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Forth Tutorial ************** This chapter is a brief tutorial about the Forth language. It will get you started with Forth, but you should probably buy a Forth book to learn more. If you already know how to program in Forth, you may skip this chapter. You may want to look at the chapter "Glossary Functional Index". Forth is a complete programming language with some unique features. In fact, it is probably unlike any other language you have ever used. Please don't let it's "strangeness" cause you to immediately reject it --- once you get used to it, you may find that you can use Forth to write working programs faster than you ever dreamed possible. Forth is often described as an assembler, compiler, operating system, editor, and command language all rolled into one. In addition, most of the Forth system is written in Forth. To find out more about your RISC OS Forthmacs system try words see words whatis words or others. Getting Started =============== If you sit down to a computer running Forth, you'll probably see the word OK right before the cursor. OK is the "prompt". It means that Forth is ready for you to type something. Suppose you type 1 2 + . Always type the cr, Return, lf, Linefeed, or whatever equivalent key your terminal has, at the end of the line. Forth will respond with 3 You have just added 1 and 2, and printed the result. I am going to assume at this point that you understand the concept of a stack. Forth uses stacks heavily. In fact, the consistent stack orientation is the key to Forth's ability to pack a lot of capability in a small amount of memory. Going back to our example, typing "1" caused the number 1 to be pushed onto Forth's main stack, which is called the data stack. Typing "2" caused the number 2 to be pushed onto the same stack, on top of the 1. The "+" caused two numbers to be popped off the stack, in this case 1 and 2. The numbers were added together and the result was put back on the stack. The "." caused a number to be popped off the stack, in this case 3, and printed on the terminal. If you have an HP calculator, no doubt this sounds very familiar to you. An important differences is that HP calculators have a stack that is 4 numbers deep, whereas Forth's stack is much larger, typically more than 64 numbers. In RISC OS Forthmacs this is PS-SIZE divided by the cell size. Other arithmetic operators that Forth knows about are - (minus), * (times), and / (divide), as well a some others that we'll worry about later. The arithmetic is performed as 32-bit integer arithmetic, although there are operators which will cope with other kinds of numbers. Defining your own words ======================= A Forth system understands a number of "words", each of which causes something to happen, usually to the stack. In our example, "+" and "." were both words. You can define your own words in terms of words the system already knows. Once defined, your words may be used just as though they they were system words. Another example: : addprint + . ; Here, the ":", which is a system word, causes a new word to be defined, in this case addprint. The definition of addprint is (B + . ), which means that when you execute addprint, two numbers will be taken from the stack and the sum printed. The ;, another system word, terminates the definition. If you were to later type 23 45 addprint Forth would respond with 68 A definition of this sort is called a "colon definition". A distinguishing characteristic is that the new word is defined in terms of other Forth words. It is also possible to define a word in terms of the assembly language of the machine you are actually running on. This kind of definition is called a "code definition", and is not used nearly as often as colon definitions. If you wished to later use addprint in the definition of another word, you could do so just as though addprint were a system-supplied word. : addprint2 addprint addprint ; 1 2 3 4 addprint2 would result in 7 3 Note the order in which the numbers were added. The reason is that, when addprint2 is executed, 4 is on top of the stack, with 3 underneath it. The first addition will thus be 3+4. It is important to keep track of what is on the stack when you are writing Forth programs. An essential part of the documentation of a Forth word is a stack diagram. A stack diagram shows what the word expects to be on the stack when it starts executing, and what it leaves on the stack after it is finished. The stack diagram for addprint would be ( n1 n2 -- sum ) showing that two numbers ( n1 and n2 ) are expected to be on the stack at the start, and that addprint leaves one number on the stack ( sum ) when it is finished. The "--" shows where the word executes, or, in other words, it separates the input parameters from the output parameters. Stack diagrams are enclosed in parentheses, because parentheses are used in Forth to enclose comments. The Forth system will ignore everything between "(" and the next ")". The space between "(" and "n1" is necessary. The space between "sum" and ")" is optional in most Forth's, but it is a good idea to include it anyway. By convention, the rightmost number (n2) is the one that is on top of the stack. Thus ( a b c -- d e f ) would signify a word which expected c on top of the stack, b under c, and a under b. After execution, f would be on top, with e under f, and d under e. Variables, @, ! =============== variable john ( -- adr ) defines a variable called john and allocates one 32-bit machines word for it. The stack diagram shows that when john is executed, an address will be left on the stack. The address is the address of the word that was allocated for john. Right now we don't know what value is contained in the variable, so let's put something in it. 123 john ! will put the number 123 in the variable. ! , pronounced 'store' , is the store operator. Its stack diagram looks like ( value adr -- ), meaning that it expects an address on the top of the stack and a value underneath the address. The value will be put in the machine word at the indicated address, and both arguments will be removed from the stack. Note in this example that the address was left by the word john. It is an important point that variables leave their addresses, rather than their values. If the value of a variable is needed, the @ (fetch) operator is used. john @ will leave the value contained in the variable, in this case 123, on the stack. The stack diagram for @ is ( adr -- value ). It is reasonable to ask why it works this way, rather than having a variable return its value. In fact, a scheme to do this was discussed in the Forth community for awhile, and the final consensus was that the the consistency and regularity of the @ and ! operators was too powerful to give up. Forth is not the only language to do it is this way. BLISS is another language which requires an explicit operator to return the value of a variable. Similar operators, C! and C@, are provided for storing and fetching bytes instead of 32-bit quantities. The stack only holds 32-bit quantities, so the bytes are stored in the least-significant part of the 32-bit number on the stack. w! ( w adr -- ) stores the least-significant 16 bits of the number on the stack into the 16-bit memory location at addr w@ ( adr -- w ) fetches the 16-bit word at addr, leaving it on the stack. The high-order bits of the stack item are zeroed. c! ( byte adr -- ) stores the least-significant byte of the stack item into the 8-bit memory location at adr c@ ( adr -- byte ) fetches the byte at addr +! ( n adr -- ) adds n to the contents of the 32-bit word at addr Constants ========= 32 constant buflen defines a constant whose value is 32. When buflen is executed, it leaves its value, in contrast to a variable, which leaves its address. There is no explicit mechanism provided for changing the value of a constant. Many Forth systems may be tricked into telling you the address where the value of a constant is actually stored, so it may be possible to change the value. However, this is non-standard and implementation dependent. Use a variable if you want to change the value. buflen buflen + . results in 64 buflen john @ + . results in 155 assuming that john was defined and initialised as before to 123. you should make sure you understand why the @ is needed after john but not after buflen. The stack diagram for a constant is ( -- value ). Remember that this is what happens to the stack when the constant is executed, not when it is defined. More generally, the word CONSTANT need not be explicitly preceded by a number. CONSTANT takes the initial value off of the stack. The stack diagram is constant \ name ( initial_value -- ) "\ name" means that when the defining word constant executes, it takes a name ( a sequence of non-blank characters ) out of the input stream and uses that name for something. In this case, the name is used as the name of the new constant. Notice that distinction between arguments that are taken from the stack and names that are taken from the input stream ( the input stream is what you type at the terminal ). Since typing a number puts that number on