Section 5 - Computer organisation and architecture Flashcards
What are some examples of internal components?
- Processor
- Main memory
- Address bus, control bus, data bus
- I/O controllers
What are internal components?
Components inside the CPU
What is the role of the CPU?
It responds to and processes the instructions that drive the computer
What does the CPU contain?
The control unit, the arithmetic logic unit (ALU) and the registers
What is the role of the control unit?
It coordinates and controls the activities of the CPU, directing the flow of data. It accepts the next instruction, breaks down its processing into several sequential steps, manages its execution and stores the resulting data back in registers or memory
What is the role of the ALU?
It performs arithmetic and logical operations on data.
What are examples of arithmetic operations?
Addition, subtraction, multiplication and division
What are examples of logical operations?
Comparing data, shifts, AND/OR
What is the role of the system clock?
It generates a series of signals used to synchronize CPU operations. Some operations take multiple clock cycles
What are general-purpose registers?
Up to 16 special memory cells that operate at a very high speed in the CPU.
What is an accumulator?
Another word for a general purpose register, usually used when there is just 1 register in which to store the result of each calculation or logical expression
What are the different types of dedicated registers?
- Program counter
- Current instruction register
- Memory address register
- Memory buffer register
- Status register
What is the role of the program counter?
It holds the address of the next instruction to be executed
What is the role of the current instruction register?
It holds the current instruction being executed
What is the role of the memory address register?
It holds the address of the memory location from which some data or an instruction is about to be fetched from, or to which the data is to be written
What is the role of the memory buffer register?
It is used to temporarily store the data read from or written to memory
What is the memory data register?
Another name for the memory buffer register
What is the role of the status register?
It contains the bits that are set or cleared depending on the result of an instruction
Where do all arithmetic and logical operations take place?
Within registers
What is a bus?
A set of parallel wires connecting 2 or more components to a computer
How is the processor connected to the main memory?
By 3 separate buses (the address bus, the data bus and the control bus)
What is the role of the address and data buses?
When the CPU wants to access a particular main memory location it sends the address to memory on the address bus. The data is then returned to the CPU via the data bus.
What is the role of the control bus?
To transmit command, timing and specific status information between system components
What are some examples of control lines?
- Memory write
- Memory read
- Interrupt request
- Bus request
- Bus grant
- Clock
- Reset
What is the function of the memory write control line?
Causes data on the data bus to be written into the addressed location
What is the system bus?
The collective name for the address, data and control buses.
How does the system bus work?
Each bus is a shared transmission medium so only 1 device can transmit along a bus at one time
What direction do signals across the address bus travel?
Only 1 way (from the CPU to main memory)
What direction do signals across the data bus travel?
Both ways
What direction do signals across the control bus travel?
Both ways
What is the function of the memory read control line?
Causes data from the addressed location to be placed on the data bus
What is the function of the interrupt request control line?
Indicates that a device is requesting access to the CPU
What is the function of the bus grant control line?
Indicates that the CPU has granted access to the data bus
What is the function of the bus request control line?
Indicates that a device is requesting the use of a data bus
What is the function of the clock control line?
Used to synchronise operations
What is the function of the reset control line?
Initialises all components
Why is the control bus important?
It needs to ensure that access to, and use of, the data and address buses by the different components of the system does not lead to conflict
How does the data bus determine system performance?
The width of the data bus is the determining factor of how much data can be transferred at once
What is a word?
A fixed group of digits which is handled as a unit by the processor (different processors have different word sizes). Each word has it’s own specific address
How is memory divided up internally?
Into words
How does the address bus affect memory capabilities of the system?
The width of the address bus determines the maximum possible memory capacity of the system
What happens during I/O operations?
The address bus is used to address I/O ports
What is an I/O controller?
A device which interfaces between an input or output device, and the processor. Each device has a seperate controller, each connected to the control bus.
How does an I/O controller work?
They receive input input and output requests from the processor, before sending device-specific control signals to the device that they control. They also manage the data flow to and from the device
What does an I/O controller consist of?
It is an electronic circuit board consisting of 3 parts:
- an interface that allows the connection of the controller to the system or I/O bus
- A set of data, command and status registers
- An interface that enables the connection of the controller to the cable connecting the device to the computer
What is an interface?
A standardized form of connection that defines things like signals, number of connecting pins/sockets and voltage levels. E.g, USB connection
What does von Neumann architecture specify?
The basic components of the computer and the process in which a shared memory and bus is used for both data and instructions
What is the stored program concept?
