Processor Components:
- Central Processing Unit: The CPU consists of many components, each with their own
role to play
- The control unit: coordinates the activities of all other components – control signals
are sent along the control bus between the control unit and the other components
- Buses: consist of a series of connectors which send signals between the internal
components. The system bus consists of the control bus, the data bus and the
address bus.
- Control signals:
o Memory read: causes data from the addressed location to be placed on the
data bus
o Memory write: causes data on the data bus to be written to the addressed
location
o Bus request: indicates that a device is requesting use of the data bus
o Bus grant: indicates that the CPU has granted access to the data bus
- The Arithmetic Logic Unit (ALU): the problem-solving part of the CPU, used for
logical, arithmetic and shift operations.
o The accumulator: results from the ALU need to be stored somewhere.
Instead of writing back to slow memory, registers are used. For simplicity, we
assume the CPU has a general use register called the accumulator
- Dedicated Registers:
o Program Counter (PC): holds the memory address of the next instruction to
be executed
o Current Instruction Register (CIR): holds the current instruction (split into
opcode and operand)
o Memory Address Register (MAR): holds the address in memory where the
processor is required to fetch or store data from or to
o Memory Data Register (MDR): temporarily holds data moving between the
processor and main memory
o Accumulator: to hold the intermediate results of an instruction
Storage Devices:
- RAM and ROM:
o When applications are loaded, they are copied into RAM from secondary
storage
o When the computer is turned on, bootstrap loads the OS into RAM
o ROM is used to store the bootstrap program and is non-volatile
- Virtual memory: acts as an extension to RAM in secondary storage. If there are too
many applications open, RAM runs out of space and virtual memory is used.
- Secondary storage is storage not directly connected to the CPU (main memory (eg
RAM) has busses which connect directly to the CPU). Secondary storage is needed
for long-term, non-volatile storage.
- Optical storage:
, o Binary stored as variations of heights on the disk’s surface
o To read data, light is shone on the surface of the disk. Light that hits a pit
reflects differently than light that hits a land
o Data is written by burning pits into the disks using a laser
▪ Fairly reliable and durable but can be easily damaged (scratches, etc)
▪ Good portability, easy to transport and carry
▪ Has a small capacity
o CD formats:
▪ CD ROM: Read-only, used for software distribution
▪ CD-RW: Rewriteable, can be written to, erased and written to again,
used for short- or medium-term backup
▪ CD-R: Recordable: can be written to once, used for copying a game,
software or video files and documents
▪ DVD and BluRay are high-capacity disks which can be used to store
films
- Magnetic storage:
o Uses read/write heads that contain electromagnets
o Parts of the surface are magnetised or demagnetised and the heads control
this and read what state they are in
▪ Large capacity and cheap to buy
▪ Usually reliable but not durable
▪ Generally not very portable
▪ Can be noisy and generate heat
- Solid state storage:
o Form of flash memory (non-mechanical)
o Consist of digital circuits that retain data which can then be changed
▪ Very fast but very expensive
▪ Very reliable as they are non-mechanical
▪ Don't use defragmentation
▪ Generally have a smaller capacity than magnetic storage
▪ Silent
- Cloud storage:
o Where data is stored in multiple servers in a remote location (generally use
magnetic and solid-state storage)
o Operates through an API
▪ Versions are kept, so is durable
▪ Service may not be reliable
▪ May be cheaper for large companies, but generally expensive for
smaller companies
Processor Performance:
Words:
, - Memory is divided up in equal units called words. Word length is usually 8, 16, 32 or
64 bits
- Each word has a separate memory address
- The width of the address bus determines the maximum possible memory addresses
of the system
- Maximum number of memory addresses = 2^width of the address bus
Data bus:
- The data bus is bi-directional as data can be sent both ways along the bus
- The width of the data bus is defined by the number of wires or lines it contains
- If the data bus is the same width as a computer word, data can be transferred to and
from memory in a single operation
Format of instructions:
- Assembly language is very closely related to machine code
- Generally, there is a one-to-one correspondence between a machine code
instruction and its assembly language equivalent
- The architecture of a computer, including the word size and the width of the address
bus determine the format of a machine code instruction for a particular type of
processor
Factors affecting performance:
- Clock Speed
o The FDE cycle is triggered by the clock pulses of the system clock
o The faster the clock speed, the faster a computer can fetch, decode and
execute instructions
o A series of regular ON/OFF signals are used to synchronise the operations of
the processor components
o Actions are usually carried out on the rising edge of the clock
o Actions each take a fixed number of cycles to complete
- The number of cores in the processor
o Each core is theoretically able to process a different instruction at the same
time with its own FDE cycle, making a quad-core computer two or four times
faster than a single core computer
o However, the software must be written to take advantage of all four
processors
o Parallel processing: each core can work concurrently on different parts of the
same task. Since instructions are processed sequentially, this is not always
possible
- The amount and type of cache memory
o Cache is a small amount of super-fast memory that stores data and
instructions that have recently been used by the processor.
