(4.7) Fundamentals of computer organisation and architecture Flashcards
The role of the processor
- Executes program instructions in order to run applications
The role of the main memory
- Includes ROM and RAM to store program instructions and frequently used data
- Faster than secondary storage, so storing frequently used data in RAM or ROM helps the processor to execute instructions quickly
The role of buses (general)
- Series of parallel wires that connect internal components of a cs, allowing signals to be passed between them
- No. of wires of bus = width
Has a direct relationship to the number of bits that can be transferred simultaneously by the bus
The role of the address bus
- Used to transport memory addresses, specifying where data is to be send to or recieved from
- Increasing the width of the address bus increases the range of addresses that it can specify, hence increasing the amount of addressable memory
- Adding a single wire doubles the number of addressable memory locations
1 wire in address bus = 2^1 =2 addressable memory locations
2 wires = 2^2 =4 addressable memory locations
3 wires = 2^3 = 8 addressable memory locations
The role of the data bus
- Sends data and instructions to and from the different components of the cs
- Increasing the width of the data bus increases the volume of data that can be transferred at one time
The role of the control bus
- Used to carry control signals that regulate the operation of the cs
- Carries the computers clock signal
The role of the I/O controllers
- (Input/output controllers) are hardware that control the communication of data between the processor and external hardware devices (e.g. keyboards, mice, monitors)
Harvard architecture and where its typically used
Harvard architecture
* Processor uses two seperate memory locations (one for instructions, one for data)
* In this way, it can give each piece of main memory different characteristics, e.g. memory used for instructions could be read-only so that instructions cannot be altered
Harvard architecture is often used in embedded systems such as digital signal processing
Von neumann architecture and where its typically used
Von neumann
* both instructions and data are stored together in the same memory
* These systems often perform worse because of this
Von neumann architecture is often used in everyday general-purpose computer systems, e.g. laptops and smartphones
The concept of addressable memory
The concept of storing data and instructions in memory with discrete, unique adresses
The stored program concept
Machine code instructions stored in main memory are fetched and executed serially (in order) by a processor that performs arithmetic and logical operations
The processor: Arithmetic logic unit (ALU)
The processor
The ALU performs arithmetic and logic operations (e.g. addition, subtraction, AND, XOR, etc)
The processor: Control unit (CU)
The processor
The control unit is responsible for controlling the components of the processor and the fetch-execute cycle
The processor: The clock
The processor
The system clock generates a timing signal which changes at a regular frequency
The signal is used to synchronise communication between the components of the processor and the rest of the cs
The processor: General purpose registers
The processor
Registers in general are small storage locations used to hold data temporarily, with high read and write speeds
General purpose registers can be used as storage for any data that is required by instructions during execution
The processor: Program counter register (PC)
The processor
Used to hold the memory address of the next instruction to be executed in the fetch-execute cycle
The processor: Current instruction register (CIR)
The processor
Holds the instruction that is currently being executed by the processor
The processor: Memory address register (MAR)
The processor
Stores the memory address of a memory location that is to be read from or written to
The processor: Memory buffer register (MBR)
The processor
Also called the memory data register (MDR).
Holds the contents of a memory location (so the data) that has been read from or data that is to be stored
The processor: Status register (SR)
The processor
Contains a number of bits, the values of which can change to indicate the occurance of an interrupt
The Fetch-Execute cycle: Fetch
The next instruction to execute is retrieved from main memory
1. The content of the PC is copied to the MAR
- The content of the MAR is transferred to main memory by the address bus
- The intruction is sent from main memory to the MBR by the data bus
- The PC is incremented by one
- The content of the MBR is copied to the CIR
The Fetch-Execute cycle: Decode
The fetched instruction is decoded
1. The content of the CIR is decoded by the control unit
- The decoded instruction is split into two parts: opcode (Specifies the operation to carry out) and operands (data on which the operation is performed)
The Fetch-Execute cycle: Execute
The instruction is carried out
1. Any data required by the instruction that isn’t present in registers is fetched
- The instruction is carried out
- Results of any calculations are stored in general purpose registers or main memory
Processor’s instruction set
a processor’s instruction set is the group of instructions that it carries out
* Each type of processor has its own instruction set, so instructions for one processor may not be compatible with other processors
* Instructions are usually stored in machine code and consist of two primary parts: opcode and operands (at least 1)
* Opcode specifies the type of operation to be carried out (e.g. additon, subtraction, logical shifting, etc)
* Operands are the pieces of data on which the operatio is performed
Immediate and direct addressing modes
Immediate addressing:
The value specified in the operand is treated as the actual data
Direct addressing:
The value specified in the operand is the address of the data. (address being a register/main memory)
Assembly: load
Assembly language operations
LDR, Rx, < memory reference >
load value < memory reference > into Rx
Assembly: store
Assembly language operations
STR, Rx, < memory reference >
store value held in Rx into < memory reference >
Assembly: add
