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` Mnkingm
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`Postscript
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`Petitioner Apple Inc. - Exhibit 1040, Cover
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`Thinking in
` Thinking in
`PostScrıpt(cid:226)
`PostScrIpF
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`Petitioner Apple Inc. - Exhibit 1040, p. i
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`Petitioner Apple Inc. - Exhibit 1040, p. i
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`Petitioner Apple Inc. - Exhibit 1040, p. ii
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`Petitioner Apple Inc. - Exhibit 1040, p. ii
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`Thinking in
`PostScrıpt(cid:226)
`
`Glenn C. Reid
`
`Addison-Wesley Publishing Company, Inc.
`Reading, Massachusetts • Menlo Park, California • New York
`Don Mills, Ontario • Wokingham, England • Amsterdam • Bonn
`Sydney • Singapore • Tokyo • Madrid • San Juan
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`Petitioner Apple Inc. - Exhibit 1040, p. iii
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`Library of Congress Cataloging-in-Publication Data
`Reid, Glenn C.
`Thinking in PostScript / Glenn C. Reid.
` p. cm.
`Includes indexes.
`ISBN 0-201-52372-8
`1. PostScript (Computer program language) I. Title.
`QA76.73.P67R46 1990
`005.26’2—dc20
`
`90-43721
` CIP
`
`Copyright © 1990 by Glenn C. Reid
`
`All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or
`transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise,
`without the prior written permission of the publisher. Printed in the United States of America. Published
`simultaneously in Canada.
`
`Many of the designations used by manufacturers and sellers to distinguish their products are claimed as
`trademarks. Where those designations appear in this book and Addison-Wesley was aware of a trademark
`claim, the designations have been printed in initial capital letters.
`
`PostScript® is a registered trademark of Adobe Systems Incorporated (Adobe). All instances
`
`The name
`
`PostScript in the text are references to the PostScript language as defined by Adobe unless
`of the name
`
`PostScript also is used as a product trademark for Adobe’s implementation of
`otherwise stated. The name
`the PostScript language interpreter.
`
`Any references to a PostScript printer, a PostScript file, or a PostScript driver refer to printers, files, and
`driver programs (respectively) that are written in or that support the PostScript language.
`
`Sponsoring Editor: Carole McClendon
`Technical Reviewer: Ken Anderson
`Cover Design: Doliber Skeffington
`Text Design: Rex Holmes
`Set in 10-Point Times Roman
`
`ABCDEFGHIJ-MW-943210
`First printing, September, 1990
`
`Petitioner Apple Inc. - Exhibit 1040, p. iv
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`

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`Table of Contents
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`Preface......................................................................... xi
`
`Chapter 1
`PostScript as a Programming Language .................. 1
`DESIGN FEATURES............................................................................2
`
`STRUCTURED PROGRAMMING TECHNIQUES ...............................3
`
`PROGRAMMING TASKS.....................................................................4
`
`WINDOW SYSTEMS, COMMUNICATIONS, AND DISPLAYS............4
`
`DATA STRUCTURES AND ALGORITHMS .........................................6
`
`CONCLUDING THOUGHTS ................................................................6
`EXERCISES .............................................................................. 7
`
`Chapter 2
`PostScript is Not Like C.............................................. 9
`COMPARISON OF LANGUAGE MECHANISMS...............................11
`
`EXPRESSING AN ALGORITHM AS A PROGRAM ...........................12
`
`THE UNIX SHELL AND OPERATING SYSTEM................................13
`
`INPUT, OUTPUT, AND THROUGHPUT ............................................14
`
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`Petitioner Apple Inc. - Exhibit 1040, p. v
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`CONCLUDING THOUGHTS ..............................................................15
`EXERCISES............................................................................. 16
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`Chapter 3
`Foundations............................................................... 17
`POSTSCRIPT LANGUAGE SYNTAX ................................................19
`
`SIMPLE PROGRAM STRUCTURE....................................................20
`
`SETTING UP TEMPLATES................................................................24
`
`DECLARING AND USING VARIABLES.............................................26
`
`ALLOCATING MEMORY....................................................................29
`
`GETTING MEMORY BACK................................................................31
`
`OPENING AND CLOSING FILES ......................................................31
`
`COMPARISONS AND EQUALITY OF OBJECTS..............................32
`
`CONCLUDING THOUGHTS ..............................................................34
`EXERCISES............................................................................. 34
`
`Chapter 4
`Some Typical Programs ........................................... 37
`A TYPICAL PAGE DESCRIPTION PROGRAM .................................38
`
`FONT PROGRAMS............................................................................40
`
`PROGRAMS THAT READ DATA.......................................................42
`
`QUERY PROGRAMS.........................................................................43
