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\ Lesson 2 - Using F-PC \ The Forth Course \ by Richard E. Haskell \ Dept. of Computer Science and Engineering \ Oakland University, Rochester, MI 48309
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Lesson 2
USING F-PC
2.1 USING SED TO EDIT FILES ......................................................................... 2-2 2.2 LOADING AND RUNNING YOUR PROGRAMS ........................................ 2-3 2.3 DEBUGGING YOUR PROGRAMS ............................................................. 2-4 2.4 EXERCISES ................................................................................................ 2-6
2.1 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 newfile 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 \ 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 2/ \ 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 5 of the F-PC's USER'S MANUAL. This manual is on disk in the F-PC package and you can type out Chapter 5 by typing type chapter5.txt Once you have finished entering the program you can leave SED by pressing the function key F10. 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 will now be able to load and run your program. You can also reenter the editor at the point you left off by just typing ED.
¡@ ¡@ 2.2 LOADING AND RUNNING YOUR PROGRAMS To load the program you typed in the file HW1.SEQ 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: 31 16 94 69 area . 3339 31 16 94 69 circum . 232 31 16 94 69 center . . 62 42 ¡@ 10 27 215 230 area . -23921 10 27 215 230 circum . 816 10 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. 10 27 215 230 area u. 41615
¡@ 2.3 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
This will take you into the BROWSE mode of the editor and display the definition of SIDES in the file LESSON2.SEQ. At this point you can browse through LESSON2.SEQ to see what else is there. Press F10 followed by ESC to get back to the ok prompt. At this point you could edit the file LESSON2.SEQ by just typing ED. 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.
10 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. 10 27 215 230 AREA [4] 10 27 215 230 12648 0 : SIDES ?> [2] 203 205 12648 2 * ?> [1] 41615 12648 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 and will provide the full screen for debugging; pressing N will nest into the word to be executed; pressing U will unnest back to the calling word.
pressing S will allow you to skip (at high speed) to a word further in the definition that you select by moving forward (using +) or backward (using -) in the definition until you reach the word and then press <Enter>. You can press ESC to cancel the skip.
As an example of nesting, type DEBUG AREA and then type 10 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.
¡@ ¡@ ¡@ ¡@ 2.4 EXERCISES
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 ¡@ 4 5 stacktest
What value in terms of a and b is left on the stack?
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