\ -*- text -*- \ A sometimes minimal FORTH compiler and tutorial for Linux / i386 systems. -*- asm -*- \ By Richard W.M. Jones http://annexia.org/forth \ This is PUBLIC DOMAIN (see public domain release statement below). \ $Id: jonesforth.f,v 1.3 2007-09-25 09:50:54 rich Exp $ \ \ The first part of this tutorial is in jonesforth.S. Get if from http://annexia.org/forth \ \ PUBLIC DOMAIN ---------------------------------------------------------------------- \ \ I, the copyright holder of this work, hereby release it into the public domain. This applies worldwide. \ \ In case this is not legally possible, I grant any entity the right to use this work for any purpose, \ without any conditions, unless such conditions are required by law. \ \ SETTING UP ---------------------------------------------------------------------- \ \ Let's get a few housekeeping things out of the way. Firstly because I need to draw lots of \ ASCII-art diagrams to explain concepts, the best way to look at this is using a window which \ uses a fixed width font and is at least this wide: \ \<------------------------------------------------------------------------------------------------------------------------> \ \ Secondly make sure TABS are set to 8 characters. The following should be a vertical \ line. If not, sort out your tabs. \ \ | \ | \ | \ \ Thirdly I assume that your screen is at least 50 characters high. \ \ START OF FORTH CODE ---------------------------------------------------------------------- \ \ We've now reached the stage where the FORTH system is running and self-hosting. All further \ words can be written as FORTH itself, including words like IF, THEN, .", etc which in most \ languages would be considered rather fundamental. \ \ Some notes about the code: \ \ I use indenting to show structure. The amount of whitespace has no meaning to FORTH however \ except that you must use at least one whitespace character between words, and words themselves \ cannot contain whitespace. \ \ FORTH is case-sensitive. Use capslock! \ Define some character constants : '\n' 10 ; : 'SPACE' 32 ; \ CR prints a carriage return : CR '\n' EMIT ; \ SPACE prints a space : SPACE 'SPACE' EMIT ; \ DUP, DROP are defined in assembly for speed, but this is how you might define them \ in FORTH. Notice use of the scratch variables _X and _Y. \ : DUP _X ! _X @ _X @ ; \ : DROP _X ! ; \ The built-in . (DOT) function doesn't print a space after the number (unlike the real FORTH word). \ However this is very easily fixed by redefining . (DOT). Any built-in word can be redefined. : . . \ this refers back to the previous definition (but see also RECURSE below) SPACE ; \ The 2... versions of the standard operators work on pairs of stack entries. They're not used \ very commonly so not really worth writing in assembler. Here is how they are defined in FORTH. : 2DUP OVER OVER ; : 2DROP DROP DROP ; \ More standard FORTH words. : 2* 2 * ; : 2/ 2 / ; \ Standard words for manipulating BASE. : DECIMAL 10 BASE ! ; : HEX 16 BASE ! ; \ Standard words for booleans. : TRUE 1 ; : FALSE 0 ; : NOT 0= ; \ LITERAL takes whatever is on the stack and compiles LIT : LITERAL IMMEDIATE ' LIT , \ compile LIT , \ compile the literal itself (from the stack) ; \ Now we can use [ and ] to insert literals which are calculated at compile time. \ Within definitions, use [ ... ] LITERAL anywhere that '...' is a constant expression which you \ would rather only compute once (at compile time, rather than calculating it each time your word runs). : ':' [ \ go into immediate mode temporarily CHAR : \ push the number 58 (ASCII code of colon) on the stack ] \ go back to compile mode LITERAL \ compile LIT 58 as the definition of ':' word ; \ A few more character constants defined the same way as above. : '(' [ CHAR ( ] LITERAL ; : ')' [ CHAR ) ] LITERAL ; : '"' [ CHAR " ] LITERAL ; \ While compiling, '[COMPILE] word' compiles 'word' if it would otherwise be IMMEDIATE. : [COMPILE] IMMEDIATE WORD \ get the next word FIND \ find it in the dictionary >CFA \ get its codeword , \ and compile that ; \ So far we have defined only very simple definitions. Before we can go