`
`ENCAPSULATED POSTSCRIPT PROGRAMS.................................43
`
`PERSISTENTLY RESIDENT PROGRAMS .......................................46
`
`CONCLUDING THOUGHTS ..............................................................49
`EXERCISES............................................................................. 49
`
`Chapter 5
`Understanding the Stack.......................................... 51
`A QUICK OVERVIEW OF DATA TYPES ...........................................51
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`NAME LOOKUP .................................................................................53
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`HOW OPERATORS USE THE STACK..............................................53
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`GROUPING AND VISUAL CHUNKING .............................................56
`
`THINKING BACKWARD AND SIDEWAYS ........................................58
`
`COMPOSITE OBJECTS ....................................................................61
`
`THE OTHER STACKS .......................................................................62
`
`CONCLUDING THOUGHTS ..............................................................63
`EXERCISES ............................................................................ 64
`
`Chapter 6
`Trusting the Stack ..................................................... 67
`SAFETY OF DATA ON THE STACK .................................................68
`
`WHERE ARE THE DATA GOING? ....................................................69
`
`REARRANGING THE STACK............................................................71
`
`CONDITIONALS AND LOOPS...........................................................73
`
`RECURSION AND LOCAL VARIABLES............................................76
`
`CONCLUDING THOUGHTS ..............................................................77
`EXERCISES ............................................................................ 78
`
`Chapter 7
`Building Conditional Statements ............................. 79
`SIMPLE CONDITIONALS ..................................................................80
`
`SETTING UP THE CONDITION.........................................................81
`
`CONDITIONALS ARE NOT MAGIC...................................................86
`
`NESTED CONDITIONALS AND ELSE CLAUSES ............................87
`
`COMPOUND CONDITIONALS ..........................................................88
`
`CONCLUDING THOUGHTS ..............................................................90
`EXERCISES ............................................................................ 91
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`Table of Contents
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`Petitioner Apple Inc. - Exhibit 1040, p. vii
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`Chapter 8
`Using Looping Constructs ....................................... 93
`LOOP BASICS ...................................................................................94
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`USING THE LOOP INDEX .................................................................95
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`LOOPS ARE PROCEDURE BODIES ................................................99
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`LOOPS OF INSTRUCTIONS ...........................................................101
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`EXITING LOOPS PREMATURELY..................................................102
`
`CONCLUDING THOUGHTS ............................................................103
`EXERCISES........................................................................... 103
`
`Chapter 9
`Procedures .............................................................. 105
`WHAT EXACTLY IS A PROCEDURE?............................................106
`
`PARAMETER PASSING ..................................................................108
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`CONSTRUCTING GOOD PROCEDURES ......................................111
`
`SELF-MODIFYING PROCEDURES.................................................114
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`CONCLUDING THOUGHTS ............................................................116
`EXERCISES........................................................................... 117
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`Chapter 10
`Using Dictionaries................................................... 119
`DICTIONARIES FOR NAME SCOPING ..........................................120
`
`LOCAL DICTIONARIES ...................................................................121
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`GLOBAL DICTIONARIES OF PROCEDURES ................................123
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`MAINTAINING THE DICTIONARY STACK......................................124
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`INTO AND OUT OF DICTIONARIES ...............................................126
`
`LOOKING INTO DICTIONARIES .....................................................127
`
`REDEFINING OPERATORS............................................................130
`
`CONCLUDING THOUGHTS ............................................................132
`EXERCISES........................................................................... 133
`
`viii
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`Petitioner Apple Inc. - Exhibit 1040, p. viii
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`Chapter 11
`Creating and Manipulating Data............................. 135
