Understanding Computer Architecture: CPU, ALU, Registers, and Memory, Slides of Computer Science

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COMPUTER ARCHITECTURE

1

• Computer Architecture:
  • The science of selecting and interconnecting hardware components that create computers
  • It will meet functional, performance and cost goals
  • It can also be described as the logical structure of the computer system
  • It forms the backbone for building successful computer system
• CPU:
  • The brain of a computer system
  • It is a VLSI chip inside the system, plugged onto the motherboard
  • It controls all internal and external devices
  • It operates only on binary data i.e 0 and 1
• Functions of CPU:
  • Carrying out arithmetic and logic functions
  • Controlling the use of main storage to store data and instructions
  • Controlling the sequence of operations
  • Controlling all the parts of the computer system 2

• Logical Unit (LU):
  • The logical unit enables the CPU to make logical operations based on the instructions provided
  • These operations are the logical comparison between the data items
  • This unit can compare numbers, letters or special characters
  • They can take action based on the result of the comparison
  • The logical operations of a logic unit test three conditions:
    • Equal to
    • Less than condition
    • Greater than condition
• Registers:
  • These are special-purpose, high-speed temporary memory units
  • These are temporary storage areas for holding various types of information such as data, instructions, addresses and the intermediate results of calculations
  • It works under the direction of the control unit to accept, hold and transfer instructions or data and perform arithmetic or logical comparisons at high speed. 4

• Registers:
• Various types of registers present inside a CPU:
• Program Counter (PC)
  • It keeps track of the next instruction to be executed
• Instruction Register (IR)
  • It holds the instruction to be decoded by the control unit
• Memory Address Register (MAR)
  • It holds the address of the next location in memory to be accessed
• Memory Buffer Register (MBR)
  • It is used for storing data either coming to the CPU or data being transferred by the CPU
• Accumulator (ACC)
  • It is a general purpose register used for storing temporary results and results produced by ALU
• Control Unit:
  • It is the heart of the CPU
  • It controls the I/O devices and transfer of data to and from the primary storage
  • It itself is controlled by the individual instructions in programs located in the primary storage 5

• Main memory unit
  • CPU accesses the main memory in a random manner
  • The CPU can access any location of this memory to either read information from it or store information in it
  • The primary memory is of two types:
  • Random access memory (RAM)
  • Read only memory(ROM)
• Random access memory (RAM):
  • It directly provides the required information to the processor
  • It is a block of sequential memory locations, each of which has a unique address determining the location and those locations contain a data element
  • It stores programs and data that are in active use
  • RAM can be further classified into two categories: DRAM and SRAM
• Dynamic Random Access Memory (DRAM):
  • It holds the data in dynamic manner with help of a refresh circuitry
  • Each second or even less than that the content of each memory cell is read
  • This read action refreshes the contents of the memory
  • Due to refreshing action, the memory is called dynamic RAM
  • DRAMs are made from transistors and capacitors
  • The capacitor holds the electrical charge if the bit contains 1 and no charge if 7 the

• Static Random Access Memory (SRAM):
  • SRAM along with DRAM is essential for a system to run optimally, because it is very fast as compared to DRAM
  • It is effective because most of the programs access the same data repeatedly and keeping all these information in the first SRAM
  • Data are first written to SRAM assuming that they will be used soon
  • SRAM is generally included in a computer system by the name of cache
• Read-only Memory:
  • It can only be read, not written
  • It stores the initial start-up instructions and routines in the BIOS which can be read by the CPU, each time it is switched on
  • The instructions in the ROM are built into the electronic circuits of the chip, which is called firmware
  • The ROM is also random access in nature, which means the CPU can randomly access any location within the ROM
• Various types of ROM:
  • Programmable read-only memory (PROM)
  • Erasable programmable read-only memory (EPROM)
  • Electrically erasable programmable read-only memory (EEPROM) 8

• Memory Hierarchy
• There are three fundamental types of memory in a computer

system:

• Internal Processor Memory:
  • This memory is placed within the CPU or is attached to a special fast bus
  • Internal memory usually includes cache memory and special registers, both of which can be directly accessed by the processor
  • This memory is used for temporary storage of data and instructions on which the CPU is currently working
  • Processor memory is the fastest among all the memories but is most expensive also
• Primary Memory:
  • Random access memory (RAM) and Read-only memory (ROM) fall under the category of the primary memory which is also known as main memory
  • Every computer comes with a small amount ROM, which contains the boot firmware (called BIOS)
  • It is volatile in nature i.e. when the power is switched off, the data in this memory is lost 10

• Memory Hierarchy
• Secondary Memory:
  • This is also known as auxiliary memory
  • The secondary memory provides backup storage for instructions and data
  • The most commonly used secondary storage devices are magnetic disks and magnetic tapes
  • These are the least expensive as well as have larger storage capacity than primary memory
  • The instructions and data stored on secondary storage devices are permanent in nature
• The secondary memory can also be used as overflow memory when

the capacity of main memory is surpassed

• Registers, Cache memory Internal Processor Memory
• RAM, ROM Primary Memory
• Hard Disk, magnetic Disk Secondary memory 11

COST

Access Speed

Storage Capacity

• These lines are broadly categorized into three functional groups:
  • Data lines
  • Address lines
  • Control lines
• Data Lines:
  • A path for moving data between the system modules
  • These are collectively known as data bus
  • Normally, a data bus consists of 8 , 16 or 32 separate lines
  • The number of lines present in a data bus is called the width of data bus
  • The width of data helps in determining the overall performance of a computer system
• Address Lines:
  • This lines are used to designate the source of data for data bus
  • For reading or writing any information on to the memory, the CPU needs to specify the address of a particular location
  • This address is supplied by the address bus
  • Thus the width of the address bus specifies the maximum possible memory supported by a system 13

• Control lines:
  • They are used to control the access to data and address bus
  • The control lines are collectively called control bus
  • These lines are used for the transmission of commands and timing signals (which validate data and address) between the system modules
  • Timing signals indicate whether data and address information is valid 14