Machine code instructions are fetched and executed serially by a processor that performs logical and arithmetic operations
How does the stored data concept work?
- A program must be resident in main memory to be executed
- The machine code instructions are fetched from memory one at a time, decoded and executed in the processor
How does Harvard architecture work?
There are physically separate memories for instructions and data. These can have different characteristics, e.g the instructions are held in read only memory but the data is held in read-write memory
Where is Harvard architecture used?
In embedded Digital Signal Processing systems
What is the advantage of Harvard architecture?
It can be faster than von Neumann architecture because data and instructions can be fetched in parallel instead of competing for the same bus
What is the fetch-execute cycle?
The sequence of operations involved in executing an instruction. The 3 phases are fetch, decode and execute
What happens in the fetch phase?
- The address of the next instruction is copied from the program counter to the memory address register. The address is sent via the address bus to main memory
- The instruction held at that address is returned along the data bus to the memory buffer register. Simultaneously, the content of the program counter is incremented so it holds the address of the next instruction
- The contents of the memory buffer register is copied to the current instruction register
What happens in the decode phase?
The instruction held in the CIR is decoded. The instruction is split into the opcode and operand, with the opcode being used to determine the type of instruction and what hardware to use to execute it. Additional data is fetched if necessary, and passed to the registers
What happens in the execute phase?
The instruction is executed and results are stored in the accumulator, general purpose registers or memory
What are the factors that affect processor performance?
- The number of cores
- The amount and type of cache memory
- The clock speed
- The word length
- The address bus width
- The data bus width
How does the number of cores affect processor performance?
Each core is theoretically able to process a different instruction at the same time. However, the software may not be able to take advantage of multiple cores.
What is cache memory?
Expensive, very fast memory inside or very near the CPU
How does cache memory work?
When an instruction is fetched from main memory it is copied into the cache so if it is needed again soon after it can be fetched from the cache (which is much quicker than the main memory). As the cache is filled up, unused data or instructions are replaced with more recent ones
How does the amount and type of cache memory affect processor performance?
Depending on the level of cache memory, some CPUs have more cache memory but it is slower (or vice verca)
How does clock speed affect processor performance?
All processor activities begin on a clock pulse. Therefore, the greater the clock speed, the faster instructions can be executed
What is the word size of a computer?
The number of bits that the CPU can process simultaneously
How does word length affect processor performance?
The higher the word length the better the processor performance as more bits can be processed simultaneously
How does address bus width affect processor performance?
If the bus width is lower the address, the address needs to be sent in 2 groups and therefore the processor is slowed down
How does data bus width affect processor performance?
The greater the data bus width, the more bits that can be transferred simultaneously
What is an interrupt?
A signal sent by a software program or hardware device to the CPU
Why might a software interrupt occur?
When an application program terminates or requests certain services from the operating system
Why might a hardware interrupt occur?
When an I/O operation is complete or an error occurs
What happens when the CPU receives an interrupt signal?
It suspends the execution of the running program/process and puts the value of each register onto the system stack. whilst an Interrupt Service Routine is called to deal with the interrupt.
What happens once an interrupt has been serviced?
The original values of the registers are retrieved from the stack and the fetch-execute cycle resumes
What is the processor instruction set?
It is the set of all instructions that are supported by its hardware. Each type of processor has it’s own instruction set
What is the instruction set of a typical computer?
- Data transfer
- Arithmetic operations
- Comparison operations
- Logical operations
- Branching
- Logic shifts
- Halt
What is the format of a machine code instruction?
It has an operation code, consisting of 2 bits defining the addressing mode and the basic machine operation. It also has an operand.
How many bits are there in the operand and opcode?
The number of bits allocated to each will vary according to the architecture and the word size of the particular processor
What is the opcode?
A set of binary digits representing the basic operation, and a 2-digit code representing the addressing mode
What is immediate addressing?
Where the operand is the actual value to be operated on
What is direct addressing?
Where the operand holds the memory address of the value to be operated on
What was the first language used to enter programs?
Machine code
What is assembly code?
A programming language using short mnemonics instead of binary codes. Each assembly language instruction translates into 1 machine code instruction
What is a parallel bus?
A set of parallel wires, with each wire carrying a single bit at a time
What is a serial bus?
A wire carrying data in a serial-bit form
What is an advantage of von Neumann architecture?
When a program is downloaded from a remote location it can be treated as data
What is a disadvantage of von Neumann architecture?
Computer viruses get treated as data while being downloaded but are programs and the CPU is tricked into executing them
