Translators and Bonus Vibes

what is assembly language?

what is assembly language?

• a low level language where each assembly code instruction is equivalent to one machine code instruction

• the instruction set is dependent on hardware

• each type of processor has a different instruction set and different assembly code

what is an assembler?

• before assembly language can be executed, it must be translated into machine code or an intermediate form called bytecode

• assembler takes each assembly code instruction and converts it into machine code

• translates source code into object code

what is a compiler?

• translates a whole program in a high level language into executable machine code (object code)

• compiler scans through source code several times, performing different checks and building up tables of information required to produce final object code

• the object code produced is hardware specific, so different hardware needs different compilers

• object code can be saved and run whenever needed without compiler’s presence

what is an interpreter?

• translates high level code to machine code

• interpreter translates code line by line into machine code and runs each line one at a time

• interpreted languages include JS and PHP

what is bytecode?

• an intermediate representation which combines compiling and interpreting

• it is then executed by a bytecode interpreter

• the bytecode may be compiled once or compiled each time a change in source code is detected before execution

• Python can also be compiled into Java bytecode and the Java interpreter can be used to run it

what are two advantages of bytecode?

• bytecode helps to achieve platform independence

  • eg any computer that can run Java has a Java Virtual Machine, which masks inherent differences between different architectures and OS

  • the JVM converts bytecode into machine code for that particular computer

  • therefore bytecode can be run on many different hardwares because the JVM can run it regardless of processor differences

• acts as an extra security layer between computer and program

  • if you download an untrusted program, you can run the Java bytecode interpreter instead of the program itself, which guards against malicious programs

advantages and disadvantages of compilers

• advantages

  • object code can be saved on disk and run whenever required without recompiling

  • object code executes faster than interpreted code because in interpreters, each line must be translated every time it is encountered, like in for loops

  • object code can be distributed or executed without compiler present

  • object code is more secure because it is harder to read by outsiders

  • appropriate for when a program is run frequently with little changes

  • appropriate from when object code needs to be distributed to external users as source code is not present so can’t be copied/amended

• disadvantages

  • compilers are not platform independent

  • each time an error is discovered recompilation is needed

what are the stages of compilation?

lexical analysis, syntax analysis, code generation and optimisation

what happens in lexical analysis?

• all comments and unnecessary spaces are removed

• simple error checking, eg illegal identifiers and assigning illegal values to constants

• all keywords, constants and identifiers are replaced by tokens representing their function in the program

• eg, numbers are converted to their run-time representation, identifiers are replaced by pointers to their addresses in the symbol table, keywords are replaced by item codes

• creates entries in the symbol table with identifiers and their run-time addresses so it can replace them by tokens in the source code

what is the symbol table?

• contains entries for every keyword and identifier in the program

• entries contain name, kind, data type, run-time address/value, and pointer to accessing information

• lexical analyser puts identifier and run-time address in, and syntax analyser puts in kind and data type

• the symbol table must be organised so entries can be found as quickly as possible to improve overall speed of computer

• the symbol table is often structured as a hash table where keyword/identifier is hashed to produce array subscript

• synonyms/collisions are inevitable so synonym is stored in next available free space

what is syntax and semantic analysis?

• syntax analysis

  • the stream of tokens from the lexer is split into phrases

  • each phrase is parsed (checking the phrase against a set of language rules to determine whether it is a valid sentence)

  • stacks are used to check bracket pairing, expressions are converted into a form from which machine code can be more easily generated

• semantic analysis

  • it is possible to create a program which has correct syntax but isn’t a correct program

  • semantic analysis checks the meaning of the code

  • eg using an identifier that wasn’t previously declared, assigning a real value to an integer variable

what is code generation?

• once the program has been checked, the computer generates machine code

• this could happen in several passes over the code because code optimisation also has to happen

what is code optimisation?

• aims to reduce execution time of object program, making it more efficient

• detects redundant instructions

• replaces inefficient code with code that achieves the same effect but is more efficient

• disadvantages

  • increases compilation time

  • sometimes produces unexpected results (like if a program was intentionally written to take a long time)

what is the linker?

• once a program has been compiled, any separately compiled subroutines must be linked into the object code

• could be input/output routines, subroutines from the language’s libraries, or subroutines written by the programmer

• the linker puts the appropriate memory addresses in all the external call and return functions so that the modules are correctly linked

what is the loader?

• copies the program and any linked subroutines into main memory to run, provided programmer hasn’t used absolute addresses and object code is in relocatable format

• when executable code was created it may assume the program will load in memory address 0

• the loader needs to relocate some memory addresses in the program because some memory is already in use

what are libraries?

• software libraries contain pre-written, pre-compiled programs which can be loaded and run when required

• most compiled languages have their own built-in libraries of pre-written functions that can be invoked from the user’s program

• programmers can also write their own libraries

• libraries can generate random numbers, provide a GUI

• library routines are tested, error-free and save the programmer time