the stack, our example works fine. It is also possible to use Forth to calculate the value of the constant, as in buflen 10 * constant bigbufsize which defines a constant whose value is 10 times buflen, or 320. This is completely general; if you can get a number on the stack by any means whatsoever, you can define a constant which returns that value. Stack Manipulation ================== The following words rearrange numbers on the stack. dup ( n -- n n ) duplicates the number on top of the stack. This is a real workhorse - dup is used a lot when you want to make a copy because you want to remember a number but the word you want to use next will "eat" it. drop ( n -- ) discards the number on top of the stack. swap ( n1 n2 -- n2 n1 ) exchanges the positions of the number on top of the stack and the number underneath it. Suppose you had 3 4 on the stack and you wanted to compute 4-3. You could do SWAP - and you would get what you wanted. over ( n1 n2 -- n1 n2 n1 ) copies the number which is directly underneath the top of the stack so that a copy of it on top. rot ( n1 n2 n3 -- n2 n3 n1 ) pulls the third stack element to the top and moves the two which were above it down to fill its vacancy. pick ( np np-1 np-2 ... n0 p -- np np-1 np-2 np-2 ... n0 np ) copies the p-th stack element to the top of the stack; 0 pick is equivalent to dup 1 pick is equivalent to over roll ( np np-1 np-2 ... n0 p -- np-1 np-2 ... n0 np ) moves the p-th stack element to the top and moves the others down to fill the vacancy. 3 roll is equivalent to rot depth ( -- n ) leaves the number of items that were on the stack before depth executed. The number that depth leaves on the stack is not included in the count. Conditionals ============ Of course Forth lets you make tests and do different things based on the results of those tests. Here are some words which are useful for comparisons. < ( n1 n2 -- flag ) Flag is true if n1 < n2 = ( n1 n2 -- flag ) Flag is true if n1 = n2 > ( n1 n2 -- flag ) Flag is true if n1 > n2 0< ( n1 -- flag ) Flag is true if n1 < 0 0= ( n1 -- flag ) Flag is true if n1 = 0 <> ( n1 n2 -- flag ) Flag is true if n1 not equal to n2 0> ( n1 -- flag ) Flag is true if n1 > 0 u< ( un1 un2 -- flag) "<" for unsigned integers, such as addresses ?dup (n-- [n] n ) duplicate the top of the stack only if it is not zero The brackets mean that the number in brackets is present only if the number on top of it is nonzero. This convention is useful because some words leave a flag on top of the stack to indicate whether or not a particular operation succeeded. If it did succeed, the flag is true and the word has left some results on the stack underneath the flag. If the operation failed, the flag is false (zero) and the results were not put on the stack. The basic conditional in Forth is IF . It's stack diagram is ( n -- ), and it is used like: if stuff_to_do_if_n_was_not_0 else stuff_to_do_if_n_was_0 then The ELSE part is optional. The use of the English word "then" to terminate the conditional is misleading, but that is the way it is. : absval ( n -- absolute_value_of_n ) dup 0 < if 0 swap - then ; Let's analyze this definition carefully. First of all, when this word executes, it expects a number to be on top of the stack. Call the number n. The first thing that happens is that DUP is executed, putting another copy of n on top of the stack. The stack now looks like ( n n ). Next we have "0", which puts a zero on top of the stack, so that stack looks like ( n n 0 ). Then we execute "<", which compares to see whether or not n is less than 0. The two numbers that "<" compares are removed from the stack and a flag is left instead. Suppose that n was -5, so that n really was less than 0. "<" would then leave a -1 on top of the stack. Any nonzero number is considered to mean TRUE, whereas zero means FALSE. So the stack is ( n 1 ). IF takes the number off the top of the stack, sees that it is -1, and proceeds to execute 0 SWAP - . This has the effect of negating n. The complete sequence of stack states is ( n ) dup ( n n ) 0 ( n n 0 ) < ( n 1 ) if ( n ) 0 ( n 0 ) swap ( 0 n ) - ( -n ) then ( -n ) The THEN terminates the conditional. If n had been positive, "<" would have left a zero, because 0 is not greater than a positive number. IF would have taken the zero off of the stack and then skipped the stuff between IF and THEN. If there had been an ELSE clause, it would have been executed. Instead of DUP 0 < , we could have used DUP 0< . As a matter of fact there is an easier way to negate a number than 0 SWAP - . You can just use NEGATE ( n -- n ). In