1.2.2

Question 1

Application software

Answer 1

• Used by an end user to perform a specific task.
• e.g. word processor or web browser

Question 2

Systems software

Answer 2

• Manages computer resources to maintain performance
• e.g. operating system or device driver.

Question 3

Utility Software

Answer 3

• Has a specific function to maintain OS performance
• e.g. backup or compression software

Question 4

Open source

Answer 4

• Provided along with the source code.
• No license required to use.

The program’s source code is freely available to view/edit by anyone. Open source software is made by many people and distributed under licenses that comply with the Open Source Definition.

Question 5

Closed source

Answer 5

• Needs a license to use.
• Source code is not available.
• Protected by Copyright

A program that legally remains the property of the organisation, group, or individual who created it. The organisation that owns the rights to the product usually does not release the source code, and may insist that only those who have purchased a special license key can use it.

Question 6

Open source advantages

Answer 6

• The source code can be compiled to work on different platforms eg Windows, macOS.
• Potentially a community of developers continuously improving the source code
• Potentially a community of users to learn from
• Can modify the source code to adapt to your needs rather than starting from scratch (saves time and money)
• Could be copied and sold for profit since source code is available
• Potentially less vulnerabilities as lots of people can view the source code (but do they and are they sufficiently knowledgeable?)

Question 7

Open source disadvantages

Answer 7

• Risk of abandonment eg community decides not to maintain the code
• Potentially less support compared to closed source
• Not always well supported or documented.
• Variable quality code.
• Less secure.

Question 8

Closed source advantages

Answer 8

• Usually professional support comes with the software
• Potentially a product that has been more robustly developed and tested
• More secure.
• Regular updates

Question 9

Closed source disadvantages

Answer 9

• Usually a financial cost
• Cannot modify to meet your needs
• Fixes to proprietary code rely on vendor rather than community so potentially slower.

Question 10

Assemblers

Answer 10

• Assembly code is platform specific, low level code.
• Translates assembly code to machine code.
• 1 line of assembly code = 1 line of machine code.
• Takes each assembly code instruction (comments are ignored) and checks syntax. Any errors are reported.
• Assuming no errors, the assembler looks up mnemonic opcodes in a static opcode table to find the operation code (eg in LMC add is 1)
• Gives memory addresses to labels (labels are symbolic representations of addresses, the dynamic symbol table maps labels to memory addresses - see right)
• Converts instructions to machine code (1s and 0s)

Question 11

Interpreters

Answer 11

• Translates and executes code line by line.
• Will error if a line contains an error.
• Slower to run than compiled code.
• Code is platform independent.
• Useful for testing.
• Takes source code line by line and analyses it.
• If an error is encountered, interpreter stops and indicates position of the error
• If there are no errors the line is run. The interpreter executes the machine code equivalent of the instruction.

Question 12

Compilers

Answer 12

• Translates code all in one go.
• Compilation process is longer.
• Produces platform specific code.
• Complied code can be run without a translator
• Scans entire source code a number of times performing different checks (see Compilation lesson for more details)
• If all checks are fine eg no syntax errors, the whole source code is translated into either executable machine code which can be saved and run whenever required OR intermediate code (between high level and machine code)

Question 13

Interpreter pros

Answer 13

• A program can quickly start running without the time spent to compile it (useful for program development where code is frequently being edited).
• Code can run on any hardware architecture on which the interpreter runs

Question 14

Interpreter cons

Answer 14

• Usually much slower than compiled program to run. For example, interpreter must translate the same statements within a loop over and over again.
• Interpreter is needed each time the code is run
• Can be harder to debug. For example if your code has multiple errors, the program stops running on the first error and doesn’t provide any reporting on further errors.

Question 15

Compiled pros

Answer 15

• The executable can be saved on disk and run whenever required without the need to recompile
• Compiled code typically executes faster than interpreted code (see below left)
• The executable can be distributed and run without needing the compiler
• Executable code is secure in that it is very difficult to reverse engineer (therefore good for programs that will be sold and closed source)
• Information on all errors immediately at compilation rather than having to execute the line (good if you have a very long program and the error is right at the end!)

Question 16

Compiled cons

Answer 16

• No code executed until everything is compiled with no errors.
• Any change/error requires code to be recompiled
• Object code is platform specific so it will not work on all platforms
• Needs to store a potentially large executable in memory.

Question 17

What are the stages of compilation?

Answer 17

Lexical analysis
Syntax analysis
Code generation
Optimisation

Question 18

Lexical Analysis

Answer 18

• Irrelevant white space and comments removed
• Allocate tokens to lexemes
• Symbol table created. Identifiers are added to symbol table
• Purpose is to determine the meaning of individual words in the source code.

Question 19

Syntax Analysis

Answer 19

• Tokens are checked against rules of the language, error raised if rule is broken
• Check meaning (semantics) for logic errors e.g. int can’t be assigned to string
• Symbol table updated eg data type, scope
• Purpose is to discover syntax errors in the tokenized program.

Question 20

Code Generation

Answer 20

• Create object code

Question 21

Code Optimisation

Answer 21

• Optimise object code
• Code is rewritten so that it runs faster and requires less resources (less memory and less processing time) while still producing the exact same effect as was intended.

Question 22

Libraries

Answer 22

• Libraries allow you to include code written by other developers (or yourself) into your projects. They facilitate reusable code components.
• Libraries can be supplied as source code or compiled code.
• If the library is source code, it is merged during compilation
• If the library code is already compiled, it is linked.

Question 23

Static Linking

Answer 23

The executable contains the libraries merged into it

Question 24

Dynamic linking

Answer 24

The libraries that are contained in separate files.
The dynamic linker part of operating system finds the files and combines them with your executable code when the program is loaded.

Question 25

The loader

Answer 25

- All operating systems (apart from embedded systems) have a subsystem called a loader. - The loader will take a compiled program and load it into memory ready to execute. If the program isn't compiled for that OS, the loader is unlikely to succeed. - Loaders do the following (not an exhaustive list...): - Validation (check permissions required to run the program, check memory requirements to ensure , etc.) - Copying the program from the hard disk/solid state drive into main memory - Initializing registers (e.g. set program counter to entry point)

Question 26

Static linking pros

Answer 26

- You guarantee the library is combined with the executable code. - Your code will be more reliable! - You can control exactly which version.

Question 27

Static linking cons

Answer 27

- The file size will be bigger as many programs might contain redundant copies of the same library. - The libraries might also be out of date.

Question 28

Dynamic linking pros

Answer 28

- The executable program itself as it does not have the libraries merged into it so saving disk space. Multiple programs can share a single copy of the library. - The operating system can maintain the latest versions of all the libraries as part of its regular updates.

Question 29

Dynamic linking cons

Answer 29

- If you've not got the dynamic library, the program can't run! You might need to ship copies of the libraries with your code just in case. - Known on the Windows platform as "DLL Hell", an incompatible updated library will break executables that depended on the behavior of the previous version of the library if the newer version is not backward compatible.