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3.2 Functions and Purposes of Translators

Computing 9691Paper 3

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Assembly language and Machine code

• An assembly language is a low-level programming language for a computer, microcontroller, or other programmable device, in which each statement corresponds to a single machine code instruction

• Assembly language uses mnemonics to represent each low-level machine operation (instruction) and opcode (Register or other resources).

• Machine code or machine language is the binary commands that can be directly executed by a processor. They are 1s and 0s. Their order tells the computer what to do.

• This code is the lowest level of software. All other kinds of software need to be translated into machine code before they can be used.

• An Assembler is the utility software used to convert Assembly language code to machine code.

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How an assembler converts assembly language code to machine code

• An assembler creates object code by translating assembly instruction mnemonics into opcodes, and by resolving symbolic names for memory locations and other entities like registers etc.

• An opcode is a single instruction that can be executed by the CPU. In machine language it is a binary or hexadecimal value such as 'B6' loaded into the instruction register. (Refer to topic 3.3)

• In assembly language mnemonic form, an opcode is a command such as MOV or ADD or JMP. – For example MOV, AL, 34h

• The opcode is the MOV instruction. The other parts are called the 'operands'.

• Operands are manipulated by the opcode. In this example, the operands are the register named AL and the value 34 hex.

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Interpretation and Compilation

• Object code is the product obtained when source code is translated by a compiler. Object code may almost be machine code or close to machine code.

Source Code

Machine Code

Object CodeCompiler

Assembly Code

Interpreter

Assembler

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Interpreters and Compilers

• Levels of Programming Languages• Low level (Machine code)• Mid level (Assembly)• High Level (C/C++, VB, Java, Fortran, etc)

• Code written in Mid level and high level languages(i.e. source code) must be translated to machine code, which is what the processor understands.

• Translators are used. The types of translators are:• Compilers• Interpreters

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Interpreters

• An interpreter takes each instruction in turn and translates it into machine code.

• The translated instruction is executed before the next instruction is translated.

• This is particularly useful when there is not enough memory to hold the compiled Program especially on old computers.

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Compilers

• A compiler translates source code into machine or object code

• A compiler translates the whole program as one complete unit to create an executable file

• The high-level language version of the program is called the source code and the resulting machine code program is called the object code.

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Compilers –Adv. And Disadv.

• Advantages.– the translation is done once only and as a separate

process. The program that is run is already translated into machine code so is much faster in execution.

– compiled programs can run on any computer• Disadvantages– A Compiler uses a lot of computer resources. It has to

be loaded in the computer's memory at the same time as the source code, and there has to be sufficient memory to hold the object code.

– When an error in a program occurs it is difficult to pin-point its source in the original program

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Interpreters – Adv. And Disadv.

• Advantages– Need less memory than compilers (useful in early computers

which had limited power and memory). – As the error messages when the error is produced on the

line it is encountered it is easier to identify / isolate the instruction causing the problem.

– Individual segments can be run without needing compile the whole program.

• Disadvantages– every line has to be translated each time it is executed, thus

interpreters tend to be slow. – As interpreter doesn't create an object file the source code

must be distributed along with the interpreter in order for the user to run the software.

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Interpreters vs. CompilersCOMPILER INTERPRETER

Fast, creates executable file that runs directly on the CPU Slower, interprets code one line at a time

Debugging is more difficult. One error can produce many spurious errors

Debugging is easier. Each line of code is analysed and checked before being executed

More likely to crash the computer. The machine code is running directly on the CPU

Less likely to crash as the instructions are being carried out either on the interpreters' command line or within a virtual machine environment which is protecting the computer from being directly accessed by the code.

Easier to protect Intellectual Property as the machine code is difficult to understand

Weaker Intellectual property as the source code (or bytecode) has to be available at run time. For example if you write a Flash Actionscript application, you can easily get de-compilers that convert the p-code back into actionscript source code (unless you use encryption, but that is another story).

Uses more memory - all the execution code needs to be loaded into memory, although tricks like dynamic Link Libraries lessen this problem

Uses less memory, source code only has to be present one line at a time in memory

Unauthorised modification to the code more difficult. The executable is in the form of machine code. So it is difficult to understand program flow.

Easier to modify as the instructions are at a high level and so the program flow is easier to understand and modify

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The Translation process

• The basic translation process is identical for both interpreters and compilers.

• Source code goes through various stages and is converted into Intermediate Language.

• From this, final object code is generated, which can be optimized.

