Computer Hardware system

Computer Hardware system

The physical parts that make up a computer, which are interconnected electronic devices are called hardware. These devices are capable of computing and manipulating information. Examples of hardware are C.P.U, input devices like keyboard, mouse, output devices like printer, monitor. Therefor Hardware constitutes :

1. Processor Unit/CPU
2. Input Devices
3. Output Devices

1. Central Processing Unit

CPU or processor is also often called the brain of the computer. All  major  calculations   and  comparisons   performed   by a computer  are carried out inside its CPU. CPU  is also responsible  for activating  and controlling  the operations  of other units of a computer system.  Hence,  no other single component  of a computer  determines  its overall  performance  as much as its CPU. The CPU is fabricated as a single Integrated chip (IC), and is known as microprocessor. The microprocesoor is plugged in to the motherboard of the computer. CPU consists of :

i. Arithmetic Logic Unit (ALU),

ii. Control Unit (CU) and

iii. set of registers which are temporary storage areas for holding data and instructions.

i. Arithmetic Logic Unit

ALU  of a CPU  is the place  where  actual  execution  of instructions  takes  place  during  data  processing  operation. When  the  control  unit  encounters   an  instruction   that  involves   an  arithmetic   operation   (such  as  add,  subtract, multiply,  divide)  or a logic  operation  (such  as less than,  equal  to, greater  than)  it passes  control  to the ALD.  The  ALU  has  some  special   purpose  registers   (whose   functions   are  described   in  a  later subsection)   and  necessary   circuitry   to  carry   out  all  arithmetic   and   logic  operations    included   in  the  set, of instructions  supported  by the CPU. For example,  control  unit might  load two numbers  into ALU registers  and then tell the ALU to add them (an arithmetic operation)     or to check  if they are equal  (a logical operation).

ii. Control Unit

The  control  unit  of the CPU selects  and  interprets  program  instructions   and then coordinates their  execution.   It has  some  special  purpose  registers   (whose   functions   are described  in a  later  subsection)   and  a decoder  to  perform  these  activities.   The  special  purpose  registers  named instruction   register   and  program    control   register,   respectively,    hold   the   current   instruction   and  the   next instruction  to be executed  and  in this  way  help  the  control  unit  in instruction  selection.  On the  other  hand,  the decoder  has  necessary  circuitry  to decode  and  interpret  the meaning  of every  instruction  supported  by the CPU. Each  instruction   is  accompanied   by  microcode   –  very  basic  directions   that  tell  the  CPU  how  to  execute  the instruction.

Although,  the control  unit does not perform  any actual  processing  of data,  it acts as the central  nervous  system  for all other components  of computer.  It manages  and coordinates  the entire  computer  system  including  its input and output  units. It obtains  instructions  from a program  stored  in main  memory,  interprets  the instructions,  and issues signals that cause other units of the system to execute  them.

iii. Registers

As  instructions.  are  interpreted   and  executed  by a computer’s   CPU,  there  is movement   of  information   between various  units of the computer.  In order to handle  this process  satisfactorily  and to speed  up the rate of information transfer,  a number  of special memory  units  called registers  are used. These  registers  are used to hold  information on a temporary  basis and are part of the CPU (not main memory). The length  of a register  equals  the number  of bits it can store. Hence,  a register  that can store  8 bits is referred  to as an 8-bit register.  Most  CPU s sold today  have 32~bit or 64-bit registers.  The length  of registers  of a computer  is sometimes  called  its word size.   The bigger the word  size, the faster a computer  can process  a set of data. With all other  parameters,  being  same,  a  CPU  with  32-bit  registers   can  process  data  twice  as  large  as  one  with  16-bit registers. Although,   the number   of  registers   varies  from  computer   to  computer,   there  are  some  registers   common  to  all computers.  Functions  of these registers  are described  below

a) Main Address Register (MAR)

It holds  the address  of the active  memory  location.  It is loaded. from program  control  register  when an instruction  is read from memory.

b) Memory Buffer  Register(MBR)

It holds   the  contents    of the  accessed    (read/written)     memory  An  instruction    word   placed   in this  register    is transferred     to  instruction    register.   A data  word   placed   in this   register    is  accessible    for  operation    with   accumulator     register . A word   to  be  stored   in a memory    location    must   be  transferred     first  to  MBR   from   where   it  is written   in memory.

c) Program Control Register (PC)

It holds the address of the next instruction to be executed. Normally, the instructions of a program are stored in consecutive memory locations and are executed in sequence unless a branch instruction is encountered. A branch instruction is an operation that caused a transfer to a non-consecutive instruction. The address part of a branch instruction is transferred to PC register to become the address of the next instruction.

d) Accumulator Register

It holds   the  data   to  be  operated    upon,   the  intermediate     results     and  the results   of processing,     It is used  during   execution    of  most   instructions.     Results   of arithmetic    operations are  returned    to  accumulator     register    for  transfer   to  main   memory   through   the  memory    buffer   register; In  many   computers,    there  are  more  than  one  accumulator     registers.

e) Instruction Register

It holds the current    instruction    being       As  soon  as the  instruction    is stored  in this  register,    the  operating   part  and .the  address    part  of  the  instruction   are separated.       The address     part  often    instruction    is sent  to  MAR   while   its operation    part  is sent  to control unit   were    It  is decoded    and   interpreted.     Finally,    the   control    unit   generates     command     signals    for’ execution    of the  task  specified    by the  instruction.

f) Input/Output Register

It   is   used    to   communicate       with    input/output      devices.     All    input information   such  as instructions  and  data  are  transferred     to this  register   by  an  input  device.   Similarly all  output   information    to be transferred    to an  output   device   is found   in this  register.

Execution of an instruction by CPU during program execution normally involves following steps:

1. 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.
2. The control unit then sends the operation part and address part of the instruction to the decoder and memory address register, respectively.
3. The decoder interpret the instruction and accordingly the control unit seeds to the appropriate unit that needs to be involved in carrying out the task specified in the instruction. For example, it is an arithmetic or logic operations, signal is sent to ALU. In this case the control unit also ensures that the data corresponding to the address part of the instruction is loaded in a suitable register in ALU before the signal is sent to ALU. The ALU performs the necessary operation on the data and signals the control unit as soon as it has finished.
4. As each instruction is executed, the address of the next instruction to be executed is loaded into the program control register automatically, and steps 1 to 4 are repeated.