unit 4: software 22 (1)

unit 4: software 22 (1)

1. high level language

   • designed to resemble a natural language such as English
     • + easier for people to read and understand + likely to be fewer errors due to the ease of use
       • uses commands that perform complex instructions - can not be executed by a CPU - requires translation into machine code
         • best to use for large, complex applications where speed of execution is not critical: word processor, video game
   • e.g. python, scratch

2. low level language

   • uses the instruction set of a CPU and does not resemble a natural language
     • - hard for people to understand and read - harder to find and correct errors
       • + executed directly by the CPU so can be made highly efficient
         • e.g. little man computer assembly, machine code
           • best to use when speed of execution or memory usage is important, or when a program interfaces directly with hardware: engine management, device drivers

3. stages of compilation

   • lexical analysis
     • symbol table construction
       • syntax analysis
         • semantic analysis
           • code generation
             • code optimisation
               • code optimisation may be employed to make it more efficient/faster/less resource intensive
             • machine code is generated
           • variables are checked to ensure that they have been properly declared and used
             • variables are checked to ensure that they are of the correct data type - operations are checked to ensure that they are legal for the type of variable being used
         • tokens are checked to see if they match the syntax for the programming language - if syntax errors are found, error messages are produced
       • a symbol table is created which holds the addresses of variables, labels and subroutines
     • comments and unneeded spaces are removed - keywords, constants and identifiers are replaced by 'tokens'

4. program construction

   • assemblers
     • translate assembly language to machine code
       • each operator and operand is replaced with its binary equivalent - no need for analysis of code or complex function creation
   • compilers
     • translate high level code to machine code
       • complex translation - each line of high-level code can require many machine code instructions
         • translate high-level code all at once - this machine code can be distributed and executed on only compatible computers
           • programs can be distributed without releasing the source code -> harder for competitors to copy their code - machine code files are fast to execute
   • interpreters
     • translate high level code to machine code
       • complex translation - each line of high-level code can require many machine code instructions
         • do not create an executable file - translate and execute one line at a time - source code is distributed and executed by the user - execution is slower, as the translation must take place before each line can run
           • programs will run on any computer with the interpreter (no need to compile multiple versions) - easier to find runtime errors, as the location of the error can be shown

5. errors

   • syntax error
     • occurs when text that is not valid code is found
       • e.g. print('hello) - missing quote - imput("name: ") - spelling error
   • linking error
     • occurs when a program tries to import a function from a file that doesn't exist
   • truncation error
     • occurs when data must be shortened (truncated) to fit
       • could result in missing text or an inaccurate value
   • runtime error
     • occurs when the program tries to perform an impossible operation
       • e.g. dividing by zero, adding a string to an int
   • rounding error
     • occurs when a value is rounded and loses accuracy
   • logical error
     • program executed and exists with no errors, but the program has not done what was expected
       • hard to detect and fix as no errors are produced
         • e.g. a prime number checker that says 20 is prime, a quiz that marks an answer incorrectly