Thursday, May 8, 2025

Hardware

Computer Architecture
- CPU (Central Processing Unit) is central to all modern computer systems.
- CPU is often installed as an integrated circuit on a single microchip.
- CPU has the responsibility for the execution or processing of all instructions and
data in a computer application.
- CPU consists of:
• Control Unit (CU)
• Arithmetic and logic unit (ALU)
• Registers and buses
- Von Neumann Architecture: John Von Neumann developed concept of ‘stored
program computer’ which has been basis of computer architecture ever since.
- Main features:
• Concept of CPU

• CPU was able to access memory directly

• Computer memories could store programs as well as data

• Stored programs were made up of instructions which could be executed in

sequential order
- Components of CPU:
• Arithmetic and logic unit (ALU)

• Control Unit (CU): reads an instruction from memory. Address of location where
instruction can be found is stored in Program Counter (PC). Instruction is
interpreted through Fetch-Decode-Execute cycle. Signals are generated along
control bus to tell other components in computer what to do. Control unit ensures
synchronisation of data ow and program instructions throughout the computer.
System clock produces timing signals on control bus to ensure this vital
synchronisation takes place, else computer will crash. RAM holds all the data and

1
fl
 programs needed to be accessed by the CPU. RAM is also referred to as
Immediate Access Store (IAS). CPU takes data and programs held in backing store
(a HDD) and puts them into RAM temporarily. Key data needed by an application
will be stored to RAM temporarily to speed up operations.

• Registers:

• Current instruction register (CIR): stores the current instruction being decoded
and executed.

• Accumulator (ACC): it is used when carrying out ALU calculations. It stores data
temporarily during the calculations.

• Memory Address Register (MAR): stores address of memory location currently

being read from or written to.

• Memory Data Register (MDR): stores data which has just been read from memory
or data which is about to be written to memory.

• Program Counter (PC): stores the address where next instruction to be read can
be found.
- Computer memory is made up of a number of partitions. Each partition consists of
an address and its contents.
- The address will uniquely identify every location in the memory and the contents will
be the binary value stored in each location.
- Address is MAR and data is MDR
- System Buses:
• The address bus carries addresses throughout the computer system. Between the
CPU and memory, the address bus is unidirectional (bits can travel in one direction
only). This prevents addresses being carried back to CPU. The wider the bus, the
more memory locations that can be addressed at any given time.

• Data bus is bidirectional. Data can be carried from CPU to memory and vice versa
and to and from input/output devices. The data can be an address, , an instruction
or a numerical value. The wider the bus, the larger the word length that can be
transported. Larger word lengths can improve computer’s overall performance.

• Control bus is also bidirectional. It carries signals from the control unit to all the
other computer components. It is usually 8 bits wide. Does not need to be wide

2
 since only carries control signals. The control bus transmits timing signals ensuring
everything is fully synchronised.
- Fetch-Decode-Execute Cycle:
• Fetch:
1. Both data and instructions can be stored in MDR.

2. The next instruction is fetched from the memory address currently stored in
MAR.

3. It is then stored in MDR.

4. Contents of MDR are then copied to Current Instruction Register (CIR)

5. The Program Counter (PC) is then incremented by 1 so that the next

instruction can be processed.

• Decode: The instruction is then decoded so that it can be interpreted in the next
part of the cycle.

• Execute: The CPU passes the decoded instruction as a set of control signals to the
appropriate components within the computer system. This allows each instruction
to be carried out in its logical sequence.
- System Clock: The clock de nes the clock cycle that synchronises all computer
operations. By increasing clock speed, the processing speed of the computer is also
increased. Although speed of computer is increased, computer’s overall
performance is not necessarily better. Other factors to consider:

• Width of address bus and data bus

• Overclocking: Clock speed can be changed by accessing BIOS and altering the
settings. However using a clock speed higher than the computer was designed for
can lead to problems like serious overheating of CPU leading to unreliable
performance. Execution of instructions outside design limits can lead to seriously
unsynchronised operations (instruction is unable to complete in time before next
one is due to be executed) - the computer would frequently crash and become
unstable.

• Use of cache memories can also improve CPU performance. Unlike RAM, cache
memory is located within the CPU itself, which means it has much faster data
access times than RAM. Cache memory stores frequently used instructions and
data that needs to be accessed faster, which improves CPU performance. When

3
fi
 CPU wishes to read memory it will rst check cache then RAM. The larger the
cache size the better the CPU performance.

• The use of a di erent number of cores can improve computer performance. One
core is made up of an ALU, a control unit and the registers. Many computers are
dual core or quad core. The idea of using more cores lessens the need to
continually increase clock speeds. However, doubling number of cores doesn’t
necessarily double the computer’s performance since we have to take into account
the need for the CPU to communicate with each core which reduces overall
performance.
- Instructions are a set of operations which are decoded in sequence. Each operation
will instruct ALU and CU. An operation is made up of opcode and operand.
- Opcode informs CPU what operation needs to be done.
- Operand is the data which needs to be acted on or it can refer to a register in the
memory.
- Since computer needs to understand operation to carry it out there is limited number
of opcodes that can be used. This is known as the instruction set.
- All software running on a computer will contain a set of instructions which needs to
be converted to binary. The fetch-decode-execute cycle is the sequence of steps
used by CPU to process each instruction in sequence.

4
ff
fi