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Lexical Analysis stage

• White spaces, blank lines and comments are removed from the code.• Using the grammar of the language being used, the lexical analyzer assigns

tokens to a meaningful string of characters. Single characters are converted into their ASCII codes.

• A token could be anything from 16-bit unsigned integers (starting from 256) to simple labels.

• Variable names require extra information. A symbol table is used to keep record of variables. This table is used throughout the translation process.

• During Lexical Analysis, only variable names are noted into the symbol table.

• The symbol table is stored as a Linked List and searching is performed using hashing.

• Some basic error reporting is done in this stage. E.g. Illegal Identifier…

When Lexical Analysis phase is finished, the code is converted into a standard format that the Syntax Analyzer can understand.

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Syntax analysis stage

• All computer languages have their specific grammar (syntax) of writing valid programming statements.

• This grammar is defined using BNF (Backus-Naur Form).

• The Syntax Analyzer (or Parser) will analyze the tokenized code against the grammar of the language.

• The parsing transforms the code into a data structure, usually a Binary Tree, which is suitable for further processing.

• Invalid command names, such as INPT instead INPUT will be identified at this point.

• Some languages require variables to be declared before they can be used. The syntax analyzer will catch variables without declarations (using the symbol table).

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Syntax analysis – Example of BNF• Taking a very elementary language, an assignment statement may be defined to be of the form

<variable> <assignment_operator> <expression>• and expression is

<variable> <arithmetic_operator> <variable>• The parser must take the output from the lexical analyser and check that it is of this form.

• If the statement is sum := sum + number, the parser will receive

<variable> <assignment_operator> <variable> <arithmetic_operator> <variable>

which becomes <variable> <assignment_operator> <expression>and then <assignment statement>, which is valid.

• If the original statement is sum := sum + + number, this will be input as<variable> <assignment_operator> <variable> <arithmetic_operator><arithmetic_operator>

<variable>and this does not represent a valid statement hence an error message will be returned.

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Syntax analysis – semantic analysis

• During syntax analysis, certain semantic checks are carried out: Label checks – make sure the line a GOTO statement passes control to exists Flow of control checks – make sure statements are used in the correct place and

order e.g. CONTINUE can only be placed inside of a loop, IF statement matched with correct END IF

Declaration checks – make sure all variables have been properly declared

• Note: although semantic checks check the logic to a certain extent, it is not the same as checking for a logic error. Remember, a logic error will not cause the program to crash, it will simply cause unexpected results. A compiler cannot find such errors. E.g. if the programmer has written a = b + c instead of a = b - c

Semantic means relating to meaning in language or logic. Something may be syntactically correct but semantically meaningless. ‘Jake ate a banana’ has

meaning and obeys the rules of English but ‘A banana ate Jake’ obeys the rules but not the semantics.

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Code Generation

• At this stage, the address of each variable is now calculated and stored in the symbol table as each is encountered.

• Intermediate code is produced which, after optimisation, is turned into executable code / machine code– All errors due to incorrect use of the language

have been removed by this stage

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Code optimisation

• Once the code generator has created machine code, it tries to optimise the code to make it more efficient.

• Consider the following lines of code x = y + 3 b = x c = b

• The code optimiser will remove redundant code so the above example could become:

c = y + 3

• A compiler’s code optimiser can favour speed or memory optimisation because in the real world you often cannot optimise both.

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Library routines• Library routines are pre-compiled modules available for use by other programs.

Programs are made up of modules Commonly used modules can be compiled and stored ready for repeated use These modules are stored in an a library They are called library routines

• Windows uses libraries in the form of DLL files (dynamic link library) • Because the variable names and memory addresses will be different from one use of the library

to the next, two programs are needed at runtime or when an executable file is created: These programs are Loaders and Linkers.

• A loader has the job of loading all the modules into memory • A linker resolves references in the main program, known as links or symbols, to library routines.

If a function in a library routine is called, the linker will match the call in the program with the function in the library routine. Libraries reduce the amount of code that needs to be written If a library routine is updated, programs using it may stop working if the interfaces

between the modules change

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Sample Questions

1. Explain why the size of the memory available is particularly relevant to the process of compilation. (4)

2. a) Explain the difference between the two translation techniques of interpretation and compilation. (2)

b) Give one advantage of the use of each of the two translation techniques. (2)

3. State any three stages of compilation and describe, briefly, the purpose of each. (6)

4. Explain, in detail, the stage of compilation known as lexical analysis. (6)