What Do I Represent?

Translators – Module Knowledge Areas Revisiting object code When we disassemble code we can view the opcodes used This is just a stage – the assembled code is typically translated into a series of hex codes What can you say about the object code below (these are from 3 different scripts)?

Translators – Module Knowledge Areas Locate a Python script and copy it to a clean directory In the same directory create a new Python file The only line you need is import filename eg import tempConvert (do not put in the extension) Give the file a sensible name and run it In your directory you will now see a folder called “_pycache_” Open this. Inside you will see a file with the extension “.pyc” Open one of the files with Sublime Text Take a screengrab of this and use it to discuss the link between source code and object code. Ensure you explain what is being shown.

Translators

Translators – Module Knowledge Areas Types of translators and their use Lexical analysis Syntax analysis Code generation and optimisation Library routines

Translators – Module Knowledge Areas Types of translators and their use Three types of translator Compiler – source code to object code. Translates all the code before running Interpreter – source code to object code BUT translates one line at a time and then executes the line Assembler – assembly code to machine code

Translators – Module Learning Objectives describe the need for, and use of, translators to convert source code to object code understand the relationship between assembly language and machine code describe the use of an assembler in producing machine code describe the difference between interpretation and compilation describe the purpose of intermediate code in a virtual machine describe what happens during lexical analysis describe what happens during syntax analysis, explaining how errors are handled explain the code generation phase and understand the need for optimisation describe the use of library routines

Translators - Assembly Understand the relationship between assembly language and machine code A particular architecture (CPU) has no way to directly read source code Each architecture has its own machine language This prevents a straight source code to machine code translation – we need to assemble the code EG running my Python code on a 64 bit Windows machine is not the same as running the code on a 32 bit Linux machine

Translators - Assembly We have previously seen that one line of source code can generate many lines of object code: Line 4 of the source code creates 2 lines of object code Line 6 of the source code creates 8 lines of object code This is referred to as one to many relationship (one source code line can give rise to many object code lines) The whole program generates 20 lines of assembly code TASK – Explain what is meant by the term ‘Assembly Language’. Ensure you make a link between assembly and source

Translators - Assembly Assembly code has a one-to-one relationship with machine code. In the diagram to the left there is a representation of how the language becomes less ‘friendly’ as the translation process is undertaken TASK – Describe the three stages in the diagram above. In your description include the architecture based reasons and human readability reasons for working in source and assembly code

Translators – Machine Code We already know that machine code is architecture specific eg 16, 32, 64 bit If an architecture is 32 bit this means that it has a word length of 32 bits This means that the address bus is 32 bits in size and that the size of each instruction is 32 bits Complex instructions – those exceeding the word length – are split into 2 or more instructions Each instruction has two parts – an opcode and data Opcode – the instruction eg ADD to add a value to the ACC (accumulator) Data – the information being manipulated eg 4 to have the instruction ADD applied to it

Translators – Assembly to Machine We already know that assembly code has a particular structure that uses mnemonics (words that look like English) to provide simple instructions EG ADD 4 to ACC is an instruction to add a value to the accumulator (notice now that we are in the territory of registers) OpcodeData MOV Value 0 to ACC The value to move into the ACC ADD 4 to ACC ADD 5 to ACC MUL 2 to ACC Task – what is the above doing? In pairs agree and be prepared to explain to the group

Translators – Machine Code KISS: Keep It Simple Son – How 2 bytes/16 bits can be characterised Bits 1-6Bits 7-16 OpcodeData OpcodeAssembly MnemonicDescription MOVWill move a value to a register ADDWill add a value and store it in the ACC SUBWill subtract a value and store it in the ACC MULWill multiply a value and store it in the ACC

Translators – Machine Code In this example of disassembled code the offset (second column) has a particular pattern – what is it? The second column refers to the instruction for the mnemonic opcode in column 3. Each instruction is exactly 3 bytes in size and therefore 3 bytes apart Task – Disassembling assembly (go to Moodle)