1.2.2

Question 1

Give two advantages of using compiled code over interpreted code

Answer 1

• Faster to execute
• Does not require compiler to run
• safer as can’t be reverse engineered

Question 2

Give two advantages of using interpreted code over compiled code

Answer 2

• It is platform independent

- Runs instantly without time waiting for compilation

Question 3

What are the stages of compilation?

Answer 3

• Lexical analysis
• Syntax analysis
• Code generation
• Optimisation

Question 4

What happens during syntax analysis

Answer 4

• Tokens are compared to the rules of the programming language
• Syntax errors are identified and reported as a list
• Symbol table is updated with more detail
• Semantic analysis

Question 5

What does a compiler do

Answer 5

Translates high level language source code into machine code which is stored in an object file. One or more object files are then combined to form the final executable code.

Same source code can be run through different compilers to generate machine code for different CPUs

Question 6

What does an interpreter do

Answer 6

An interpreter translates source code one line at a time, once translated the machine code is executed immediately.

Question 7

What does an assembler do

Answer 7

Converts assembly language source code into machine code. The assembler is usually designed to produce machine code for a specific CPU.

Question 8

When is assembly language usually used

Answer 8

When the program needs to interact very closely with the details of the hardware, for example device drivers.

It is also used when code needs to be high speed and efficient

Question 9

What happens during lexical analysis

Answer 9

• The compiler breaks down the ASCII into its component parts called lexemes
• Compiler scans the source code letter by letter looking for keywords (IF, WHILE)
• Removes redundant characters such as tabs, whitespace and comments
• lexemes are converted into binary sequences of fixed length called tokens

Question 10

What happens during code generation

Answer 10

• Tokens are converted into machine code instructions
• Allocates memory locations to tokens that were noted as variables
• Works out relative addresses for jumping around within the program (IF)
• Results stored as an object file
• Tries to optimise the code (makes it more efficient and fast, removes redundant code)

Question 11

What is intermediate code

Answer 11

simplified code that is between high level & machine code

It is produced by compiler
and runs on any computer

Question 12

Benefits of intermediate code

Answer 12

• Allows portability between machines

- Allows sections of code to be written in different languages by different programmers

Question 13

What is a virtual machine

Answer 13

A theoretical computer which provides an environment in which a translator is available

Uses an interpreter to run the intermediate code

Question 14

Issue with virtual machines

Answer 14

Security, the current version of the VM may have security vulnerabilities that allow rogue code to take control of the host computer. This is why VMs are patched frequently.

Question 15

Advantages of using an interpreter when writing code

Answer 15

Errors are reported as they occur rather than together and are easier to correct

Programs can be restarted from any point after correcting an error

Breakpoints (for debugging) can be inserted in the code to halt the program at any point at which point the values of any variables used can be displayed

It is possible to set variable values while running the program

Less memory is needed as only the object code from a single source code instruction in use is held and interpreters are usually smaller programs than compilers

Question 16

Disadvantages of using an interpreter

Answer 16

The end user needs an interpreter

The program runs more slowly because the program has to be translated each time it is run

The end user has to be given the source code