Program Construction

Question 1

What is an assembler

Answer 1

A simple translation program

Converts low level assembly language into machine code (binary)

Question 2

What is a compiler

Answer 2

Convert high level language (source code) into machine code (object code)

Question 3

What is an interpreter

Answer 3

Runs a high level language program without converting it into a binary executable

Takes a line of the high level program (source code), translates it and runs it, then runs the next line etc.

Question 4

Advantages of an assembler

Answer 4

Just as efficient as machine code

No extra unnecessary lines of code generated

Can be used for programs which require maximum efficiency/performance

Question 5

Advantages of a compiler

Answer 5

Although compilation may take mins, it produces an executable that will be fast to run over and over again

Executable program can be given to the end user without them needing to have a compiler

Very difficult for the end user to edit the code as it is in binary format

Shows a list of errors in the program which must be corrected before compilation (can also be disadvantage)

Question 6

Disadvantages of compiler

Answer 6

Compiling can be slow and tedious when developing a program (as each time program needs to be ran it has to be recompiled)

A compiler targets a particular platform and cpu instruction set e.g. programs compiled for android ARM processors won’t work on Intel x86 chips etc.

Commonly used for a number of languages e.g. python, javascript

Question 7

Advantages of an interpreter

Answer 7

No delay in waiting for compilation, program can begin to run immediately (ideal for program development)

Should run on any computer as long as there is an interpreter available for the language

Question 8

Disadvantages of an interpreter

Answer 8

Program will run much more slowly as each line of code needs to be translated each time before it can run

Needs an interpreter to run it

End user has access to source code (can be stolen)

Only finds errors when it gets to a line with an error in, so program could have errors thst are never discovered until experienced by the end user

Question 9

What is the ideal solution for compiler and interpreter

Answer 9

Assuming both exist for the language

Use an interpreter during development phase

When fully worked and tested, compile it for distribution to end-user

Question 10

What is intermediate code

Answer 10

Similar to machine code but not processor specific

Potentially allows code to be created to run on multiple architectures

Question 11

Advantage of intermediate code

Answer 11

Code runs very quickly as the conversion from intermediate code is much more straightforward than from high-level language

Question 12

What are the stages of compilation (compiler)

Answer 12

Source code -> lexical analysis -> syntax analysis -> semantic analysis -> code generation -> optimisation -> object code (machine code)

Lexical analysis - code generation stored in symbol table

Question 13

What does lexical analysis entale

Answer 13

Removes all comments and spaces + unnecessary characters

Tokenisation - converts source code into tokens which refer to entries in the symbol table

Add entries to the symbol table for variables, procedure names etc.

Does some basic error checking (e.g. flag up error if variable name was too long)

By the end of Lexical Analysis the source code program has been turned into a set of tokens which relate to entries in symbol table.

Question 14

What are the two tables for lexical analysis

Answer 14

Reserved word table

User identifier table

Look at examples

Question 15

What does the syntax analysis stage entale

Answer 15

Checks that the structure of the program is correct, that it follows grammar rules of the language

does this using a meta language

at this stage a list of syntax errors could be produced and compilation may stop

Question 16

What does the semantic analysis stage entale

Answer 16

Checks for various types of error such as

Checks that all variables have been declared

Checks that real values are not being assigned to integers, strings assinged to booleans etc.

Checks there is no illegal mixed mode arithmetic (e.g. multiplying a string variable by an integer variable)

Question 17

What does the code generation and optimisation stage entale

Answer 17

Generates machine code

Each line of high level language usually become many lines of machine code

Sometimes routines try to improve efficiency by making the program smaller or making it run quicker

Question 18

What is a translation error

Answer 18

Error spotted by the compiler which are either:

Syntax errors

Semantic errors

Question 19

What is an execution error

Answer 19

Errors thst occur when the program is running and that cause the program to crash (run-time error)

E.g. a variable being divided by 0 or reading a record past the end of a file

Errors that the compiler itself cannot spot as the code is correct. Occurs due to particular circumstances

Question 20

Why does a compiled program take more time to debug than an interpreted one

Answer 20

Errors are reported after compilation is finished

One error may cause many other spurious errors

Recompiling after fixing an error adds time to the process

Question 21

Define linking error

Answer 21

Calling a standard function where the correct library has not been linked to the program

Question 22

What are the main objectives of code optimisation

Answer 22

Achieve the required output of the program

Increase the speed of the program

Decrease demand on resources

Not delay the overall compilation process