WJEC Computing CG3

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Why is switching necessary in a networked computer system?
Switching prevents all data from being sent to all parts of a network.
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Why is data usually transmitted in digital form rather than in analogue form?
Digital transmission is preferred as it is less likely to suffer corruption/degradation/interference.
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Describe Circuit Switching
Circuit Switching: Path is set up between the sender and receiver, all data follows the same path in order, path cannot be used by any other data.
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Describe Packet Switching
Packet Switching: Data is split into packets, Each packet may be transmitted via different routes, Packets may arrive out of order and are re-assembled.
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Advantages of Packet Switching over Circuit Switching
Better security as it is very difficult to intercept, Makes more efficient use of data lines as there is no waiting during gaps, Less likely to be affected by network failure because multiple paths used.
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Explain what is meant by the term router and describe the function of a router in a computer network.
A router is a device in a network which holds information about the addresses of computers in the network, and can direct data to the correct computer.
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State the items of data you would expect a packet to contain.
The actual data, Source Address, Destination Address, The order number of the Packet, Control Signals/bits, Error control signals/bit
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Explain the term Serial Transmission, and give one advantage.
Data is sent one bit at a time along the same data line. Advantage: Requires only two wires compared with 8/16 in parallel, serial can travel longer distances than parallel.
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Explain the term Parallel Transmission, and give one advantage.
All bits in a byte are sent simultaneously along separate lines. Advantage: Transmission is faster than serial transmission.
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Describe what is meant by Simplex
Data transmission is possible in one direction only.
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Describe what is meant by Half-Duplex
Data transmission is possible in both directions, but only in one direction at a time.
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Describe what is meant by Full-Duplex
Data transmission is possible in both directions simultaneously.
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Why do computer systems sometimes need to deal with data in analogue form?
Because many input devices (e.g. microphone) produce an analogue signal.
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Giving an example, explain why are network protocols necessary.
Protocols are necessary to specify data formats, and to enable devices to communicate with each other. E.g. Linking a printer to a computer.
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Describe what is meant by a data collision on a network. What should happen once a data collision is detected?
Data collision occurs when two sets of data are detected on the network simultaneously. Once detected, a computer waits for a random time then sends again.
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Explain what is meant by the term web log, and outline how a politician might use a web log.
A web log is a set of entries on the world wide web. A politician could add items to keep their readers up to date with current political news.
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Briefly describe one feature which any web page include to make it easy to use.
All links should be correct.
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Explain what is meant by the term interrupt in a computer system.
An interrupt is a signal generated by a device/software which requires attention / attempts to cause a break in execution. Afterwards, execution of the original routine may continue or another high priority interruput may be serviced.
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Describe situations where an interrupt might arise.
Operating system request, e.g. clock. Input/output process. Run time error, e.g. array index exceeded. User request, e.g. mouse click. Hardware condition, e.g. printer fault. Security update. Power failure.
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Examples of an interrupt generated by software.
Run-time error e.g. array index exceeded. Input/output request. Software fault.
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Examples of an interrupt generated by hardware.
Hardware fault / Power failure. Peripheral, e.g. keyboard key pressed.
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Describe what will happen if, while one interrupt is being processed, another with a higher priority arises.
1. The OS suspends current interrupt routine. 2. It runs the new higher priority interrupt routine. 3. The OS returns to original interrupt routine and continues.
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Describe a situation in which a high-priority interrupt may be generated.
Impending data loss. Impending hardware/software failure. Detection of imminent power failure.
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Explain the term buffering, and give an example.
Buffering: Using an area of memory to store data while transferring to/from a peripheral / waiting to be processed. Example: Single buffering in a keyboard.
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Explain the term double buffering, and give an example.
Double Buffering: While one buffer is being emptied, another can be filled. Example: Double buffering in a printer.
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State the advantage of double buffering over single buffering.
Double buffering avoids waiting for the data transfer / allows for faster / more efficient data transfer.
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Explain the term Ready, in context of a process state.
The process is waiting to be executed by the processor.
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Explain the term Blocked, in context of a process state.
The process is waiting for some event, such as input/output/error.
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Describe in detail a suitable application which might use batch processing, making it clear why batch processing is the most appropriate approach in this case.
Payroll. Time sheets are collected for e.g. a monthly operation. Process is carried out with no user interaction. Batch processing may avoid using computer resources at times when demand is high (i.e. uses at off-peak hours).
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Describe what is meant by the term scheduling in a computer's operating system.
Scheduling is the method by which the processor time is allocated in a multi-programming/multi-tasking operating system.
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Giving an example, explain what is meant by the term multi-tasking.
Multi-tasking occurs when more than one task or application is available to the user at the same time. Example: Could be using a spreadsheet and an internet browser at the same time on a single computer.
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Explain what is meant by the term multiprogramming and explain in detail the functions provided by the operating system which allow the system to achieve multiprogramming.
Method of benefitting from speed of CPU compared with slow peripherals|More than one job in memory at same time|More than one job processed at same time|Time-slice is the amount of time allocated to each job by the OS
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Multiprogramming Q Continued...
Scheduling allocates time-slices to jobs|Polling is the sequential checking of jobs so that each gets its appropriate share of time|Partitioning is a division of computer's memory for different jobs|It is achieved by the use of interrupts
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Multiprogramming Q Continued........
One job is halted if waiting for a peripheral - other jobs can now be processed|Paging jobs in and out makes better use of memory|This promotes efficient use of CPU
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Describe the difference between rounding and truncation in computer programs.
Rounding: Number is approximated to nearest whole number/tenth/hundredth etc. i.e. 34.7 = 35. | Truncating: Number is approximated to whole number/tenth/hundredth etc. NEARER ZERO i.e. 34.7 = 34
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Describe a serious error that can arise as a result of rounding in computer programs.
Successive use in further calculations may seriously increase inaccuracy.
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Describe why rounding is generally more appropriate than truncating.
In many cases, rounding tends to give an answer closer to the original number.
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Describe what is meant by the term data structure.
A data structure is a group/set/collection of related data items/elements.
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Give examples of data structures.
Stack, Queue, Array, Linked List, Binary Tree, String, Record
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Why are data structures useful in computing?
Best way of organising data related to a real problem.
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State the essential features of an array.
An array: is a set of data elements of the same type|has its elements accessed via indexes|has a fixed number of elements.
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Explain what is meant by the term record.
A record is a set of data items all related to a single individual/entity. It can contain data of different types.
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Explain what is meant by the term linked list.
A linked list is a set of data elements, where each element contains: 1. The data itself. 2. A pointer to the next element.
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Describe one benefit and one drawback of using a linked list compared with using an array.
Benefit: New iterms can be inserted into a linked list without rearranging all other elements|If programmed dynamically, uses memory more efficiently. Drawback: A linked list is more complex to program
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State a computer application where a queue is the most appropriate data structure, and explain why.
A printer queue / a keyboard buffer. Why: Because the natural/desirable processing order is first in first out,
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Describe a computer application where a stack is the most appropriate data structure to use, and explain why it is the most appropriate data structure in this case.
Subprogram return addresses / Undo & Back. | Why: Idea of winding back nesting of subprograms. AND Because the natural/desirable processing order is first in last out.
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State one advantage and one disadvantage of using a binary tree to store ordered data compared with using an array.
Advantage: Faster to search / add a value. | Disadvantage: More complex to program/process.
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Give one drawback of using 3D arrays.
More complex to program/process.
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Name and describe the problem often caused by an arithmetic shift to the left.
Overflow: The resulting number is too large to be contained in the 8 bits available.
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What is the difference between an arithmetic shift and a logical shift?
An arithmetic shift maintains the sign bit; a logical shift does not.
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Describe, using an example, how the two's complement of a binary number is derived.
1. Flip the bits. 2. Add one. | e.g. 00001000 -> 11111000
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Explain why hexadecimal is often used to represent binary numbers.
It acts as a shorthand for binary.
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Describe advantages of storing numbers in integer form rather than floating point form.
Numbers are stored completely accurately | Takes up less storage space | Allows for an exact representation of zero.
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Describe advantages of storing numbers in floating point form rather than integer form.
Numbers with decimals can be stored.
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What sign bit means positive/negative?
0 = +, 1 = -
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Why is it useful to use character sets such as ASCII?
Enables computer/software systems to communicate with each other easily.
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Describe in detail what is meant by an indexed sequential file.
Records are stored in key sequence order. An index allows data to be accessed directly.
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Give the main advantages of using an indexed sequential file compared with both standard sequential and random access files.
Over standard sequential: Allows for faster access because you can move DIRECTLY to individual records. | Over random access: Allows for faster sequential processing of the whole file.
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Why is data held on computer files often encrypted?
Encrypted data is more secure - cannot easily be read without the decryption key.
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Explain what is meant by the term archived and explain why archiving is necessary.
Archiving is the process of storing data which is no longer in frequent use. It is held for security/legal/historical/backup. Frees up resources on the main computer system.
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Explain what is meant by a sequential file.
In a sequential file, records are stored in key sequence order.
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Describe how records are added and how records are deleted from a sequential file.
Addition of record: Record added (probably at end of file), Then file sorted. | Deletion of record: Make a new copy of the records until in the correct place for deletion, Do not copy the record to be deleted, Continue until the end of the file.
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What is the purpose of encryption and how does it operate?
Files are often encrypted to safeguard the data, by making it impossible to read without the encryption key.
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Describe in detail how a random access file operates.
The physical location of the record is calculated from the data in the key field|The calculation is carried out by a hashing algorithm|A data collision occurs when two data items are hashed in the same location
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Random Access Q Continued...
In this case there needs to be an overflow area where the latest data is stored, usually in a linear structure|When there are many items in the overflow area, access may become slow|In which case a new hashing algorithm and a larger file is required.
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Advantages of Sequential Files.
Easier to program. | Particularly suitable if access only ever needs to be sequential.
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Advantages of Indexed Sequential Files.
Allows faster access than sequential. | Avoids overheads of random. | If only sequential access is required, should be faster than random.
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Advantages of Random Files.
Allows very fast access irrespective of position in file - very suitable for large files which need this sort of access.
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Name 2 methods of defining algorithms.
Structured English | Flowcharts | Pseudocode
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Explain what is meant by the term parameter, and explain how passing parameters by reference works.
A parameter is a variable than can be passed to/from the procedure. When passing by reference, the address of the required data is passed to the procedure.
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Name and describe one other method by which you can pass parameters to and from a procedure.
The other method is by value: Where a local copy of the data is created for the procedure and discarded later.
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Give one disadvantage of passing parameters by reference, compared with by value.
Passing by reference may lead to unintended side effects where the parameter has its value changed in the main program as well as in the procedure.
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Briefly describe how a bubble sort operates. Name a recursive sort algorithm which is usually faster than a bubble sort.
A pass is made through the data, comparing each value with the following one and swapping them if necessary; a number of passes are made until the data is in order. Faster: Quicksort.
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Outline how an insertion sort operates.
Item is compared with sorted list to find correct position. Items in the sorted list are moved up/down to enable new items to be added in the correct place.
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State why quicksort is often used, and briefly describe how a quicksort operates.
Often used: It is the fastest/most efficient type of sort algorithm. Description: 1. A pivot is selected 2. Produce two new lists of smaller and larger numbers. 3. Repeat above points on new sub lists until sorted.
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Explain the term recursive algorithm.
A recursive algorithm is one which calls itself. It must also have a 'base case' to allow it to terminate.
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Explain what is meant by the term algorithm
An algorithm is a finite set of rules to solve a specific problem.
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COMPILATION: Describe Lexical Analysis, making clear its output.
1. Input stream is broken down into tokens. 2. Comments and unneeded spaces are removed. 3. Error messages are generated if appropriate.
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COMPILATION: Describe Syntax Analysis, making clear its output.
1. Symbol table/dictionary is produced. 2. Tokens are checked for fit to the grammar, using BNF-type rules. 3. If not the case, error message(s) are produced.
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COMPILATION: Describe Semantic Analysis, making clear its output.
1. Checks that all variables are declared and used. 2. Checks that e.g. real values are not being assigned to integers. 3. Checks that operation is legal/no mixed mode arithmetic.
