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-=== Lesson 2 ===
-<code>
-\       Lesson 2 - Using F-PC
-\       The Forth Course
-\       by Richard E. Haskell
-\          Dept. of Computer Science and Engineering
-\          Oakland University, Rochester, MI 48309
-comment:
-                                Lesson 2
-                               USING F-PC
-.1  USING SED TO EDIT FILES            2-2
-.2  LOADING AND RUNNING YOUR PROGRAMS  2-3
-.3  DEBUGGING YOUR PROGRAMS            2-4
-                     EXERCISES                          2-6
-.1  USING SED TO EDIT FILES
-        The full-screen editor SED is used to edit your programs and
-        store them in permanent disk files.  For example, to write the
-        program for solving Exercise 1.1 in Lesson 1 enter F-PC
-        and in response to the ok prompt type
-        sed hw1
-        This will create a new sequential file called HW1.SEQ.
-        All F-PC source files have the extension SEQ that is automatically
-        appended to your filename if you do not specify it.
-        The first line of your program should begin with a backslash \
-        and followed by the name of your program or file.  This first
-        line will be printed at the top of each page when you print out a
-        listing of the program.  You do this by typing FPRINT HW1.
-        On the second line of the file type a backslash \ and a Tab.
-        Then press Alt-O P.  This will paste the date and time.
-        Now type in the complete program as follows:
-comment;
-\       Homework #1
-\        07/02/89 08:25:20.35
-\       Exercise 1.1 -- Lesson 1
-\       Find area, circumference and center of rectangle
-: sides         ( t l b r -- r-l b-t )
-                ROT                     \ t b r l
-                -                       \ t b r-l
-                -ROT                    \ r-l t b
-                SWAP - ;                \ r-l b-t
-: area          ( t l b r -- area )
-                sides * ;
-: circum        ( t l b r -- circum )
-                sides + 2* ;
-: center        ( t l b r -- xc yc )
-                ROT                     \ t b r l
-                +                       \ t b r+l
-/                      \ t b xc
-                -ROT                    \ xc t b
-                + 2/ ;                  \ xc yc
-comment:
-        Note that an intermediate word SIDES was defined that leaves
-        the values (right-left) and (bottom-top) on the stack.  The
-        word SIDES is then used in the definitions of both AREA and
-        CIRCUM.  F-PC is case insensitive which means that you can use
-        upper or lower case interchangeably.  We will normally follow
-        the convention of defining our own words in lower case and use
-        upper case for F-PC words that we use in our definitions.  This
-        makes it easy to recognize in a definition which words are F-PC
-        words and which ones are words that we have previously defined.
-        Note also that in the definitions of SIDES and CENTER we have
-        shown the stack picture as comments on the right side of each
-        line.  You will find this to be very useful when considerable
-        stack manipulation is going on.
-        The SED editor has a full range of editing capabilities that are
-        described in CHAPTER 4 of the F-PC's USER'S MANUAL.  This manual
-        is on disk in the F-PC package and you can type out Chapter 4 by
-        typing
-        type chapter4.txt
-        Once you have finished entering the program you leave SED by
-        pressing ESC followed by <Enter>.  At this point you can edit
-        another file by just typing its name.  If you don't want to edit
-        another file at this point press ESC.  You are now in F-PC with the
-        ok prompt.  You should generally be able to load and run your
-        program at this point.  However, the SED editor has not released
-        the memory that it used in editing your program and if you have
-        a large program that allocates lots of memory you may have trouble
-        loading it at this point.  I have therefore adopted the practice
-        of leaving F-PC at this point by typing BYE and then entering it
-        again by typing F from DOS.  At this point you can load any size
-        file without problems.
-.2  LOADING AND RUNNING YOUR PROGRAMS
-        To load the program you typed in the file HW1.SEQ just type
-        fload hw1
-        If you didn't make the file HW1.SEQ you can accomplish the same
-        thing by just loading this lesson by typing
-        fload lesson2
-        This is because everything is a comment except the definitions of
-        the words SIDES, AREA, CIRCUM and CENTER.
