Programming Language Concepts CIS 280

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Programming Language Concepts (CIS 280)

• Elsa L Gunter
• NJIT
• Fall 2001

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WWW Addresses for SML

• http//www.cis.njit.edu/elsa/280/110-smlnj.exe
• ftp//ftp.research.bell-labs.com/dist/smlnj/releas
  e/110/110-smlnj.exe
• http//cm.bell-labs.com/cm/cs/what/smlnj/index.htm
  l
• http//cm.bell-labs.com/cm/cs/what/smlnj/doc/basis
  /pages/sml-std-basis.html

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Language Paradigms

• Imperative languages
• Main focus machine state the set of values
  stored in memory locations
• Command-driven Each statement uses current
  state to compute a new state
• Syntax S1 S2 S3 ...
• Example languages C, Pascal, FORTRAN, COBOL

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Language Paradigms (continued)

• Applicative (functional) languages
• Programs as functions that take arguments and
  return values arguments and returned values may
  be functions
• Programming consists of building the function
  that computes the answer function application
  and composition main method of computation
• Syntax P1(P2(P3 X))
• Example languages ML, LISP, Scheme

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Language Paradigms (continued)

• Rule-based languages
• Programs as sets of basic rules for decomposing
  problem
• Computation by deduction search, unification and
  backtracking main components
• Syntax Answer - specification rule
• Example languages (Prolog, Datalog,BNF Parsing)

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Language Paradigms (continued)

• Object-oriented languages
• Classes are complex data types grouped with
  operations (methods) for creating, examining, and
  modifying elements (objects) subclasses include
  (inherit) the objects and methods from
  superclasses
• Computation is based on objects sending messages
  (methods applied to arguments) to other objects
• Syntax Varies, object lt- method(args)
• Example languages Java, C, Smalltalk

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Programming Language Implementation

• Develop layers of machines, each more primitive
  than the previous
• Translate between successive layers
• End at basic layer
• Ultimately hardware machine at bottom

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Basic Machine Components

• Data basic data types and elements of those
  types
• Primitive operations for examining, altering,
  and combining data
• Sequence control order of execution of primitive
  operations

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Basic Machine Components

• Data access control of supply of data to
  operations
• Storage management storage and update of program
  and data
• External I/O access to data and programs from
  external sources, and output results

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Basic Computer Architecture
External files
Main memory
Cache memory
CPU
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Typical Hardware Pipeline

• Fetch next instruction in program
• Decode op-code to determine next operation to
  perform
• Load data from specified registers into operator
  buses
• Execute operations
• Store results in specified registers
• One or more steps performed each clock cycle

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Hardware Machine Components Data

• Bits 0, 1
• Bytes 8 bits
• Words 16 bits, 32 bits, 64 bits
• Op-codes typically one or two words long, not
  all words are codes (usually)

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Operations

• Correspond to (named by) op-codes
• Copy data between main registers and main memory
  (or cache)
• Perform arithmetic and logic operations on data
  in registers, putting results in registers
• Alter flow of execution, conditionally or
  unconditionally
• Copy data between I/O busses and memory

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Sequence Control

• Most instructions cause the program counter to
  increment by one as a side effect
• Goto, aka jump, reset program counter
• Gosub, aka, jump-save-return, same as goto,
  except save previous program counter to register
  first
• Conditional operations allow a single step to be
  skipped
• Conditional gotos, and gosubs

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Data Access

• Instructions specify data directly (as part of
  the word) or indirectly
• Indirect access
• Specify a register that contains data
• Specify a memory location that contains data
• Specify a register that contains a memory
  location that contains data

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Storage Management

• Basically all computes need to move data between
  registers and main memory
• Increasingly, most computers have cache in
  between registers and main memory for temporary
  storage
• Secondary caches becoming more common
• Multiple processors accessing same memory, but
  with separate caches need coherent view of memory

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External Environment (I/O)

• Computer (CPU) must control access to external
  devises
• Extended memory
• Hard drives
• CD-ROM, floppies, etc.
• Input devices, eg, keyboards, mice,
  analog-to-digital converters
• Output devices, eg, consoles, sound boards

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Virtual (Software) Machines

• At first, programs written in assembly language
  (or at very first, machine language)
• Hand-coded to be very efficient
• Use layers of software (eg operating system)
• Now, no longer write in native assembly language
• Each layer makes a virtual machine in which the
  next layer is defined

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Example Layers of Virtual Computers for a C
Program
Input data
Output results
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Virtual Machines Within Compilers

• Compilers often define layers of virtual machines
  
• Functional languages Untyped lambda calculus -gt
  continuations -gt generic pseudo-assembly -gt
  machine specific code
• May compile to intermediate language that is
  interpreted or compiled separately
• Java virtual machine, CAML byte code

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To Class

• Name some examples of virtual machines
• Name some examples of things that arent virtual
  machines

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Interpretation Versus Compilation

• A compiler from language L1 to language L2 is a
  program that takes an L1 program and for each
  piece of code in L1 generates a piece of code in
  L2 of same meaning
• An interpreter of L1 in L2 is an L2 program that
  executes the meaning of a given L1 program
• Compiler would examine the body of a loop once
  an interpreter would examine it every time the
  loop was executed

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Bindings of Program Elements

• A binding of a program element is the selection
  of a particular value for it from a set of
  possible values
• The binding time is when that selection is made
• Binding time can depend on whether we use a
  compiler or an interpreter

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Binding Times

• Language definition time language syntax and
  semantics
• Language implementation time interpreter versus
  compiler, aspects left flexible in definition,
  set of available libraries

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Binding Times

• Compile time data layout, internal data
  structures
• Link time (load time) binding of values to
  identifiers across program modules
• Run time (execution time) actual values assigned
  to non-constant identifiers

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To class

• In expression x y 5 what bindings can you
  identify, and when are they most likely made?

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Program Aspects

• Syntax what valid programs look like
• Semantics what valid programs mean what they
  should compute
• Compiler must contain both information

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Major Phases of a Compiler

• Lex
• Break the source into separate tokens
• Parse
• Analyze phrase structure and apply semantic
  actions, usually to build an abstract syntax tree
• Semantic analysis
• Determine what each phrase means, connect
  variable name to definition (typically with
  symbol tables), check types

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Major Phases of a Compiler

• Translate to intermediate representation
• Instruction selection
• Optimize
• Emit final machine code

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Major Phases of a Compiler
Source Program
Lex
Relocatable Object Code
Instruction Selection
Tokens
Linker
Parse
Unoptimized Machine-Specific Assembly Language
Abstract Syntax
Machine Code
Semantic Analysis
Optimize
Optimized Machine-Specific Assembly Language
Symbol Table
Translate
Emit code
Intermediate Representation
Assembly Langague
Assembler
Modified from Modern Compiler Implementation in
ML, by Andrew Appel
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Example of Optimization

• Program code X Y Z W
• Load reg1 with Y
• Load reg2 with Z
• Add reg1 and reg2, saving to reg1
• Store reg1 to tmp
• Load reg1 with tmp
• Load reg2 with W
• Add reg1 and reg2, saving to reg1
• Store reg1 to X
• Eliminate two steps marked