4.7 Computer organisation and architecture Flashcards
Name the 3 main internal components of a computer system (three box model)
- Processor
- Main memory
- Input/output (I/O) controllers
What is a system bus? What sort of information can it represent?
- A system bus is a set of parallel wires connecting independent components of a computer system
- They are used to pass signals between components, which can represent data, address or control information
What is data bus used for? What’s its direction?
The data bus is used to transport data between components.
The data bus is bidirectional
What is address bus used for? What’s its direction?
The address bus is used to address memory and I/O (input output) locations.
The address bus is unidirectional
What is control bus used for? What’s its direction?
The control bus is used to transport control signals between components.
The control bus is ← bidirectional →
3 examples of control signals
- Clock signals
- Memory read
- Memory write
What are I/O controllers?
The circuits that connect a system bus and an I/O device. (Providing correct currents/voltages etc.)
Difference between volatile and non-volatile memory
- Volatile maintains its data only while the device is powered.
- Non-volatile retains stored information even after power is removed
Example of volatile memory
RAM (Random Access Memory)
Example of non-volatile memory
ROM (Read Only Memory)
What is the stored program concept?
- A program has to be resident in main memory to be executed
- Machine instruction codes are fetched sequentially and executed one at a time by the processor
4 differences between Von Neumann and Harvard architecture
VN: Uses one memory unit for both instructions and data
H: Has separate memory units for instructions and data
VN: Uses one set of buses for both instructions and data
H: Has separate buses for instructions and data
VN: Commonly used in general purpose computers
H: Commonly used in embedded systems (e.g. digital signal processing)
VN: Enables a more flexible use of main memory, enabling the processor to run a variety of programs that aren’t necessarily known in advance
H: For systems with a predetermined use, instruction memory can be implemented as ROM which protects the programs from hacking
What happens in each of FDE stages? (short answer)
Fetch - the next instruction is copied from main memory into the processor
Decode - the instruction gets decoded based on the processor’s instruction set
Execute - The instruction gets executed (carried out)
5 steps for fetch in FDE
- Contents of the PC is copied into the MAR
- The address bus is used to transfer this address to main memory
- The instruction held at that address is transferred via the data bus into the MBR
- The contents of the MBR are copied into the CIR
- The PC is incremented to hold the address of the next instruction to be executed (not always +1 e.g. if word length is two bytes)
What happens in decode stage of FDE?
- The instruction held in the CIR is decoded by the control unit
- (by checking its binary value correlates to an operation in its instruction set)
What is the instruction set? (2*)
- The set of binary codes for the machine operations that a processor has been designed to perform.
- It is specific to the processor
3 examples of what might happen in the execute stage of FDE
- The ALU is used for maths and logic instructions
- The MAR and MBR are used to access data from main memory for
LOADandSTOREinstructions - The general purpose registers and status register are updated during this step
What is the purpose of main memory?
Main Memory holds the instructions and data currently being used by the processor.
What is the ALU?
The Arithmetic Logic Unit is the part of the CPU that is responsible for arithmetic operations (e.g. addition, subtraction, binary shifts) and logic comparisons (e.g. Or / And / Not)
What is a register?
Extremely fast, very small, on chip memory for temporary storage of binary values
What does the program counter do?
Holds the address of the next instruction to be executed
What does the MAR do?
- Holds the address of the memory location that data will be read/written to
- The MAR is the processor’s direct connection to the address bus for accessing main memory
What does the MBR do?
- Temporarily stores data read from or written to main memory
- The MBR is the processor’s direct connection to the data bus for accessing main memory
- Holds the instruction in the fetch stage before it is copied into the CIR
What is the role of the CIR in the FDE cycle?
- Stores the current instruction
- The control unit performs the decode step on the CIR by checking its binary value correlates to an operation in its instruction set
What information is stored in the status register?
- The status register contains many bits (binary flags) that are set (1) or cleared (0) depending on the result of an instruction
- Each bit represents a boolean value, for example if an overflow error has occurred
What is the purpose of general purpose registers?
- Have no specific purpose
- Usually used as temporary storage while performing arithmetic operations
- Often store the operands of processor instructions
What is a processor interrupt?
- A signal that is sent to the processor to request immediate attention
- When the processor receives an interrupt, it suspends what it is doing and runs the programs associated with the interrupt
4 causes of processor interrupts
- A hardware device that has data ready to process
- A hardware device that has completed a task it was asked to do
- A pre-set timer expiring
- A hardware failure occurring that needs to be addressed
What is a ISR?