o Level 1 cache is smaller but faster than Level 2 cache
, o It is split into instruction cache and data cache, so that data and instructions
can be fetched simultaneously. The more cache memory a computer has, the
more likely it is that it will not have to fetch the next instruction or data from
RAM, as it will already have been loaded into the superfast cache memory
from which it can be retrieved much more quickly
Pipelining:
- This is a technique used to improve performance by overlapping stages of the FDE
cycle or by breaking down the stages in an arithmetic instruction
- An instruction enters the pipeline, and as soon as one stage has been completed,
another instruction enters the pipeline. A third instruction then enters before either
of the others is completed.
Types of Processors:
Von Neumann:
- Instructions and data are stored in a common main memory and transferred using a
single shared bus
Harvard architecture:
- Separates the data and instructions into separate memories using different buses
- Program instructions and data are no longer competing for the same bus
- Harvard principles are used with specialist embedded systems and digital signal
processing where speed takes priority over the complexities of design
Advantages of von Neumann architecture:
- Almost all general purpose computers are based on von Neumann’s principles
- It simplifies the design of the control unit
- Data from memory and from devices are accessed in the same way
Von Neumann Harvard
Used in PCs, laptops, servers and high Used in digital signal processing,
performance computers microcontrollers and in embedded systems
such as microwaves and watches
Data and instructions share the same Instructions and data are held in separate
memory. Both use the same word length memories which may have different word
lengths. Free data memory can’t be used
for instructions and vice versa
One bus for data and instructions is a Separate buses allow parallel access to data
bottleneck and instructions
One bus is simpler for control unit design Control unit for two buses is more
complicated and expensive
CISC and RISC:
- Central Processing Unit: The CPU consists of many components, each with their own
role to play
- The control unit: coordinates the activities of all other components – control signals
are sent along the control bus between the control unit and the other components
- Buses: consist of a series of connectors which send signals between the internal
components. The system bus consists of the control bus, the data bus and the
address bus.
- Control signals:
o Memory read: causes data from the addressed location to be placed on the
data bus
o Memory write: causes data on the data bus to be written to the addressed
location
o Bus request: indicates that a device is requesting use of the data bus
o Bus grant: indicates that the CPU has granted access to the data bus
- The Arithmetic Logic Unit (ALU): the problem-solving part of the CPU, used for
logical, arithmetic and shift operations.
o The accumulator: results from the ALU need to be stored somewhere.
Instead of writing back to slow memory, registers are used. For simplicity, we
assume the CPU has a general use register called the accumulator
- Dedicated Registers:
o Program Counter (PC): holds the memory address of the next instruction to
be executed
o Current Instruction Register (CIR): holds the current instruction (split into
opcode and operand)
o Memory Address Register (MAR): holds the address in memory where the
processor is required to fetch or store data from or to
o Memory Data Register (MDR): temporarily holds data moving between the
processor and main memory
o Accumulator: to hold the intermediate results of an instruction
Storage Devices:
- RAM and ROM:
o When applications are loaded, they are copied into RAM from secondary
storage
o When the computer is turned on, bootstrap loads the OS into RAM
o ROM is used to store the bootstrap program and is non-volatile
- Virtual memory: acts as an extension to RAM in secondary storage. If there are too
many applications open, RAM runs out of space and virtual memory is used.
- Secondary storage is storage not directly connected to the CPU (main memory (eg
RAM) has busses which connect directly to the CPU). Secondary storage is needed
for long-term, non-volatile storage.