fact, absval duplicates the function provided by system-supplied Forth word ABS ( n -- |n| ). IF ... ELSE ... THEN may be nested to any depth, provided of course that there is a THEN for every IF. IF may only be used within colon definitions, which means that you can't just type in IF blah-blah-blah THEN unless you are in the middle of defining some other word. Loops ===== The basic looping construct is the DO LOOP. The stack diagram is (end+1 start -- ). Examples are necessary. : printlots ( n9 n8 ... n1 n0 -- ) 10 0 do . loop ; will print ten numbers from off of the stack. The loop will start at 0 and end after 9. You can get at the loop index by using I . It leaves the loop index on the stack (where else?). : sumit010 ( -- sum ) 0 11 1 do i + loop ; will add up the numbers from 1 to 10 inclusive. The "0" is there because the current running sum is kept on the stack, and the sum needs to be initialised to 0. LOOP limits don't have to be positive. Try out various limits and see what happens. : bigjumps ( -- n ) 0 1000 0 do i + 10 +loop ; will sum the multiples of 10 less than 1000. +LOOP increments the loop index by a number that it takes from the stack, in this case 10. You can run loops backwards if you wish, by using +LOOP with a negative increment. You may prematurely exit from a DO LOOP by executing LEAVE within the loop. You probably want to enclose the LEAVE in an IF ... THEN structure, because it doesn't make sense to construct a loop which always exits after the first times through. More general loops may be constructed with begin ... until or begin ... while ... repeat In the case of BEGIN ... UNTIL, the stack is checked by UNTIL. If if is zero, looping continues at BEGIN. If it is one, execution continues after UNTIL. For BEGIN ... WHILE ... REPEAT, WHILE checks the stack. If it is 1, looping proceeds, first executing the stuff between WHILE and REPEAT, then starting over at BEGIN. If WHILE found a zero on the stack, the stuff between WHILE and REPEAT is skipped and execution continues after "repeat". Well, campers, it's example time again. This time we'll assume that there is a table somewhere in memory. This table consists of 8-word entries, and the end of the table is flagged by a -1 in the first word of the last entry. We would like to scan through the table and add a number to the third word in each entry. This third word represents the age of an employee, so we'll call it AGE . Our operation will make everybody older. Study this example carefully. It shows how Forth programs should be constructed with simple words. : age ( entry_adr -- age_adr ) \ converts entry adr to age adr 12 + ; : addin ( n adr -- n adr ) \ adds n to the number at adr over over \ copy both n and adr age +! \ increase age by n ; : more? ( adr -- adr flag ) \ flag is false iff adr points to a -1 dup @ -1 <> ; : +entry ( adr1 -- adr2 ) \ advances to the next entry 32 + ; : older ( n adr -- ) \ adds n to each age in table at adr begin more? while addin +entry repeat drop drop ; Supposing that there is a word EMPLOYEES which returns the address of such a table, we could make a everybody 100 years older by typing 100 employees older Notice that I was careful to define each word before I used it inside another word. This is a requirement with typical Forth systems. There is no "forward referencing", so everything must be defined before it is used. This may seem archaic, but in fact it is seldom ever missed. The payoff is great, in terms of reduced compiler complexity and conceptual simplicity. There are Forth "cross-compiler" and "meta-compiler" programs which allow forward references to varying degree, but they are generally used to build entire Forth kernels from scratch. Now suppose we want to print out multiples of 2 until the user types a key on his keyboard, at which time we want to stop. There is a word KEY? which returns true if the user has typed something since the last time we checked, 0 otherwise. : mults-of-2 ( -- ) 0 begin dup . 