- ISRs are small programs designed to respond to each interrupt
- The operating system has interrupt service routines to handle each interrupt
- ISRs are provided by the OS or I/O device drivers
What needs to happen for an ISR to be run safely?
The processor’s volatile environment needs to be saved while the ISR is running
What is the volatile environment (of a processor)?
The volatile environment is a snapshot of the information needed to execute a process from its current instruction (e.g. the values of register such as the program counter).
6 factors affecting processor performance
- number of cores
- cache memory
- clock speed
- word length
- address bus width
- data bus width
What is clock speed?
The frequency at which the system clock ticks
What is word length?
Word Length is the number of bits in a word
(A word is the fixed size unit of data used across the computer)
What is address bus width?
- The number of bits that can be used to form an address of a memory location
- Or the number of parallel wires allocated to the address bus
What is the data bus width?
- The number of bits that can be transferred to or from memory in one operation
- Or the number of parallel wires allocated to the data bus
How is fetch time affected if the data bus isn’t big enough for a piece of data?
- the piece of data will be split into two parts
- meaning fetch time will be doubled.
What is cache memory? What does it contain?
Cache memory is a small capacity of very fast memory that sits on or close to the processor.
Cache memory contains:
1. Copies of data from frequently used memory locations
2. Pre-fetched instructions
3. Data to be written to main memory
How does increasing cache memory size affect processor performance?
- Increasing cache size means more instructions and data can be stored in cache
- So main memory needs to be access less frequently
- Cache memory is faster to access than main memory
- So throughput and response time is increased
How does increasing the number of cores affect processor performance?
- Multiple tasks can be run at the same time
- A single task can be split over multiple processors
- Many programs are now written to optimise the use of parallel processing
- This increases throughput
- Although the efficiency depends on the nature of the required task
- However, increasing the number of cores greatly increasing complexity
What is a barcode?
- A barcode is a sequence of parallel black and white bars that encodes binary information
- Sometimes a check digit is added to the end of the barcode for validation
Operation of a barcode reader
- 💥 Light from a laser illuminates the barcodes
- 🪞 More light is reflected from the white areas than from the black bars
- 🦠 The reflected light is captured by photoelectric cells
- ⚡️ The photoelectric cells generate a set of electrical pulses that correspond to the black and white stripes in the barcode.
- These pulses are processed and converted to a binary number that represents the code
- 🚨 Most barcode readers indicate that the barcode has been read successfully, for example by sounding a beep or showing a green light
What are QR codes? How are they different from regular barcodes?
- Two-dimensional barcodes
- They have a higher storage capacity
- QR codes are read using images, not lasers
Operation of a digital camera
- The shutter opens, allowing light to enter the camera through the lens
- 🔍 The lens focuses the light onto an image sensor
- An image sensor is an array of millions of photosites (light-sensitive elements)
- ⚡️ Each photosite produces an electrical signal and each signal represents a current
- The photosites only measure the intensity of the light. To determine the colour of each pixel, a ‘Bayer filter’ is used
- 📊 The Bayer filter provides an array of red, green and blue filters
- The final value for each pixel is derived from its own colour intensity and those of its immediate neighbours through the application of a demosaicing algorithm
Operation of a laser printer
- 🥁 The print drum is coated in a positive static charge
- 💥 A laser beam is directed at the print drum
- The laser is modulated (turned on & off)
- ↩️ The laser reverses electric charge on drum, where image should be dark / black
- ➕ The toner is given a positive charge
- The charged drum picks up toner
- 🌈 There are four different toner cartridges, one for each colour (cyan, magenta etc)
- The toner is transferred from drum to paper
- 🛼 Toner is fused to paper by heated rollers
What does RFID stand for and what is its purpose?
RFID (radio frequency identification) allows data to be transmitted wirelessly over radio waves.
What are the two parts to an RFID system?
Tag
Reader
What is in a passive RFID tag?
- chip, which contains a small amount of memory
- antenna
How does an RFID reader work?
- The reader emits radio waves which are picked up by the tag’s antenna
- The power induced in the tag’s antenna from these waves is enough to power the chip
- The chip uses its antenna to emit its own radio wave which contains the information held on the chip
- This wave is picked up by the reader which decodes the information and returns the information to a computer
Advantage of RFID over barcodes or digital cameras
The use of radio signals means that the system does not require a line of sight between the tag and the reader
What is a passive RFID tag? Range?
- An RFID tag that doesn’t contain a power supply
- The device is powered by radio energy transmitted by the reader
- Range of up to 1m
What is an active RFID tag? Range?
