Lecture #3

CIS400 9/15/99

Language evaluation cont.

per Tucker:

· Expressivity- readability

· Well defined- syntax, semantics

· Data types & structures

· Modularity

· I/O facilities

· Portability

· Efficiency

· Pedagogy- easy to teach & learn

· Generality- useful in wide range of applications

per Sebesta:

· Readability :

- Simplicity

- Orthogonality

- Control statements

- Data structures

- Syntax

· Writeability:

- Simplicity

- Orthogonality

- Support for abstraction

· Reliability:

- Type checking

- Exception handling

- Readability/ writeability

- Aliasing – 2 different ways to refer to the same spot in memory/storage

· Cost:

- Training

- Code development

- Compilation

- Execution

- Maintenance

Language design

per Hoare:

· Focus on one well known feature at a time, (could be basic as data type)

· Examine many alternative features designed by others & choose the best, rejecting those that are inconsistent

per Horowitz:

· Choose specific application (logic, financial, etc.)

· Keep design committee small

· Choose precise design goals

· Release versions to small sets of interested people

· Revise language definition

· Attempt to build compiler & write formal language definition- semantics

· Revise language definition

· Produce clear and concise manual

· Provide "production quality" compiler and wide distribution

· Write primers explaining language

Language processors

· Computers- integrated set of algorithms & data structures capable of storing and executing programs

· Composite ® actual (hardware) computer-wires and circuits

· Computer ® software simulated computer

programming language is implemented by developing a translator capable of transforming programs in one language into machine language of some computer where it can be executed correctly

STATE

· Instructions- represent state transition via program execution

· You have a choice: implement with hardware, (quick yet inflexible)

implement with software, (slow and flexible)

COMPUTER ORGANIZATION

Operations: Sequence control how do we decide which instruction to take next

Note: control sequence changes accomplished by the program address register

Interpreter

· Fetch op code

· De-code op code

· Fetch necessary operands

· Branch to primitive (OP_k)

· Then repeat until the end of the program

COMPUTER AS A MULTI-LEVEL MACHINE:

Each level supported by the level below it:

level 5 problem oriented language

translated by compiler

level 4 assembly language level

translated by assembly

level 3 operating system

partial interpretation by OS

level 2 conventional machine language

interpreted by micro-program

level 1 micro-programming

directly executed by hardware

level 0 digital logic

gates & transistors,

program address registers

TRANSLATION VS. INTERPRETATION

· Translation: program written for level n machine translated to level 1 machine

· Advantages:

-statements decoded ONCE

-efficient execution

· Disadvantages:

-space consumption

· Interpretation: program written for level n + 1 is executed on level n machine

· Advantages:

-space conservation

· Disadvantages:

-execution

TRANSLATORS

· Compiler: high level® machine

· Assembler: one to one, assembly ® machine

· Loader: relocatable version of machine code ® machine code

· Link editor: combines collections of relocatable programs ® single relocatable machine program

· Pre-processor: extended language ® standard language