2 + key? until drop ; Of course, this could have been done as : mults-of-2 ( -- ) 32766 0 do i . key? if leave then 2 +loop ; but the real point here was to illustrate the BEGIN ... UNTIL . key? ( -- flag ) input and output flag is true if a key has been typed. key ( -- char ) key waits until the user types a character, then returns the character emit ( char -- ) emit displays the char on the terminal space ( -- ) displays a space character. cr ( -- ) displays a carriage return, line feed spaces ( n -- ) displays n spaces type ( adr n -- ) displays n characters, beginning at adr ." string of characters to display" here you actually have to type the double-quote characters. The string starts after the space following the first quote, and continues up to but not including the last quote. expect ( adr n -- ) accepts n characters from the terminal, storing them at addr. If a carriage return is typed before n characters have been accumulated, expect fills out the rest of the n locations with nulls -trailing ( adr nl -- adr n2 ) reduces the character count n1 so that trailing blanks are omitted count ( adr -- adr+1 n ) converts a packed string to an address and character count A word about COUNT is in order. Forth stores strings as a length byte followed by a number of characters. The length byte contains the number of characters in the string, not including the length byte. COUNT takes the address of a packed string, which is the address of its length byte, and returns the address of the actual characters in the string, which is 1 more than the address of the length byte. On top of that, it returns the value of the length byte. Note that Forth addresses are byte addresses. If you add 1 to a Forth address, you get the next byte. To get the next word, you have to add 2. Normally, when you are typing Forth words for it to execute, they are being stored in a buffer called the "terminal input buffer". To execute the next word, it first has to be fetched from the terminal input buffer. To do this, a word called WORD is executed. WORD takes the next word from the terminal input buffer and stores it as a packed string, then returns the address of the packed string. The boundary between one word and the next is denoted by a space character. Actually, WORD accepts an argument which tells it what to use as the separator character. Normally, space is used, but if you want to use WORD yourself, you are free to chose your own delimiter. word \ word ( delimiter_char -- adr ) accepts the next word, or sequence of characters not including the delimiter_char, from the input stream. Stores it as a packed string and returns its address (the address of the length byte). Numeric I/O =========== We have already seen how to use "." Also, we have seen that numbers may be put on the stack simply by typing the number. More options are available, such as accepting or typing hexadecimal numbers. base ( -- adr ) a variable which contains the value of the current numeric input and output base. hex ( -- ) sets base to 16 ( default when running stand alone ) decimal ( -- ) sets base to 10 ( the default when running under unix ) . ( n -- ) displays n according to the current BASE u. ( u -- ) displays u as an unsigned number .r ( n r -- ) displays n right-justified in a field of width r u.r ( u r -- ) like ".r", but unsigned ? ( adr -- ) displays the contents of the memory location at address adr. Equivalent to "@ ." .s ( -- ) displays the entire stack contents without removing anything from the stack Note that executing BASE @ . is useless as a way to find out what the current value of BASE is. You will always get 10 as the answer because whatever BASE is, it will be typed as "10" according to its own base. If you want to find out what the base is according to the decimal numbering system, this will work: : decbase ( -- ) base @ dup decimal . base ! ; Can you figure out why this works? Arrays ====== Forth doesn't have very many explicit data structures, but it provides good primitives for efficiently creating whatever kind of data structures you want. Let's build an array. We need to know a couple more words first. create \ name ( -- adr ) defines a new word which will return its address when executed. The name of the new word is taken from the input stream. allot ( n -- ) allocates n bytes of storage for the most recently defined word. CREATE is the like VARIABLE, except that variable automatically allocates /N bytes of storage. variable xxx is like the CREATE xxx /cell allot). Okay, here we go. : arrayvar \ name ( size-in-words -- ) cells create allot ; : a@ ( index adr -- value ) swap cells + @ ; : a! ( value index adr -- ) swap cells + ! ; ARRAYVAR will allow us to create an array to hold size words. a@ and a! will allow us to fetch and store elements of an array we have created. Let's make an array and initialise it to all zeroes. 50 arrayvar ourarray : init ( n adr -- ) swap 0 do 0 over i a! loop ; 50 ourarray init Now let's put a 5 in the 23rd element. \d 5 23 ourarray a! Is it there? 