- Optical storage:
, o Binary stored as variations of heights on the disk’s surface
o To read data, light is shone on the surface of the disk. Light that hits a pit
reflects differently than light that hits a land
o Data is written by burning pits into the disks using a laser
▪ Fairly reliable and durable but can be easily damaged (scratches, etc)
▪ Good portability, easy to transport and carry
▪ Has a small capacity
o CD formats:
▪ CD ROM: Read-only, used for software distribution
▪ CD-RW: Rewriteable, can be written to, erased and written to again,
used for short- or medium-term backup
▪ CD-R: Recordable: can be written to once, used for copying a game,
software or video files and documents
▪ DVD and BluRay are high-capacity disks which can be used to store
films
- Magnetic storage:
o Uses read/write heads that contain electromagnets
o Parts of the surface are magnetised or demagnetised and the heads control
this and read what state they are in
▪ Large capacity and cheap to buy
▪ Usually reliable but not durable
▪ Generally not very portable
▪ Can be noisy and generate heat
- Solid state storage:
o Form of flash memory (non-mechanical)
o Consist of digital circuits that retain data which can then be changed
▪ Very fast but very expensive
▪ Very reliable as they are non-mechanical
▪ Don't use defragmentation
▪ Generally have a smaller capacity than magnetic storage
▪ Silent
- Cloud storage:
o Where data is stored in multiple servers in a remote location (generally use
magnetic and solid-state storage)
o Operates through an API
▪ Versions are kept, so is durable
▪ Service may not be reliable
▪ May be cheaper for large companies, but generally expensive for
smaller companies
Processor Performance:
Words:
, - Memory is divided up in equal units called words. Word length is usually 8, 16, 32 or
64 bits
- Each word has a separate memory address
- The width of the address bus determines the maximum possible memory addresses
of the system
- Maximum number of memory addresses = 2^width of the address bus
Data bus:
- The data bus is bi-directional as data can be sent both ways along the bus
- The width of the data bus is defined by the number of wires or lines it contains
- If the data bus is the same width as a computer word, data can be transferred to and
from memory in a single operation
Format of instructions:
- Assembly language is very closely related to machine code
- Generally, there is a one-to-one correspondence between a machine code
instruction and its assembly language equivalent
- The architecture of a computer, including the word size and the width of the address
bus determine the format of a machine code instruction for a particular type of
processor
Factors affecting performance:
- Clock Speed
o The FDE cycle is triggered by the clock pulses of the system clock
o The faster the clock speed, the faster a computer can fetch, decode and
execute instructions
o A series of regular ON/OFF signals are used to synchronise the operations of
the processor components
o Actions are usually carried out on the rising edge of the clock
o Actions each take a fixed number of cycles to complete
- The number of cores in the processor
o Each core is theoretically able to process a different instruction at the same
time with its own FDE cycle, making a quad-core computer two or four times
faster than a single core computer
o However, the software must be written to take advantage of all four
processors
o Parallel processing: each core can work concurrently on different parts of the
same task. Since instructions are processed sequentially, this is not always
possible
- The amount and type of cache memory
o Cache is a small amount of super-fast memory that stores data and
instructions that have recently been used by the processor.
o Level 1 cache is smaller but faster than Level 2 cache
, o It is split into instruction cache and data cache, so that data and instructions
can be fetched simultaneously. The more cache memory a computer has, the
more likely it is that it will not have to fetch the next instruction or data from
RAM, as it will already have been loaded into the superfast cache memory
from which it can be retrieved much more quickly
Pipelining:
- This is a technique used to improve performance by overlapping stages of the FDE
cycle or by breaking down the stages in an arithmetic instruction
- An instruction enters the pipeline, and as soon as one stage has been completed,
another instruction enters the pipeline. A third instruction then enters before either
of the others is completed.
Types of Processors:
Von Neumann:
- Instructions and data are stored in a common main memory and transferred using a
single shared bus
Harvard architecture:
- Separates the data and instructions into separate memories using different buses
- Program instructions and data are no longer competing for the same bus
- Harvard principles are used with specialist embedded systems and digital signal
processing where speed takes priority over the complexities of design
Advantages of von Neumann architecture:
- Almost all general purpose computers are based on von Neumann’s principles
- It simplifies the design of the control unit
- Data from memory and from devices are accessed in the same way
Von Neumann Harvard
Used in PCs, laptops, servers and high Used in digital signal processing,
performance computers microcontrollers and in embedded systems
such as microwaves and watches
Data and instructions share the same Instructions and data are held in separate
memory. Both use the same word length memories which may have different word
lengths. Free data memory can’t be used
for instructions and vice versa
One bus for data and instructions is a Separate buses allow parallel access to data
bottleneck and instructions
One bus is simpler for control unit design Control unit for two buses is more
complicated and expensive
CISC and RISC:
