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Computer science > Program construction > Flashcards

Program construction Flashcards

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1
Q

What are regular expressions?

A
  • Lexical analyser looks through code for specific patterns of letters, numbers or symbols. These are known as regular expressions
  • Once recognised they are applied to source code + a stream of tokens is generated
  • Can be represented as diagrams
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2
Q

What is lexical analysis?

A
  • Views source code and replaces keywords and symbols with tokens
  • spaces and comments are removed and tokens have replaced keywords, variables etc.
  • Invalid tokens pull up error
  • Once completed stream of tokens sent to syntax analysis
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3
Q

What are abstract syntax trees?

A
  • As code is parsed, a tree structure is formed
  • Abstract syntax trees (AST) are syntactically correct and only readable in one way
  • Operators are at root node
  • Numbers are operands
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4
Q

What its semantic analysis?

A

Tries to imply meaning of code
Checks against aspects that are not related to syntactic form or that are not easily determined during parsing
Typical semantic errors:
-Type mismatch
-Undeclared variable
-Reserved identifier misuse
-Multiple declaration of variable in scope
-Accessing an out of scope variable
If error is found then report generator displays an error message.

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5
Q

What is code mapping?

A

Matching common patterns from AST to block of machine code
Repeated until all parts of AST are mapped
Allocate blocks of code to registers

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6
Q

What is code optimisation?

A

Code improvement e.g constant folding(e.g replace x= 4*4 with x= 16)
Remove redundant code
Tries to improve code by making it consume less resources and deliver high speed

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7
Q

What are the stages of compilation?

A

Lexical analysis
Syntax analysis
Semantic analysis
Code generation and optimisation

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8
Q 
What are translation and execution errors?
#Needs more (look at P.C test)
A

Translation:
Occurs when program is being compiled
e.g Syntax, linking and semantic errors

Execution(runtime):
Occurs when program is running
e.g. division by 0, reading past end of file, stack overflow ect.

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9
Q

What is machine code generation and optimisation?

A
  • Final phase of compilation
  • Once AST is created + checked against rules of language, code generation can then happen
  • When program can be lexically and syntactically analysed without errors, machine code can be generated
  • Tweaks code so it can run as quickly and with as little memory as possible.
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10
Q

What is the assembler?

A
  • Assembler is a program that translates assembly code into machine language
  • Relatively simple process as in general one assembly code statement translates into one machine code statement
  • It can allocate memory to variables
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11
Q

Compare compiler and interpreter

A

Compiler

  • Source code hidden from other developers- intellectual property protected
  • Compiled code will only run on CPUs with same instruction sets and matching OS
  • Executable files can easily be distributed on CD or over internet

Interpreter:

  • Source code shown to all developers and users- intellectual property risk
  • will run on any platform from same source code base interpreter is required to run
  • Systems need to be set up before code can run. Can include installing library files as well as interpreter
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12
Q

Disadvantages of interpreter?

A
  • Must have interpreter running on computer to be able to run the program
  • As source code must be compiled each time, programs can sometimes run slowly
  • Have to rely on interpreter for machine level optimisation rather than programming them yourself
  • Developers can view source code which means they could use your intellectual property
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13
Q

Advantages of interpreter

A
  • Source code has to be written only once, then can run on any computer with a compiler
  • Can easily inspect contents pf variables as running
  • Can test code more efficiently as well as being able to test single lines of code
  • code can be run on many different types of computers and OS
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14
Q

What is interpreter?

A
  • Converts high level language program to machine code line by line
  • Error found will be reported immediately to user + halts further execution
  • Any machine coed produced while interpreter is running will not be saved meaning it will have to be interpreted each time it wants to run code
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15
Q

What are compilers?

A
  • Convert high level language source code into machine code
  • Produces standalone executable file which can be released to others without the need for further compilation
  • Protects intellectual property of developers
  • Code is converted all at once
  • If compiler encounters syntax error, it cannot translate statement so no object code is produced
  • When errors are found they are placed in error file and error message displayed to user.
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16
Q

What is syntax analysis?

A
  • Checks against allowable patterns / in an acceptable order
  • Known as parsing the code
  • String of tokens split into phrases
  • Phrases are checked in accordance with rules (syntax) of programming language
  • If a match cannot be made then the report generator generates an error
17
Q

What is intermediate code?

A
  • Code that is compiled and can be run on different machines/platforms
  • Needed as normal compiled code has incompatibilities when run on different machines
  • Used with Virtual machines (A programming environment that allow s a program written to run on other types of machine without any changes being necessary)
  • If code is translated directly then a different translator would be used for each high-level language and each version of machine code. (as each processor architecture requires a different machine code)
18
Q

What is symbol table construction(Lexical analysis)?

A
  • When a label/variable is encountered it is mapped to. a memory address
  • Variables map to a memory location
  • labels map to a position where instruction jumps to
  • Data structure used to store keyword, variables etc. which were removed during lexical analysis.

Computer science (5 decks)

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