23 ourarray a@ . Of course it is. Would I lie to you? Data structures in Forth can be as sophisticated as you can imagine. It's up to you; the primitives are all present. Vocabularies ============ Note: This section is incomplete. The stuff that is here is correct, but there are some words that I don't list. Read the "Vocabularies and Search Order" chapter in the manual for more information. Forth words may be grouped together in sets called VOCABULARIES . This is somewhat analogous to directories in other operating systems, whereby files are grouped together in different places. When Forth is looking for a word, it may search several vocabularies. The vocabulary where Forth looks first is called the "context" vocabulary, and there is a system variable called CONTEXT which keeps track of which vocabulary is the "context". All Forth systems have a vocabulary whose name is FORTH. If the word that is sought is not found in "context", a list of other vocabularies is searched. The "context" vocabulary specifies where to look for words. It is also necessary to know where to put the new words created by the user. This vocabulary is called the "current" vocabulary (also known as the "compilation vocabulary), and again there is a system word CURRENT to keep track of which vocabulary is selected as the "current" one. Typical vocabularies present in most systems include FORTH, and ASSEMBLER. Others may be present, and you can create your own if you wish. After you have created a vocabulary, you make make it the "context" vocabulary by typing the vocabulary's name. The only way to make a vocabulary "current" is to first make it "context" (by typing its name). Then you may use the word "definitions" which sets the "current" vocabulary to be the same as the "context" vocabulary. vocabulary \ name ( -- ) creates a new vocabulary whose name is taken from the input stream. Executing that name later will cause the new vocabulary definitions ( -- ) makes the "CURRENT" vocabulary the same as "the "CONTEXT" one order ( -- ) displays the list of vocabularies to be searched So, if you want to make a new vocabulary named "myvoc", execute vocabulary myvoc If you want your new vocabulary to be the one that is searched first (i.e. the "context"), just type its name. myvoc If you wished new word definitions to go into your new vocabulary (i.e. make it "current"), all you have to do is type definitions assuming that you have already made "myvoc" the "context" by typing "myvoc". If not, just execute myvoc definitions It is possible to create vocabularies within vocabularies, but it is most of the time you should create new vocabularies only within the "forth" vocabulary. To insure that this is the case, type forth definitions before creating new vocabularies. This way, you always know how to find your vocabularies ( just type Forth then the vocabulary name). Forgetting ========== forget \ name ( -- ) forgets the named word from the dictionary. All words which have been defined after the named word are forgotten too. This is necessary because the subsequent words may depend on the word you are forgetting. words ( -- ) tells you what words the system knows Arithmetic Summary ================== + ( n1 n2 -- sum ) integer addition - ( n1 n2 -- difference ) integer subtraction, n1-n2 1+ ( n -- n+1 ) add 1. This is provided as an alternative to "1 +" because it is such a frequent operation and should be optimized 1- ( n -- n-1 ) subtract 1 2+ ( n -- n+2 ) add 2. 2- ( n -- n-2 ) subtract 2 * ( n1 n2 -- product ) integer multiply 2* ( n -- n*2 ) times 2. Usually implemented by shifting / ( n1 n2 -- quotient ) integer portion of quotient 2/ ( n -- n/2 ) halve. Usually implemented by shifting mod ( n1 n2 -- remainder ) integer remainder /mod ( n1 n2 -- quotient remainder ) quotient and remainder u* ( un1 un2 -- ud ) like "*" but unsigned u/mod ( u1 u2 -- uremainder uquotient ) unsigned /mod max ( n1 n2 -- max ) maximum min ( n1 n2 -- min ) minimum abs ( n -- |n| ) absolute value negate ( n -- -n ) arithmetic negation and ( n1 n2 -- and ) bitwise logical and or ( n1 n2 -- or ) bitwise logical or xor ( n1 n2 -- xor ) bitwise logical exclusive-or not ( n1 -- complement ) bitwise logical complement Further Topics and books ======================== There is a lot more to Forth than I have described here. However, this should be sufficient to get you a long way. For a complete introduction to the complete Forth language, I recommend the following books: Mastering Forth Anita Anderson and Martin Tracy Mountain View Press, P.O. Box 4656, Mountain View, CA 94040 Starting Forth Leo Brodie of Forth,Inc. Prentice-Hall,Inc. Englewood Cliffs, NJ 07632 Thinking Forth Leo Brodie Prentice-Hall,Inc. Dr. Dobbs Toolbook of Forth, Parts I+II