Assembly language operations
ADD, Rx, Ry, < operand >
Add < operand > to Ry and store in Rx
Assembly: subtract
Assembly language operations
SUB, Rx, Ry, < operand >
subtract < operand > from Ry and store in Rx
Assembly: Move (copying)
Assembly language operations
MOV, Rx, < operand >
copy < operand > into register x
Assembly: compare
Assembly language operations
CMP Rx, < operand >
compare value in Rx with < operand >
Assembly: branch (unconditional)
Assembly language operations
B < label >
always branch to < label >
Assembly: branch (conditional)
Assembly language operations
B< condition > < label >
branch to intruction at < label > if the last comparison meets < condition >
conditions:
* EQ - equal to
* NE - not equal to
* GT - greater than
* LT - less than
Assembly: logical AND
Assembly language operations
AND Rx, Ry, < operand >
perform bitwise logical AND between value in Ry and < operand > and store result in Rx
Assembly: logical OR
Assembly language operations
ORR Rx, Ry, < operand >
perform bitwise logical OR between value in Ry and < operand > and store result in Rx
Assembly: logical NOT
Assembly language operations
MVN Rx, < operand >
perform bitwise logical NOT on value < operand > and store result in Rx
Assembly: logical XOR
Assembly language operations
EOR Rx, Ry, < operand >
perform bitwise logical XOR between value in Ry and < operand > and store result in Rx
Assembly: logical shift left
Assembly language operations
LSL Rx, Ry, < operand >
logically shift left the value in Ry by the value in < operand > and store the result in Rx
Assembly: logical shift right
Assembly language operations
LSR Rx, Ry, < operand >
logically shift right the value in Ry by the value in < operand > and store the result in Rx
Assembly: halt
Assembly language operations
HALT
stops execution of program
The role of interrupts and interrupt service routines (ISRs)
An interrupt is a signal sent to the processor by another part of the cs requesting attention.
* volatile environment is saved on stack
* Source of interrupt is identified
* Appropriate interrupt service routine (ISR) is called
* Volatile environment restored
Factors affecting processor performance
Processor perfomance factors
multiple cores:
* tasks split up between cores, increased speed of execution
cache memory:
* increased amount of very fast memory, close to processor increases execution speed
clock speed:
* processor executes more instructions per unit time
word length:
* processor can handle more data per instruction
address bus width:
* address more unique memory locations
data bus width:
* move more data around per unit time
Principles of operation of a barcode reader
Consist of:
* Laser light source
* lens
* photodiodes
* mirror
Operation:
1) Mirror directs light from laser onto barcode
2) Light reflected by barcode passes through the lens (light portions reflect the most light, the dark sections absorb incident light), and is incident on the photodiode which turns light into electrical charge
3) The electrical charge is measured and processed to form a digital signal, representing the content of a barcode
Principles of operation of a digital camera
Consist of:
* a lens that focuses light onto a sensor
* a shutter which controlls the path of light between the lens and the sensor
Operation:
1) sensors convert incident light into electrical charge
2) charge builds up in each cell, representing a pixel in the image (in colour cameras there are multiple cells for each pixel)
3) when the photo is taken, the charge in each of the cells is measured and converted to a digital value which is processed by the camera and stored as a digital image
Principles of operation of a laser printer
Consists of:
* laser light source
* mirror
* drum
* toner roller
* fusers
Operation:
1. The drum is electrically charged all over before the laser is directed at it’s surface by a mirror
2. Areas which the laser shines are discharged, leaving an impression of the page, in electrical charge, on the drum
3. The toner roller dispenses charged toner(a plastic powder) onto the drum, which is attracted to the charged portions of the drum
4. The toner is applied to the paper by the drum
5. paper is heated by fusers, fixing the toner to the paper
Principles of operation of an RFID
(Radio frequency identification)
Method of transferring information wirelessly between a tag and a reader.
Consists of:
* a chip which contains a small amount of memory
* attached to the chip is a coil of wire which acts as an antenna
Operation:
1. The reader emits radio waves, picked up by the tag’s antenna
2. The power induced powers the chip
3. The chip uses its antenna to emit a radio wave, containing the info on the chip
4. the wave is picked up by the reader, which decodes the info and returns it to a computer
Principles of operation of a hard disk drive
Operation:
1. Has circular platters, made of magnetic material
2. above each platter, an actuating arm hovers, on which a read/write head is mounted
3. The read/write head changes the magnetic polarity of parts of the platter
4. Data is written in concentric tracks, which are each divided into sectors
5. The platter rotates thousands of times per minute, allowing for good read and write speeds
Capacity can be increased by:
* adding more platters
* decreasing the width of tracks
Principles of operation of an optical disk
Read-only disks
1. Store data using lands and pits
2. pits burnt into the disk by a high-power laser
3. a low-power laser is passed over the surface of the disk:
- when the laser is incident on a land, the light reflects back onto a photodiode
- when the laser is incident on a pit, the light is scattered in different directions
\4. the patern of reflections and scatters is converted into a digital signal
Recordable and rewritable disks
1. a pattern of reflections and scatters is created by a dye on the disk’s surface
* no dye: laser reflected off the surface of the disk
* dye: laser absorbed by the dye and not reflected
Recordable optical disks use photosensitive dye which changes from opaque to transparent under a high-power laser
Rewritable disks use a phase-change dye that can be converted multiple times between transparent and opaque, depending on the temperature it is heated to