`CONSTRUCTING AN ARRAY .........................................................135
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`CONSTRUCTING A STRING...........................................................137
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`MANIPULATING DATA WITH PUT AND GET.................................137
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`CONCATENATING ARRAYS AND STRINGS .................................139
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`INPUT AND OUTPUT OF STRING DATA .......................................140
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`ARRAYS VERSUS DICTIONARIES ................................................141
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`ADVANCED TECHNIQUES .............................................................142
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`CONCLUDING THOUGHTS ............................................................143
`EXERCISES .......................................................................... 144
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`Chapter 12
`Storing and Using Data........................................... 145
`CLASSICAL DATA STRUCTURES..................................................151
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`CONCLUDING THOUGHTS ............................................................156
`EXERCISES .......................................................................... 156
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`Chapter 13
`Program Data and Instructions .............................. 157
`TURNING DATA INTO INSTRUCTIONS .........................................159
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`TURNING INSTRUCTIONS INTO DATA .........................................161
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`DATA CONVERSIONS ....................................................................163
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`CONCLUDING THOUGHTS ............................................................165
`EXERCISES .......................................................................... 165
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`Chapter 14
`Using Files and Input/Output Techniques ............ 167
`OPENING AND CLOSING FILES ....................................................170
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`READING AND WRITING FILES .....................................................172
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`Table of Contents
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`WRITING FORMATTED DATA TO FILES .......................................175
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`FILE STATUS INFORMATION.........................................................178
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`RANDOM VERSUS SEQUENTIAL ACCESS ..................................179
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`CONCLUDING THOUGHTS ............................................................180
`EXERCISES........................................................................... 180
`
`Appendix
`Answers to Exercises ............................................. 185
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`Index of Example Programs ................................... 209
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`Subject Index ........................................................... 217
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`Petitioner Apple Inc. - Exhibit 1040, p. x
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`
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`Preface
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`This book is intended to provide a practical, intriguing, and fresh look at
`the PostScript programming language. PostScript is a mysterious
`language, powerful and cryptic. It is expressive and complicated and yet
`surprisingly simple. In order to master a programming language, you have
`to learn to think like the compiler or interpreter, and instinctively know
`how to solve problems. You develop a “tool kit” of useful approaches,
`proven solutions, and techniques. You reach an understanding that is
`based on analogy and connections with other things that you know.
`
`This book helps you build a solid foundation of understanding for the
`PostScript language. It teaches you to become an expert programmer and
`to have confidence that you have written the best possible PostScript
`program. It shows you how to combine the elements of the language into
`a strong, well-designed, modular program that is easy to develop and
`maintain. It is not a problem-solving book, nor simply a reference to the
`language; it is a guide to developing programming techniques and to
`learning how to use the PostScript tool kit, which is filled with hundreds
`of individual operators.
`
`Comparisons are drawn to other programming languages throughout the
`book, particularly to C, which is a very common language and one that is
`often used in the same environments where the PostScript language is
`found. If you are a competent C or Pascal programmer but you have had
`limited exposure to PostScript, this book should be exactly what you
`need. If you are a BASIC, Forth, or Lisp programmer, you should find
`this book at just the right level for introducing you to the PostScript
`
`xi
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`language. If you think of yourself as an expert PostScript programmer,
`you will still find some useful techniques in this book, and it is a very
`worthwhile addition to your library of PostScript books, since it overlaps
`very little with existing material.
