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Unit 2 Basic architecture
2.1 building blocks of a PC
2.2 CPU
2.3 RAM, DRAM, SDRAM, ROM, EPROM
2.4 Input/output
2.1 Building blocks of a PC :
Even the size, shape, performance and cost of computer have been changing over the last several years, the basic logical structure as propose by Von Neuman has not changed.
Basic operations for converting raw input raw data into useful information and presenting it to a user.
- Inputting : process of entering data and instructions into a computer system.
- Storing : saving data and instructions to make them readily available for initial or additional processing as and when required.
- Processing : performing arithmetic operations (add, subtract, multiply, divide, etc) or logical operations (comparisions, like equal to, less than, greater than, etc) on data to convert them into useful information.
- Outputting : process of producing useful information or results for a user, such as printed report or visual display.
- Controlling : directing the manner and sequence in which the above operations are performed.
Internal architecture of computer differs from one system model to another. However, basic organization remains the same for all computer systems. Figure below shows the block diagram of basic computer organization. It displays the five major building blocks (functional units) of a digital computer system. These five units correspond to the five basic operations performed by all computer systems.
Figure: Basic organization of computer
Details of the building blocks of a computer:
- Input Unit :
Data and instruction must enter a computer system before the computer can perform any computation on the supplied data. The input unit that links a computer with its external environment performs this task. Data and instruction enter a computer through the input unit in a form that depends upon the input unit used.
For example: data can enter using a keyboard, mouse or any other types of input device.
All input device must transform the input signal to binary code. The input units performing these transformations is called input interfaces.
- Output UNIT :
Output unit supplies the information obtained from data processing to outside world. Hence it links computer with outside world. Result are produced in binary form, therefore before supplying the result to outside world the system must convert them to human acceptable form. Unit called output interfaces accomplish this task. Output interface match the unique physical or electrical characteristics of output devices ( terminals, printers, etc) to the requirements of an external environment.
In short, functions of output units:
unit does not perform any actual processing or jobs but acts as central nervous system for other components of the computer system. It manages and co-ordinates the operation of all the other compone nts.
Control unit: Control unit does not perform any actual processing or jobs but acts as central nervous system for other components of the computer system. It manages and co-ordinates the operation of all the other components of the computer system. It manages and co- ordinates the operation of all other components. It obtains instructions from the program stored in main memory, interprets the instructions and issues signals causing other units of the system to execute them. Arithmetic logic unit. ALU of a computer system is the place where actual execution of instruction takes place during processing operation.ALU executes all arithmetic and logical operation. It can perform four kinds of arithmetic operations; addition, subtraction, division and multiplication. It also performs logic operations or comparisons such as less than, equal to and greater than.
2.2 Central processing unit
CPU is the brain of the computer system. All major calculations and comparisons performed by a compute are carried out inside its CPU. CPU is also responsible for activating and controlling the operations of other unit of a computer system. Hence no other single component of a computer system determines its overall performance as much as its CPU. In order to be able to evaluate a computer’s capabilities quickly, it is important to know how CPUs are internally structured, how different CPUs differ each other and how CPU speed is evaluated. Figure below shows the processor and memory architecture of a computer system.
i
Figure: processor and memory architecture of a computer system
Control unit : explained early…..
ALU: explained early……
Instruction set :
Similarly all output information to be transferred to an output device is found in this register.
Other registers are: A, B, C, D, E, H, L….etc
Execution of an instruction by CPU
Steps:
- The control unit takes the address of the next program instruction to be executed from program control register and reads the instruction from corresponding memory address into the instruction register of control unit.
- The control unit then sends the operations part and address part of the instruction to the decoder and memory address register respectively.
- The decoder interprets the instruction and accordingly the control unit sends signals to the appropriate unit that needs to be involved in carrying out the task specified in the instruction.
- As each instruction is executed, the address of the next instruction to be executed is fetched or loaded into program control register automatically and steps 1 to 4 are repeated.
Or
in small steps:
The CPU executes each instruction in series of small steps.
The steps are as follows:
- fetch the next instruction from memory into the instruction register
- change the program counter to point to the following instruction
- determine the type of instruction just fetched
- if the instruction uses a word in a memory, determine where it is
- fetch the word , if needed into a CPU register
- execute the instruction
- go to step 1 to begin executing the following instruction
this sequence of steps is frequently referred to as the fetch-decode- execute
cycle.
Instruction execution in detail
1.Fetchingtheinstruction…(fetchcycle)
The next instruction is fetched from the memory address that is currently stored
in the program counter (PC), and stored in the instruction register (IR). At the
end of the fetch operation, the PC points to the next instruction that will be read
at the next cycle.
2.Decodetheinstruction……(decodecycle)
The decoder interprets the instruction. During this cycle the instruction inside
the IR (instruction register) gets decoded.
3.In case of a memory instruction (direct or indirect) the execution phase will be in the next clock pulse. If the instruction has an indirect address, the effective address is read from main memory, and any required data is fetched from main memory to be processed and then placed into data registers(Clock Pulse: T 3 ). If the instruction is direct, nothing is done at this clock pulse. If this is an I/O instruction or a Register instruction, the operation is performed (executed) at clock Pulse.
4.Executetheinstruction…..(execute cycle ) The control unit of the CPU passes the decoded information as a sequence of control signals to the relevant function units of the CPU to perform the actions required by the instruction such as reading values from registers, passing them to the ALU to perform mathematical or logic functions on them, and writing the result back to a register. If the ALU is involved, it sends a condition signal back to the CU. The result generated by the operation is stored in the main memory, or sent to an output device. Based on the condition of any feedback from the ALU, Program Counter may be updated to a different address from which the next instruction will be fetched.
The cycle is then repeated.
