taken out of its normal circuit and placed in front of a special ultraviolet eraser for several minutes.

The disadvantages of EPROM are

1. It must be taken out of the circuit to erase it
2. The entire chip must be erased
3. The erasing process takes 15 to 20 minutes.

The inconvenience and other technical problems associated with the removal of EPROM from its normal circuit of operation are taken care of in Electrically Erasable PROMs (EEPROM). The EEPROM can be erased and reprogrammed in normal conditions.

This makes EEPROM ideal for applications where some of the parameters need to be changed over a period of time. Non-volatile RAM is also one type of EEPROM. The diagram below shows the various types of semiconductor memories discussed above.

Note: The booting program, basic input-output system (BIOS) program, and vector addresses for interrupts are stored in ROM.

Input/Output Device

The input and output devices are the ears and eyes of the computer. These are the communication channels to the external world. The data and instructions can be entered into the memory through the keyboard or simple switches.

These devices are known as input devices. The devices that are used to display or print the results are known as output devices. The monitor, seven-segment LEDs, printer etc., are a few examples of the output devices.

Input/output devices can be interfaced with the processor in two modes:

1. Parallel I/O mode.
2. Serial I/O mode.

Data can enter or exit in a group of certain bits, which is called parallel I/O mode. In serial mode, one bit is transformed using on-line. Both modes have their own advantages and disadvantages. The mode of communication is chosen according to the application.

System Interconnections (Bus)

The mechanism that provides the medium for communication among the processor, memory, and input/output. A common example of a system interconnection is by means of a system bus, consisting of a number of conducting wires to which all the other components are attached. There are three system buses.

Address Bus

The processor issues the address of the instruction byte or word to the memory system through the address bus. Therefore, the address bus is a collection of conducting wires that transfers the address of the memory/peripheral from the processor to the memory/peripheral.

The processor execution unit, when required, issues the address of the data or code through the address bus. The address bus fetches the code byte or data byte from an address specified. For example, let a processor at the start reset the program counter at address 0.

Then the processor issues an address 0 on the address bus, and the instruction at address 0 is fetched from the memory. This is a unidirectional bus.

Data Bus

The data bus is a group of lines used for data flow. This is a bidirectional bus, meaning that data can flow in both directions between the processor and the input/output or memory. When the processor issues the address of the instruction, it retrieves the information through the data bus.

For example, when an instruction is given to store the data from one register to the memory address m, the processor issues the address m on the address bus and sends the data through the data bus.

In the same manner, when the processor issues address M for an instruction, it fetches the instruction through the data bus from the address M.

Control Bus

The control bus consists of various single lines. These lines are used to communicate the timing and control signals. These signals synchronize the subsystems and are also known as synchronization signals.

These control lines may be address latch enable (ALE), memory read, memory write, I/O read, I/O write, interrupt acknowledges, and other control signals as per the processor design.

Note: The control bus is not a real bus since it is not a group of lines like the address bus or data bus, but individual lines.

Memory system bus and interconnections for a simple bus structure are shown in the image below.

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