`
`The most fundamental level of a program is the set of techniques used in
`the development of the program itself, regardless of what the program
`does. This book addresses that area of PostScript programming in a way
`rooted partly in computer science theory, encompassing algorithms and
`data structures, but it is very practically oriented toward real-world
`programming and programmers.
`
`Each chapter contains a section of Exercises. You should find these
`problems easy to solve if you have understood the concepts in the chapter.
`Many concepts do not gel fully without some direct application to a
`problem. In these cases, the exercises should stimulate you to think about
`the issues that have been presented and give you a chance to confront a
`situation that requires you to apply your knowledge. The appendix
`contains answers to the exercises.
`
`To give you a feel for what this book is about and to whet your appetite,
`the following code segment illustrates a common (though advanced)
`technique that is used in many PostScript programs. If you instantly
`recognize what it does and could have written it yourself, then you may
`use this book to discover other techniques that you may not have yet
`encountered. If this example seems mysterious or you have to spend a lot
`of time analyzing it, then you need this book to help you start thinking this
`way.
`
`% print out all the elements in an array or procedure
`dup type /arraytype eq 1 index type /packedarraytype eq or { %ifelse
`0 exch { %forall
`exch dup (XXX) cvs print (: ) print 1 add exch ==
`} forall pop
`}{ pop } ifelse
`
`This particular example is a bit advanced and special-purpose, but it
`should not be intimidating even to a beginning or intermediate
`programmer. A typical question in an end-of-chapter exercise might be
`“Rewrite the previous program fragment using local variables rather than
`using the operand stack.”
`
`xii
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`Preface
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`Petitioner Apple Inc. - Exhibit 1040, p. xii
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`In order to get the maximum benefit from this book, you probably should
`have written at least one program—from scratch—longer than 100 lines
`of code (and gotten it to work correctly). This program might have been
`written in virtually any programming language. If you then modified the
`program for an additional 100 hours after it first started to work, if you
`understand variables, arrays, and strings—and if you want to become a
`very good PostScript programmer—then you are an ideal candidate for
`this book.
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`Preface
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`Petitioner Apple Inc. - Exhibit 1040, p. xiii
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`Petitioner Apple Inc. - Exhibit 1040, p. xiv
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`Chapter 1
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`PostScript as a
`Programming Language
`
`It probably is true that PostScript is not everyone’s first choice as a
`programming language. But let’s put that premise behind us, and assume
`that you need (or want) to write a program in the PostScript language.
`How do you become an expert in this language, and how do you develop
`the instincts and learn the techniques necessary to tackle challenging
`problems with confidence?
`
`As a programming language, PostScript is somewhat unusual—and
`frankly, many of the programming instincts you’ve developed from using
`other languages may not be directly applicable with the PostScript
`language. The first few chapters of this book help you put PostScript into
`perspective by comparing it to languages you probably already know,
`such as C. There are stronger similarities between PostScript and, say,
`Forth or Lisp, but if you are expert in either of those languages you
`probably won’t have much trouble mastering PostScript.
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`Petitioner Apple Inc. - Exhibit 1040, p. 1
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`Every programming language was designed with a specific purpose, even
`if that purpose was to be a “general-purpose programming language.” To
`use a language effectively, you should understand its design and learn its
`strengths and weaknesses. PostScript is a full-fledged programming
`language, and it is possible to accomplish almost any task with it. It
`includes many high-level language constructs and data structures. The
`PostScript language is a tool kit filled with hundreds of special-purpose
`tools. Mastery of the language teaches you to select just the right
`instrument, swiftly and instinctively, and to use it effectively.
`
`DESIGN FEATURES
`
`PostScript’s most obvious language features are that it is an interpreted
`language, it is stack-based, and it is heavily oriented toward graphics and
`typography. These design choices were all made in order to make it useful
`as a device-independent page description language for imaging on raster
`devices. These language features also make it a fairly unusual language in
`which to write programs.
