computer architecture Flashcards
What are the components of the three box model of a computer system?
The processor, main memory and I/O all connected by a bus
What is hardware?
Electrical circuits that a computer is assembled from, the platform on which software executes
What does the processor do?
Executes programs and supervises the function of other parts of the system
(essentially a very big finite state machine)
What is a microprocessor?
A processor on a single chip
What is a chip?
An integrated circuit (IC) encased in plastic
What is CPU?
Central processing unit, the processor as well as the main memory
However since microprocessors were invented it often refers to just a microprocessor
What is the main memory?
The memory directly addressable by the processor
What is another name for main memory?
Immediate access store (IAS)
What is the main memory used for?
Storing program instructions and data
How are values stored in main memory?
Each BYTE is stored in a separate MEMORY LOCATION
What is main memory made up of?
RAM chips
can also include one or more ROM chips
What is a memory location?
A separately addressable area of main memory
What does RAM stand for?
random access memory
What does random access mean?
Locations can be accessed in any order without having to cycle through each location from the start (ie not serial)
What is RAM?
volatile random access memory
What is volatile memory?
Data stored will be lost when power is turned off
What does ROM stand for?
read only memory
What is ROM?
non volatile main memory which cannot be written over once set up
What does EEPROM stand for?
electronically erasable programmable main memory
What is EEPROM?
Contents may be altered but writing is ~100x slower than reading
How are the main three components connected within a system?
Through the system/external bus
What does the system/external bus include?
- data bus
- address bus
- control bus
What is a computer bus (generally and physically)?
A set of parallel wires that connect independent components of a computer system in order to pass signals between them
What is the data bus?
A bi-directional bus, typically with 32 wires, transports data between components
What is the address bus?
A uni-directional bus, typically with 32 wires, addresses memory and I/O locations
What is the control bus?
A bi-directional bus, typically with 8 wires, transports control signals between components
What does I/O stand for?
input/output
What kind of signals does the control bus carry?
- clock signal for timing
- reset signal for initialising components
- Memory read for asserting that the memory loaction in use is being read
- memory write
- I/O used to show that processor wants to use an I/O not main memory (when they share memory addresses)
Which is the uni-directional bus?
Address bus
What are peripherals?
Any computer device not part of the CPU (processor and main memory)
How are peripherals connected to the processor?
The system bus is connected to an I/o controller and the I/O controller is connected to the device
What does I/O do?
Allows the CPU to communicate with peripherals
Give examples of peripherals
keyboard visual display unit magnetic disk CD-R and DVD-R drives network cards speakers etc
What does an I/O controller consist of?
- Electronics to interface the device to the system bus
- data, command and status registers
- Electronics to send control signals to the device connected to the computer
Why do peripherals have to be connected via control buses and not directly to the processor?
Each peripheral operates in a different way so a processor can’t be designed to be able to control so many types of peripheral. The processor doesn’t need to be redesigned each time there is a new peripheral
What is an I/O port?
Part of an I/O controller where the processor exchanges data with the peripheral device (data, command and status registers)
What is a register?
A storage location that can be read from and written to very fast
Inside the processor and I/O controllers
What is an I/O device?
A hardware unit that sends, receives or stores data by communicating with the processor and main memory through an I/O controller
What is an I/O controller?
An electronic circuit that connects a system bus to an I/O device, providing the correct voltages and currents for both
Give examples of external peripherals
mouse, keyboard, printer, monitor, memory stick, scanner etc
Give examples of an internal peripheral
CD-ROM drive
What is secondary storage?
permanent storage memory not directly connected to the processor
What is another name for secondary storage?
backing store
Give examples secondary storage
magnetic hard disk (not removable if internal)
CD-R, CD-ROM, CD-RW
DVD-R, DVD-ROM, DVD-RW
memory stick
What is the von Neumann bottleneck?
A large amount of the data transferred between processor and main memory is not useful, it is just the addresses of the data needed
What is a main memory address?
A unique numeric code given to each location in memory so it can be identified
What is the stored program concept?
A program must be resident in main memory to be executed
Machine code instructions are fetched one by one in sequence from the main memory and executed in the processor one by one
What are the two different types of stored program computer?
von neumann and harvard
What is von neumann architecture?
