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53 Cards in this Set
- Front
- Back
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What is a computer? |
A device that can perform operations by executing a list of instructions. It is defined by the mathematical model of Turing machines. |
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What should a general-purpose computer be equivalent in capability to? |
A universal Turing machine - it should be able to execute programs with conditional branching. |
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What must computer memory have? (2) |
Discrete states and a way to change/measure the states (read/write memory). |
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What are the desirable properties of memory? (6) |
Low error rate, small size, high capacity, high speed, low power consumption, low cost. |
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What does CMOS stand for and what is it used to construct? |
Complementary-Symmetrical Metal Oxide Semiconductor. It is used to construct digital logic circuits and fast memory. |
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What transistors does CMOS technology use? (2) |
p-type and n-type. |
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What are logic gates? |
Binary devices (they perform a boolean function). |
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What is a transistor? |
A semiconductor that can be used for electronic switches or amplifying signals. They are small, light, robust & energy efficient. |
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What is a CMOS inverter? |
A binary device using complementary & symmetrical p-type and n-type transistors. |
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With a CMOS inverter, what happens when the 'in' voltage is HIGH? |
The (upper) p-type transistor is in a high resistance state and the (lower) n-type transistor is in a low resistance state, so the output is at the ground (LOW) voltage. |
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How is a NAND logic gate set up (in terms of transistors)? |
It has 2 p-type transistors in parallel, followed by 2 n-type transistors in series. |
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What does it mean to say "NAND and NOR gates are universal"? |
Using only gates of these two types, any boolean function can be implemented. |
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What is a flip-flop? |
A simple circuit that can store & retain binary state information. |
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How is a flip-flop set up? |
Two inverters are connected together, with a SET and a RESET input, so that the feedback maintains a constant state & the state can be changed by pulsing one HIGH and keeping the other LOW. |
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How are integrated circuits made? |
By depositing materials onto a wafer of silicon (up to 300mm across). |
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What is main memory used for? |
Holding instructions & data for running programs. |
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What is main memory made from? |
Transistors & capacitors on silicon - many million per mm^2. |
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How fast can main memory transfer data? |
10 Gbytes/s, with 50 nanoseconds latency. |
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What do SRAM and DRAM (respectively) need to store 1 bit? |
SRAM needs 6 transistors.DRAM needs 1 transistor & 1 capacitor. |
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What is the cache? |
It is a static RAM part of the main memory, located close to the CPU - a 'middle man' between the CPU (even faster speeds) and the main memory (slower speeds). |
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What are cache hits & misses? |
A cache hit is where the CPU requests data and finds it in the cache. A cache miss is where the data is not in the cache and so it must be accessed from main memory. |
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What is virtual memory? |
More addresses are available to a programmer than there is space in the working memory. Data is 'paged' back-and-forth between the main memory and the disk, depending on what is needed. |
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How large is a megabit and a megabyte? |
A megabit is 2^10 bits, while a megabyte is 10^6 bytes. |
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What is secondary storage? |
It is larger storage than primary storage, but it is not random access, and it is not volatile (when the computer is switched off, the data stored in secondary storage is retained). |
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What are the access times (order of) for secondary and tertiary storage? |
Milliseconds vs seconds. |
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How do you write to a magnetic disk? |
Logic signals in the computer produce an electric current, which produces a current when passed through the coil in the head. This changes the magnetisation of a patch of the alloy on the disk platter. |
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How do you read from a magnetic disk? |
The same head is used - As the platter rotates, the magnetised patches induce a current in the coil, which can then be interpreted by logic circuits in the computer. |
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How is a magnetic disk organised? |
Into tracks & sectors. The density of bits around a track is ~100,000 bits per cm & there are around 10,000 tracks per cm. |
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What is RAID? |
It stands for Redundant Array of Independent Disks & it is a storage virtualisation method whereby multiple magnetic disks appear as one storage device on a computer. |
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What does RAID provide? (4) |
Larger capacity, increased performance, better fail tolerance (no data is lost if a single disk fails) & improved availability (still usable if a single disk fails). |
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What techniques does RAID use? (3) |
Striping - each single bit/byte/block on a separate disk. Mirroring - identical data on 2+ disks. Parity - separate error correction information is stored with the data (may be on a separate disk). |
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What are the different versions of optical disk? (3) |
non-rewritable - data encoded as pits & bumps on the surface of the disk. rewritable (once) - data encoded as changes in the properties of a dye. rewritable - data encoded as the crystalline state of an alloy coating. |
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How do you read data from an optical disk? |
Light from a laser is scattered across the disk & is picked up by a photocell, which converts the light level into binary logic signals. The amount of light scattered depends on the surface the laser is pointing at. |
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How do you write data to an optical disk? |
A high-power laser controlled by logic signals is used to change the properties of the dye or alloy coating. |
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How is data encoded onto the disk? |
As a single spiral - the disk spins at different speeds to maintain a constant read speed. |
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What determines the smallest patch of the disk that the laser can illuminate? |
The wavelength of the laser. |
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How far across are the pits and bumps on a CD or DVD? |
Less than 1 micron. |
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How long is the spiral & how fast is it read on a CD? |
5.6km read at 120cm/s. |
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What is the capacity of a CD & how fast can it be read from/written to? |
~650Mbytes (storage) read/written at 150,000 - 175,000 bytes/s. |
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DVDs have smaller pits & bumps and a more precise laser. What is their capacity? |
4.7 Gbytes (single-layer) up to 17 Gbytes (XL quad-layer). |
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Blu-rays use a blue laser (shortest wavelength). What is their capacity? |
25Gbytes (single-layer) up to 128 Gbytes (XL quad-layer). |
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What is the data transfer speed of a blu-ray disk? |
4.5 - 72 Mbytes/s. |
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How is information read from a tape? |
Using an array of lights & photocells (linearly). |
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What is the read speed of a tape? |
10-2000 bytes/s. |
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What is magnetic tape? |
A long strip of coated plastic film pulled between 2 reels in a cartridge, across read/write heads. |
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How does a magnetic tape work? |
Patches of the coating are magnetised, either in long tracks parallel to the length of the tape (linear) or slanted across the tape (scanning). |
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What are magnetic tapes good for? (3) |
Off-site backups & archives (because of their 10+ year lifespan & good portability). Good for sequential access, bad for random access. |
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What are file systems? |
Virtual storage devices with characteristics determined by the underlying physical storage device, its logic circuitry and the OS. |
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What is a file (from a user's perspective)? |
A continuous sequence of bytes that can be interpreted in different ways by different programs. It can be moved between main memory and secondary storage devices. Each one has a name & associated metadata. They can be organised hierarchically in a directory. |
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What is a file (from an OS perspective)? |
A continuous sequence of bytes that may be scattered across many blocks or sectors on a hard drive, and that will not share a block with any other file (meaning files may take up more space than their size suggests). |
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How are blocks used by the OS? |
They are used to efficiently address and assemble all the information that constitutes a file. |
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How are blocks kept track of? |
The OS maintains a file index on disk - this is a table recording which blocks make up which files which allows efficient assembly of files & tracking of the free blocks on the disk. |
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What does organising a disk into larger blocks accomplish? |
An increase in speed but a reduction in capacity. |