`
`The PostScript language is stack-based (resulting in “backwards” syntax)
`primarily so that a program can be evaluated with very low overhead in
`processing and storage. Each instruction can be acted upon immediately,
`without the need to buffer large amounts of the program.
`
`PostScript is interpreted (rather than compiled) for a couple of reasons.
`The most obvious one is device independence. If the program remains in
`source form until it reaches its final destination, it is extremely portable to
`new computer environments, processors, etc. It is an interpreted language
`also because documents are often transmitted long distances, archived, or
`saved and printed many times, and this enables the documents to remain
`in ASCII text source form for better life expectancy and resiliency in a
`device-independent environment.
`
`And finally, PostScript is heavily biased toward graphic imaging and
`typography because it was originally conceived as a mechanism to control
`raster imaging devices like laser printers and display screens.
`
`The design choices made in the PostScript language have quite an impact
`on the programmer’s experiences in mastering the language, but they
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`2
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`Chapter 1: POSTSCRIPT AS A PROGRAMMING LANGUAGE
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`Petitioner Apple Inc. - Exhibit 1040, p. 2
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`were not made arbitrarily, nor intended to make the programmer’s life
`more difficult.
`
`STRUCTURED PROGRAMMING TECHNIQUES
`
`If you have had any formal training in programming, you are familiar
`with the various structured programming techniques, up through and
`including object-oriented programming. These techniques vary, but are
`largely based on the concept of a procedure, or a modular functional unit
`
`black box). A black box is typically any set of
`(sometimes called a
`operations that has a specific input and output and performs a single task.
`The mechanics of the operation are hidden from one who uses the black
`box. This is the notion behind writing procedures, creating software
`libraries, object-oriented programming, block-structured code; it is a
`widely recognized technique in computer science.
`
`to structured
`isn’t well suited
`To start out, PostScript simply
`programming, and if you try too hard to formalize the interfaces, declare
`local variables, and observe many tried and true programming practices,
`you may end up with a complicated, inefficient, and ill-designed program.
`On the other hand, if you ignore structured programming techniques and
`write code off the top of your head, you will probably have something
`worse. The trick is to borrow enough of the useful concepts from
`structured programming to make programs readable, easy to maintain,
`easily debugged, and efficient, while using the expressive power of the
`language to your advantage.
`
`goto
` in the PostScript language, or even explicit
`There is no concept of
`control transfer, so you don’t have to worry about that particular dogmatic
`concern. You do have to be careful about procedure-calling interfaces,
`where they exist, since all program checking is done at run-time.
`
`Many of the concepts in structured programming can be used quite
`effectively in PostScript. You can define procedures to help you share
`similar code across parts of your program. You can use the operand stack
`for your temporary data rather than having many global variables. You
`can design interfaces between components of your program and pass
`parametric information on the stack. You can even define functions that
`return results onto the stack for use by other procedures.
`
`Chapter 1: POSTSCRIPT AS A PROGRAMMING LANGUAGE
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`Petitioner Apple Inc. - Exhibit 1040, p. 3
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`PROGRAMMING TASKS
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`There are many different kinds of tasks associated with programming.
`There are data-crunching tasks, user interface tasks, communication with
`other computers or programs, file operations, graphics, and various
`others. In some tasks, such as numerical computation, the algorithm by
`which the task is carried out is of the utmost importance, both for
`accuracy and for performance. Other tasks, such as user interfaces and
`graphics, are much more results oriented: what is important is whether it
`looks good and/or works well. In file operations, arguably the accuracy
`with which the operations are carried out is the most important aspect of
`the task, and the algorithm and performance of the task are secondary.
