Application Generaton

Question 1

1) What is machine code?

2) What are Opcodes and Operands?

Answer 1

1)
-Processors only understand machine code
-Machine code is binary sequences that represent instructions and data
-Originally no choice but to code in machine code
2)
-Opcodes are the sequences contain instructions
-Operands are the sequences containing data

Question 2

What is an instruction set?

Answer 2

• Each processor has its own instruction set
• They are the instructions that are available for a certain processor to process
• Code has to be written for each processor

Question 3

1) What is assembly language?

2) What is an assembler?

Answer 3

1)
-Written in mnemonics making code easier to read and write
-Not very portable (different chips will have different languages)
2)
-Assembler converts assembly into object code

Question 4

Why were HLL’s introduced?

Answer 4

• in the 19950’s due to limitations on assembly code, more money was spent on developing software than purchasing hardware. Even with the relatively higher cost of hardware compared to today
• Fortran the first HLL made code more-readable
• HLL needs interpreters and compilers in order to convert it into something the CPU can utilise

Question 5

What is an interpreter? +/-

Answer 5

*Reads code line by line
*Each line is converted into machine code
*Then Machine code is executed
+Useful when debugging as the execution will terminate at the line where the error occurs
-Runs slower
-May involve the translation of the same code multiple times
-User needs to have access to an interpreter to execute the code which takes up more memory

Question 6

What is a compiler?

Answer 6

• Takes HLL source code and converts it to object code
• Compiled code can be run as often as required
• Faster than interpreted code after complication. The complication takes longer
• Hides the original code as compiled code is machine code
• small programs can compile in seconds, more complex can take a while
• Unlike interpreted code you do not have to release source code for other people to run it meaning other people cannot steel it

Question 7

What is object code?

Answer 7

• Intermediary step taken before producing ‘pure’ machine code
• contains placeholders where library code should be placed
• Passing the object code through a linker generates machine code that can be executed by the processor

Question 8

What are utilities? Examples?

Answer 8

• Relatively small program with one purpose usually concerned with system maintenance
• Anti Virus programs: detects and removes viruses
• Disk defragmentation: Groups all parts of paged/segmented data so they can be read in one go
• Compression: Reduce amount of space data takes up in storage. Makes use of regularly repeated patterns in data
• File managers: Allow files and directories to be moved, copied, deleted and renamed
• Backup utilities: Allow backups to be automatically made of specific data

Question 9

What are applications? Examples?

Answer 9

• Software which allow user to perform a task or produce something
• Word processors, spreadsheet packages, presentation software, desktop publishing software, image editors, web browsers and computer aided design packages

Question 10

How to compilers work? (In general terms)

Answer 10

-go through a defined sequence of steps

Each step brings the high level code closer to machine code

Question 11

What is lexical analysis?

Answer 11

• All comments and whitespace stripped from code
• HHL converted into tokens
• tries to ID the reserved words (IF, THEN etc), operators, variables and constants
• all separate entities become their own tokens

Question 12

What is a symbol table used for?

Answer 12

• Throughout compilation the complier must track the variables and subroutines within the code
• the names of these are added to the symbol table
• later on, info such as data type and scope (global/local) are added

Question 13

What is syntax?

Answer 13

-set of rules that govern its structure

Question 14

What is syntax analysis?

Answer 14

• compiler checks the code has been written in line with syntactic rules of the language
• if code doesn’t, a list of syntax errors will be generated
• generates an abstract syntax tree (AST) that will represent program
• if tokens generated in lexical analysis do not fit into this tree than we have a syntax error

Question 15

What is the code generation step of compilation?

Answer 15

• the code is generated as an AST

- complainer now takes this and converts into object code

Question 16

What is the optimisation step of compilation?

Answer 16

• we want code to run efficiently (as fast as possible or as low memory usage as possible)
• optimisation can remove redundant/unreachable code or replace instructions with more efficient alternatives
• occurs during and after code generation

Question 17

What are libraries?

Answer 17

• Reusable, complex code written by other programmers to simplify programming tasks
• Accessed through API’s (application programming interface)
• not uncommon for a library to be written in one language and have API’s designed to allow other languages to work with it (different languages will be suited to different problems)

Question 18

What are linkers/loaders?

Answer 18

• if we have used libraries we need to ensure they are included with final code
• a linkers is required to include the library code and our compiled code into a single exe
• linkers can be static or dynamic (loaders)

Question 19

What is static linking?

Answer 19

• all library code is inserted directly into the program when it is complies
• final exe very large
• can have multiple exe’s with all the same library embedded

Question 20

What is dynamic linking?

Answer 20

• attempt to bypass the size of static linking
• compiled versions of the library are stored
• OS links a program to the library when it is run
• loaders are core parts of the OS and are tasked with loading a program into memory
• reduces duplication of library packages