- An RFID tag that has a small battery within the tag
- The tag will transmit its identifier at regular intervals
- Range of up to 200m
4 reasons secondary storage is needed
- Main memory is volatile, so secondary storage needed to store files that are needed multiple times
- Secondary storage can be used to store larger files, as it usually has a much higher capacity than main memory
- Secondary storage can be used for virtual memory
- Main memory is expensive
What type of storage are Hard Disk Drives?
Magnetic storage
What are the parts of a HDD?
- at least one metal platter
- a spindle which spins the platter
- a read-write head on an actuator arm
Operation of HDD
- The read-write head is used to store and retrieve data on the metal platters
- 💿 Each platter is made up of concentric tracks which have many polarised dots
- Binary data is represented on the surface of the metal platters with magnetised dots
- 🧲 Magnetised/Unmagnetised represent 0s/1s
4 pros of HDD
- High read-write access speed (but slower than SSD)
- Greater Capacity than SSD
- … at a lower cost per Gb than SSD
- Reliable and don’t degrade over time
4 cons of HDDs
- Not as portable as CD/DVD/USB memory sticks
- HDDs are more likely to fail as they are made up of lots of moving parts
- HDD’s are fragile (bumps and knocks damage disk)
- Slower access speeds that SSDs
What type of storage are Solid State Drives?
solid state storage / NAND flash memory
Operation of SSD
- SSDs use floating gate transistors that allow an electrical charge to be trapped, representing a 0 or a 1
- Transistors are organised into pages and blocks
- Blocks are made of many pages
- A whole block of data must be written at once
- Cannot overwrite pages
- Page must be erased before it can be written to
- In actual fact, technology requires whole block to be erased
4 pros of SSDs
- Faster data transfer (read-write) speeds than HDD
- No moving parts, so are more robust
- More portable than magnetic storage
- Smaller SSD devices available, e.g. SD cards and USBs
When is an SSD suitable?
- When files need to be read or changed frequently (e.g. in a server)
- Or when the system needs to be portable
2 cons of SSDs
- The number of times SSD devices can be written to is limited (hence unsuitable for long term archiving of data)
- Higher cost per Gb than HDD
What sort of storage are optical drives?
Optical storage
Name three optical discs (e.g. CD), in increasing capacity
- Compact Disc (CD)
- Digital Versatile Disc (DVD)
- Blu-ray Disc
3 types of optical drive (e.g. ROM)?
- ROM (read only)
-
R (recordable)
- Can be written onto once. Good for archiving data
-
RW (rewritable)
- Has a chemical dye layer on top of the reflective layer
- The chemical dyes allow the burning process to be reversed
How is an optical disc created? Operation of optical drive?
creation
- A special machine stamps pits and flats onto a reflective aluminium layer.
- The pits and flats represent binary values.
- Pits represent 0 and flats represent 1.
optical drive
1. The disc spins at constant linear velocity
2. Laser in disk drive is shone at disk
3. Light is reflected back from disk
4. The pits and lands reflect different amounts of light
5. Light sensor detects reflection and records 0s/1s
3 pros of optical discs
- Extremely light and portable
… so they are good for media distribution - Relatively cheap (for lower storage capacities)
- Reliable (unless scratched)
5 cons of optical drive
- Data can be ruined if disc is scratched
- Lower capacity than HDD and SSD
- Slow seek time
- Not all disks are rewritable
- Not compatible with very much anymore
Effect of increasing data bus width (3*)
- more bits can be transferred between main memory and the processor in one operation
- so fewer read/write operations are needed
- so there are fewer delays when retrieving data
Effect of increasing address bus width
- The processor can access a larger number of memory locations
- so the CPU doesn’t have to access secondary storage as often
- which reduces the reliance on virtual memory
- Increased scalability and increased throughput
Effect (and downside) of increasing word length
- Longer word length means registers can store more bits, so the processor can process more bits in one go
- Also, complicated value splitting is required if the operand size is larger than the word length
- however if word length is increased, more memory is wasted when storing small values
Effect (and drawbacks) of increasing clock speed
- Higher clock speed means more instructions can be executed per unit time
however - All machine code instructions take a certain amount of time to execute anyway
- And heat dissipation becomes more of an issue as clock speed increases
4 advantages of Harvard architecture
- ⏰️ Instruction and data can be accessed simultaneously
- 📝 Instruction and data memory can have different word lengths
- 🤖 Different technologies can be used to implement instruction and data memory
- 🧑💻️ For systems with a predetermined use, instruction memory can be implemented as ROM which protects the programs from hacking
Explain what happens when a processor interrupt occurs
- Volatile environment saved on stack
- Source of interrupt identified
- Appropriate ISR run
- Volatile environment resumed