`
`The nature of the task has a strong influence on the programming style
`and technique. If your purpose is to draw graphics, your programming
`efforts should be minimal, clear, and used only to streamline the process
`of drawing. In
`the PostScript
`language
`in particular, graphics
`programming is the means to the end, not the end in itself. The
`PostScript
` mentions that any marks made on the page
`Language Reference Manual
`are a side effect of the execution of the program, but from your
`perspective as a programmer, it is the other way around. The fact that you
`have to write PostScript programs is a side effect of the fact that you are
`trying to draw pictures on a PostScript-based system.
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`WINDOW SYSTEMS, COMMUNICATIONS, AND DISPLAYS
`
`Recently, the PostScript language has evolved from a printer control
`language into a communications medium on host computers. There are
`window systems and networks based on the PostScript language. In light
`of this, there are many new kinds of programming tasks that must be (or
`can be) carried out in the PostScript language.
`
`interesting aspects of a window system or
`the more
`One of
`communications environment based on a programming language is that
`you can send an algorithmic description of the picture you want drawn (or
`of the operation you want carried out) instead of the individual bits or
`instructions. For instance, if you wanted to draw a grid of parallel lines,
`both horizontally and vertically, you might express this graphic operation
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`4
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`Chapter 1: POSTSCRIPT AS A PROGRAMMING LANGUAGE
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`Petitioner Apple Inc. - Exhibit 1040, p. 4
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`(which has a strongly repetitive basis) as two loops of instructions. (See
`Example 1.1. The results of Example 1.1 are shown in Figure 1.1).
`
`Example 1.1: Repetitive Algorithms
`
`% 18-pt spacing, 24 lines
`0 18 18 24 mul { %for
`dup 0 moveto
`600 lineto
`} for stroke
`
`% 18-pt spacing, 36 lines
`0 18 18 36 mul { %for
`dup 0 exch moveto
`436 exch lineto
`} for stroke
`
`Figure 1.1: Output of Example 1.1
`
`This is quite a bit more compact than it would be to draw 60 individual
`
`moveto and
`
`lineto instructions.
`lines, each with its own coordinates and
`0
`; just
`In this particular example, 25 percent of the data end up being
`avoiding the transmission of those zeroes is significant. The loops above
`consist of 30 PostScript language tokens to be transmitted, whereas the
`
`moveto and
`
`lineto instructions would require
`same program done with
`360 PostScript tokens.
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`Chapter 1: POSTSCRIPT AS A PROGRAMMING LANGUAGE
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`5
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`Petitioner Apple Inc. - Exhibit 1040, p. 5
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`In addition to providing power and compactness of representation, having
`an interpreted programming language at the network and graphics layer of
`a computer system provides a very neat solution to problems of
`transportability and machine independence, since many of the decisions
`are delayed until run-time. One important concept in object-oriented
`programming in general is to keep the interface between objects clean
`enough that there are not any problems when you hook them together.
`This is especially important where you have to write programs at the
`places where objects meet. It is important for you, the reader and
`programmer, to understand PostScript well enough to create programs
`that behave well in the nested, shared, cut-and-pasted environments that
`exist today.
`
`DATA STRUCTURES AND ALGORITHMS
`
`The classic view of a computer program is a synthesis of data structures
`and algorithms. Books often have titles that contain these two words, and
`every program probably has one or the other in it somewhere. If you use
`PostScript to describe pages and to drive laser printers, your need for real
`data storage and computation should be minimal. However, if you use
`PostScript in a windowing environment to arbitrate input events from the
`keyboard (as an extreme example), you will definitely need to create data
`structures and implement some potentially difficult algorithms.
`
`CONCLUDING THOUGHTS
`
`The next several chapters help you build some solid approaches to
`PostScript data structures and help you master the language elements
`thoroughly so that you will feel comfortable implementing your favorite
`algorithms. In particular, the data types unique to PostScript are explored
`(dictionaries and names) as well as traditional data types such as strings,
`arrays, and numbers. Other chapters are devoted entirely to seemingly
`
`ifelse operator or looping. However, since
`simple things like the
`PostScript was not originally designed for heavy programming, these
`traditional programming
`techniques have never been
`thoroughly
`addressed. And you simply can’t write a serious PostScript program
`
`ifelse or writing a procedure or two.