-        The process of loading either of these files causes all of the
-        colon definitions to be added to the dictionary.  It is just as
-        if you typed in all of the colon definitions when in the
-        interpretive mode of F-PC.  Except that all the colon definitions
-        are now saved on disk and you can go in and edit them at any time.
-        If you test the program by using the values given in Exercise 1.1
-        you will obtain the following results:
-16 94 69 area . 3339
-16 94 69 circum . 232
-16 94 69 center . . 62 42
-27 215 230 area . -23921
-27 215 230 circum . 816
-27 215 230 center . . 112 128
-        The second value of area equal to -23921 makes no sense.  What
-        has happened is that the area is greater that 32767 and thus
-        the 16-bit signed value has gone into the negative region because
-        bit 15, the sign bit, is set to 1.  We can print out the real
-        value for the area by using the Forth word U. (U-dot) rather than
-        . (dot).  The word U. prints out the unsigned 16-bit integer value
-        on top of the stack.  This will produce the following result.
-27 215 230 area u. 41615
-.3  DEBUGGING YOUR PROGRAMS
-        F-PC has several useful words to help you debug your programs.
-        The word SEE will let you decompile a word.  For example, after
-        FLOADing the file HW1 (or LESSON2) type
-        see area
-        see sides
-        see circum
-        see center
-        Note that each colon definition is shown.  This is done by looking
-        up each word in the dictionary and then looking up the name of each
-        word in its definition.
-        The word VIEW allows you to find the file in which a given word is
-        defined and to display the actual definition as written in the file.
-        Type
-        view sides
-        You can use the word VIEW to find the definition of any F-PC word.
-        The F-PC word DEBUG is a powerful debugging tool that allows you
-        to single step through the definition of a word while watching
-        what is being stored on the stack.
-        After FLOADing LESSON2 type
-        debug area
-        The next time the word AREA is executed, it will pause at each word
-        in its definition and display the stack contents.  Pressing any key
-        (other than Q, C, N, U, X or F) will continue the single stepping.
-        For example, if you type
-27 215 230 area
-        the definition of AREA will be displayed on the top of the screen
-        and the following will be displayed on the bottom of the screen as
-        you press the space bar three times.
-27 215 230 AREA  [4]    10     27      215     230
-  0 :   SIDES      ?>  [2]    203  205
-  2     *          ?>  [1]  41615
-  4     UNNEST     ?>  ok
-        After each name in the definition is executed by pressing the
-        space bar, the number of items on the stack is shown in the
-        brackets [ ] and the values of the top four items on the stack
-        are displayed.  Note in this case that when the two values 203
-        and 205 were multiplied to produce 41615 the reason for the
-        displayed value of -23921 when we tested the program becomes clear.
-        In fact, after single stepping through the above definition of AREA
-        the value 41615 will be left on the stack.  If you then type . (DOT)
-        the value -23921 will be displayed.  This, of course, is just the
-        signed value of the 16-bit integer 41615.
-        Type UNBUG to cancel debug so that future executions of AREA will
-        not be debugged.  While single stepping,
-                pressing Q will quit DEBUG and execute UNBUG;
-                pressing C will continue execution to the end of the
-                definition or until <Enter> is pressed;
-                pressing F will return temporarily to Forth
-                (press <Enter> to return to single stepping of the
-                debugged word);
-                pressing X will toggle the source listing on and off;
-                pressing N will nest into the word to be executed;
-                pressing U will unnest back to the calling word.
-        As an example of nesting, type DEBUG AREA and then type
-27 215 230 AREA
-        Then press N.  This will nest into the definition of SIDES.
-        Single step through this definition and watch how it returns
-        to the definition of AREA.
-EXERCISE 2.1
-        Make a file called HW2.SEQ and write the following colon definition
-        in this file:
-        : stacktest     ( a b -- ? )
-                        DUP *
-                        SWAP DUP
-                        * + ;
-        Add a stack picture to the right of each line in this definition.
-        FLOAD the file and use DEBUG to single step through the word when
-        you type
-5 stacktest
-        What value in terms of a and b is left on the stack?
-comment;
-</code>