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COMPILATION: Describe Code Generation, making clear its output.
1. Machine code is generated. 2. Code optimisation may be employed.
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Explain, using an example, what is meant by the term standard module.
A standard module is one which carries out a common task and can be used for a standard situation in many programs. Examples: Print function / Input validation
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Give the benefits of using standard modules.
No need to write again as has already been writted. | Less likely to have errors because it has already been tested/used. | Likely to be efficient as may have been written by experts in the field.
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Describe why a natural language (such as English or Welsh) is not suitable for defining the syntax of a programming language.
Natural languages are normally ambiguous/imprecise.
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What is the function of a translation program when applied to high-level source code in a computer?
Converts the source code into machine code.
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What is the function of an assembler program in a computer? In what circumstances might a programmer choose to use an assembler program?
An assembler program converts a program written in assembler code into machine code. Reason: If the application requires maximum computer efficiency.
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Discuss the features of visual programming languages.
1. High level language. 2. Might be used for developing in a GUI. 3. Such applications lend themselves naturally to VP. 4. Enables production of buttons & icons.
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Discuss the features of 4th generation languages.
1. High level language. 2. Useful in a database query situation. 3. Many features such as query, manipulation features. 4. May have report and application generators. 5. May attempt a natural language interface. 6. Requires less programming skill.
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Discuss the features of Object Oriented languages.
1. Uses objects/classes etc. - includes both data and associated processing. 2. Enables production of buttons/icons etc. 3. Uses features such as inheritance, encapsulation.
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Discuss the features of computer packages with programming capabilities.
1. Requires less programming skill. 2. Cheaper since most facilities are provided by package. 3. Can import/export to/from other packages. 4. Less likely to contain errors 5. More help available in package. 6. Users probably familiar with interface.
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Explain what is meant by the term relocatable code in a computer system.
Relocatable code is program code which can be moved from one area of computer memory to another.
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Explain why it is often useful to use subprogram libraries.
They contain well-tested utilities and can be used by any user, avoid re-writing.
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Explain why it may be useful to compiler modules of a program separately.
Modules do not need to be compiled each time they are required.
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Explain the role of a link loader and give one example of a linking error.
A link loader is a software tool which combines already compiled modules into the executable program. Example of error: The number or type of parameters provided is wrong.
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Describe in detail the operation (features) of a debugger.
Program trace: Enables the coder to see the progress through the program. Breakpoint: Allows the coder to temporarily halt execution in order to see the value of variables at that point. Variable watch: Lists the value of a variable at specific point
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Debugger Q continued...
Store dump: Lists the entire contents of memory at a specific point. Error diagnostics: Provision of messages relating to errors in the program.
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Explain why careful version control is necessary when developing computer programs.
Any amendments must be made to the most recent versions.
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Explain the different between a procedural and a non-procedural program.
Procedural: Carrying out actions/calculations. | Non-procedural: To do with facts/rules/making queries.
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What is the purpose of BNF?
BNF is used to describe unambiguously the syntax of a programming language.
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Give one reason why it is useful to standardise computer languages.
Program written in a certain language on one computer is likely to run easily on a different computer.
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Explain the meaning of Parallel Processing, and give an example where it may be useful.
The simultaneous use of several processors/cores to perform a single task. Example: Where an extremely large calculation is being carried out.
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Explain the difference between an interpreter and a compiler.
An interpreter translates each line of code of the source program, and then executes it. | A compiler translates a whole program prior to execution.
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Describe and give an example of the relevant situation of Special Purpose Languages.
Desc: Used to simulation, control applications etc. Why: Time analysis elements
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Other cards in this set

Card 2

Front

Why is data usually transmitted in digital form rather than in analogue form?

Back

Digital transmission is preferred as it is less likely to suffer corruption/degradation/interference.

Card 3

Front

Describe Circuit Switching

Back

Preview of the front of card 3

Card 4

Front

Describe Packet Switching

Back

Preview of the front of card 4

Card 5

Front

Advantages of Packet Switching over Circuit Switching

Back

Preview of the front of card 5
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