`without using
`
`6
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`Chapter 1: POSTSCRIPT AS A PROGRAMMING LANGUAGE
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`Petitioner Apple Inc. - Exhibit 1040, p. 6
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`EXERCISES
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`Each chapter in this book comes with some exercises at the end of it, to
`help you to test your understanding of the concepts presented. Since this
`first chapter has taken a philosophical overview of PostScript as a
`programming language, the exercises are more like essay questions than
`problems.
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`1.
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`2.
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`3.
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`4.
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`Why do you want to write a PostScript program? (This is a serious
`question, and one which should be given some thought; reflection on
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`why may provide you with good horse sense throughout your pro-
`gramming experience.)
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`Why is PostScript a “backwards” language, in which the operators
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`after the operands?
`come
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`Why is PostScript an interpreted language?
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`Why is it silly to write fractal programs in PostScript?
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`Chapter 1: POSTSCRIPT AS A PROGRAMMING LANGUAGE
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`7
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`Petitioner Apple Inc. - Exhibit 1040, p. 7
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`8
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`Chapter 1: POSTSCRIPT AS A PROGRAMMING LANGUAGE
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`Petitioner Apple Inc. - Exhibit 1040, p. 8
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`Chapter 2
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`PostScript is Not Like C
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`Many programmers try to treat a new language as if it were an old
`language with which they are already familiar. “If you know one
`programming language, you know them all.” That is somewhat true with
`procedural languages like C, Pascal, Modula 3, or maybe FORTRAN.
`
`if ...
`then
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`else, and once you learn
` ...
`There are only so many ways to say
`one language, you can learn the others without too much difficulty. These
`languages typically only have a dozen or so language elements; if you
`were to name all the “reserved words” in Pascal or C, how many would
`you come up with? If you named the built-in features of the language, you
`wouldn’t get much past control structures, procedure definitions, data
`types, and input/output capabilities. These languages are general-purpose
`procedural languages.
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`But PostScript has over 300 individual special-purpose operators, it is
`interpreted rather than compiled, it uses post-fix notation, and is quite
`different from C. This is good, to a large extent, but again, you need to
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`9
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`Petitioner Apple Inc. - Exhibit 1040, p. 9
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`understand the differences rather than to take the posture that it’s just
`another language.
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`In a procedural language, the interface between components of a program
`is formalized in the notion of a procedure call. (For more information
`about procedures, see Chapter 9). The program is typically written as a
`series of modular procedures or functions, with a well-defined calling
`mechanism for transferring control and passing data between the
`modules. This calling mechanism is artificial, and is apparent only at the
`programming language level. When compiled, the data (or pointers to the
`data) are in fact placed on the operand stack of the hardware processor,
`and an instruction to jump to a subroutine (JSR) is issued to transfer
`control to the other procedure, which then uses the data on the stack. The
`notion of type checking is carried out at the language level by requiring
`that the data be declared to be of an explicit type as the data are being
`passed to or from a procedure or function.
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`When writing C programs, the programmer designs the modules, the
`interfaces, and the interaction between them. The data structures are
`designed to work cleanly within the modular design of the program, and
`the parameters to the various procedures are set up to pass data
`appropriately between the modules or procedures. Data types and
`structures should be appropriate for the task at hand; some of them are
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`globally visible (and therefore can be used throughout the entire
`made
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`local to a particular procedure (and can be used only
`program); others are
`within that procedure). A fairly high percentage of coding time is spent
`just selecting names for data types, structure elements, procedures,
`variables, and modules. The names given to these program elements
`contribute heavily to the readability and maintainability of the code.
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`The PostScript language is an interesting hybrid between high-level and
`low-level langua