A single main memory is used for instructions and data, which travel along a shared bus
What is harvard architecture?
Separate instruction and data memories with their own buses
Which form of stored program architecture has a faster overall processing speed?
Harvard
The instructions and data can be transferred in parallel and are not competing for resources.
What is a microcontroller?
A complete computer (processor, memory, I/O) on one chip, usually used as embedded computer systems in other devices such as mp3 players, designed to form only a few functions
Which stored program architecture do microcontrollers normally use?
Harvard
What does the processor contain?
- program control unit
- arithmetic and logic unit
- registers
- internal clock
- internal buses
- logic gates
In a processor, what does the program control unit do?
Fetches program instructions from memory, decodes them and executes them
In a processor, what does the program control unit consist of?
- instruction decoder
- current instruction register
- program counter
- control circuits connecting to the system clock
What does the program counter register store?
the address of the next instruction to be fetched and executed
In a processor, what does the arithmetic and logic unit (ALU) do?
performs arithmetic and logical operations such as addition and subtraction and boolean logic.
Consists of circuitry
In a processor, what are logic gates used for?
flow of control
What is a general purpose register?
A register not assigned a specific role by the designer
Accessed with load, store, add etc
All may be used by programmers
What is a dedicated register?
A register assigned a specific role by the processor designer
Programmers can use some but not all
What does the stack pointer register do?
points to a stack holding return addresses, procedure/function parameters and local variables
accessed when procedure or function is called or an interrupt is serviced
What does the status register do?
Holds one bit ‘flags’ with condition codes to indicate the outcome of operations (eg. whether there is an overflow in an arithmetic operation). Also status information such as whether interrupts are enabled or disabled. Lots of instructions check values in the status register, involved in the flow of control
What does the accumulator register do?
Holds the result of the current set of calculations
What does the current instruction register do?
Hols the current instruction to be executed whilst it is decoded and excuted
What does the memory address register do?
Holds the address of the memory location currently being accessed
What does the memory buffer register do?
Holds the data item being transferred to or from the memory location being accessed
i.e everything entering or leaving the processor goes through here
What is clock speed of a processor?
The frequency in mega or gigahertz at which the processor executes instructions. One instruction will always be executed within a certain number of clock cycles (‘ticks’)
What is the system clock?
Every computer has a quartz controlled oscillator that supplies timing at a fixed rate. Timing signals are derived from here and used to decide clock rate and synchronise the function of other components
What is the internal clock?
Linked to the control unit, if a higher frequency than the normal clock rate is required then the system clock’s frequency can be raised inside the processor with frequency multiplying circuits
What is word length?
The number of digits in a binary word
What is bus width?
The number of signal wires or lines allocated to a bus
In a processor what does performance mean?
How quickly a task can be completed
How can performance of a processor be measured approximately?
Running standard programs on the processor and seeing how many machine operations are performed per unit time
What units are used for processor performance?
giga-ops per second (GOPS) or mega-ops per second (MOPS)
What effect does increasing the clock speed have on a processor?
The performance will be increased - more instructions for more ‘ticks’ can be carried out in the same amount of real time
Why are there limits on clock speed?
Higher clock frequencies generate more heat and this cannot be removed fast enough (especially when there are lots of transistors on a single chip)
What is a multicore processor?
Putting more than one processor on a single chip
processors = cores
How do multicore processors avoid overheating?
The cores operate at a lower frequency than a normal processor would
What are the advantages of multicore processors?
Multiple tasks can be run at the same time or the same task can be split across several processors
useful for improved multimedia work (moviemaker, photoshop)
What effect does increasing word length of registers have on a processor?
Performance is improved
If a word is too big for a register extra processing has to be done to split and store it across more than one register
What effect does increasing word length have on addressable memory?
Registers are used to store the binary codes for main memory addresses, being able to store a longer word means more addresses can be referenced by the registers, meaning that more of main memory can be used by the processor (is ‘addressable’)
What are typical word lengths in a modern processor?
32 and 64 bit
What effect does increasing bus width have on a processor?
performance is improved
The system bus acts as a bottleneck, if it is not wide enough to carry an instruction in one go it has to be sued multiple times, slowing performance
Increasing data bus width would also improve performance
increasing address bus width would lead to more addressable memory
