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6. Program Translation

6. Program Translation. CS100: The World of Computing John Dougherty Haverford College. Overview. The Problem The “Source” – high-level code The Target – low-level (machine) code Types of translation The translation algorithm/process PIPPIN. The Problem.

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6. Program Translation

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  1. 6. Program Translation CS100: The World of Computing John Dougherty Haverford College

  2. Overview • The Problem • The “Source” – high-level code • The Target – low-level (machine) code • Types of translation • The translation algorithm/process • PIPPIN

  3. The Problem • People communication in ambiguous, high-level languages, using experience, context, and can ask for clarity interactively • e.g., Thoreau threw through the tunnel. • Machines have no “sense” of context or experience, and need unambiguous instructions

  4. The Source • High-level programming languages close to natural language (but not quite) • Alice, Javascript, C++, Java, C# • Known as Source code • Each instructions implies many lower-level instructions (as we’ll see …)

  5. The Target • Low-level instructions that are clear and simple – typically fixed in size, with a command and some reference to data • Opcode • Operand • Known as … • Machine code || binary code || executable

  6. Program Translation • From High- to Low-Level • Recall “divide and conquer” in programming • Input  Process  Output • … then details of Process, then details of … • Typically many low-level operations per high-level instruction • From source code to machine/binary code • Two ways to translate …

  7. Interpretation • Works with the source always • Translates and executes “on the fly” • Like a language translator at the UN • Easier to debug • Executes slower

  8. Compilation • Works with executable • Translates the entire program from source to machine code once • Executes the machine code as many times as needed • Recompile often during development • Executes substantially faster • Most software is distributed (except open source) • Hides algorithm

  9. Phases of Translation • Scanning – breaking text sequence into tokens (i.e., meaningful chunks) • “while”, “=”, “For all together” • Parsing – organizing the tokens to discover the meaning of the program • Code Generation – writing the sequence of machine level operations • Opcodes, operands

  10. Language Levels • High-level: one-to-many relation to machine language (e.g., z = x + y is 4 PIPPIN ops) • Assembly language: one-to-one (roughly) relation to machine language (PIPPIN) • Low-level: machine, or binary, language of 0s and 1s

  11. Arithmetic Instructions • To demonstrate this process, we’ll look at standard arithmetic expressions and statements in a high-level language • Expressions have a pattern, or (recursively-defined) form Var = exp Where exp = value | exp + exp | exp – exp | … (demonstration of Rosetta)

  12. PIPPIN instruction layout opcode operand Each box contains a byte

  13. Sample PIPPIN Opcodes

  14. Example PIPPIN program ; PIPPIN code for Z = X + Y [1] LOD X ; acc <= X [2] ADD Y ; acc <= acc + Y [3] STO Z ; acc => Z [4] HLT ; halt ; other examples AE pp. 252-4

  15. Programming Paradigms • Imperative: procedures as abstractions, details of how to do a task (e.g., FORTRAN, Pascal) • Functional: mathematical approach of input-process-return value – functions can be composed of other functions (including themselves), and can be evaluated (e.g., LISP) • Declarative: describe the information, but not the way it is processed (e.g., Prolog) • Object-Oriented: interacting objects (e.g., Java, C++, C#, Smalltalk, Javascript, Alice)

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