further, we really need to \ make some control structures, like IF ... THEN and loops. Luckily we can define arbitrary control \ structures directly in FORTH. \ \ Please note that the control structures as I have defined them here will only work inside compiled \ words. If you try to type in expressions using IF, etc. in immediate mode, then they won't work. \ Making these work in immediate mode is left as an exercise for the reader. \ condition IF true-part THEN rest \ -- compiles to: --> condition 0BRANCH OFFSET true-part rest \ where OFFSET is the offset of 'rest' \ condition IF true-part ELSE false-part THEN \ -- compiles to: --> condition 0BRANCH OFFSET true-part BRANCH OFFSET2 false-part rest \ where OFFSET if the offset of false-part and OFFSET2 is the offset of rest \ IF is an IMMEDIATE word which compiles 0BRANCH followed by a dummy offset, and places \ the address of the 0BRANCH on the stack. Later when we see THEN, we pop that address \ off the stack, calculate the offset, and back-fill the offset. : IF IMMEDIATE ' 0BRANCH , \ compile 0BRANCH HERE @ \ save location of the offset on the stack 0 , \ compile a dummy offset ; : THEN IMMEDIATE DUP HERE @ SWAP - \ calculate the offset from the address saved on the stack SWAP ! \ store the offset in the back-filled location ; : ELSE IMMEDIATE ' BRANCH , \ definite branch to just over the false-part HERE @ \ save location of the offset on the stack 0 , \ compile a dummy offset SWAP \ now back-fill the original (IF) offset DUP \ same as for THEN word above HERE @ SWAP - SWAP ! ; \ BEGIN loop-part condition UNTIL \ -- compiles to: --> loop-part condition 0BRANCH OFFSET \ where OFFSET points back to the loop-part \ This is like do { loop-part } while (condition) in the C language : BEGIN IMMEDIATE HERE @ \ save location on the stack ; : UNTIL IMMEDIATE ' 0BRANCH , \ compile 0BRANCH HERE @ - \ calculate the offset from the address saved on the stack , \ compile the offset here ; \ BEGIN loop-part AGAIN \ -- compiles to: --> loop-part BRANCH OFFSET \ where OFFSET points back to the loop-part \ In other words, an infinite loop which can only be returned from with EXIT : AGAIN IMMEDIATE ' BRANCH , \ compile BRANCH HERE @ - \ calculate the offset back , \ compile the offset here ; \ BEGIN condition WHILE loop-part REPEAT \ -- compiles to: --> condition 0BRANCH OFFSET2 loop-part BRANCH OFFSET \ where OFFSET points back to condition (the beginning) and OFFSET2 points to after the whole piece of code \ So this is like a while (condition) { loop-part } loop in the C language : WHILE IMMEDIATE ' 0BRANCH , \ compile 0BRANCH HERE @ \ save location of the offset2 on the stack 0 , \ compile a dummy offset2 ; : REPEAT IMMEDIATE ' BRANCH , \ compile BRANCH SWAP \ get the original offset (from BEGIN) HERE @ - , \ and compile it after BRANCH DUP HERE @ SWAP - \ calculate the offset2 SWAP ! \ and back-fill it in the original location ; \ FORTH allows ( ... ) as comments within function definitions. This works by having an IMMEDIATE \ word called ( which just drops input characters until it hits the corresponding ). : ( IMMEDIATE 1 \ allowed nested parens by keeping track of depth BEGIN KEY \ read next character DUP '(' = IF \ open paren? DROP \ drop the open paren 1+ \ depth increases ELSE ')' = IF \ close paren? 1- \ depth decreases THEN THEN DUP 0= UNTIL \ continue until we reach matching close paren, depth 0 DROP \ drop the depth counter ; ( From now on we can use ( ... ) for comments. In FORTH style we can also use ( ... -- ... ) to show the effects that a word has on the parameter stack. For example: ( n -- ) means that the word consumes an integer (n) from the parameter stack. ( b a -- c ) means that the word uses two integers (a and b, where a is at the top of stack) and returns a single integer (c). ( -- ) means the word has no effect on the stack ) ( With the looping constructs, we can now write SPACES, which writes n spaces to stdout. ) : SPACES ( n -- ) BEGIN DUP 0> ( while n > 0 ) WHILE SPACE ( print a space ) 1- ( until we count down to 0 ) REPEAT DROP ; ( c a b WITHIN returns true if a <= c and c < b ) : WITHIN ROT ( b c a ) OVER ( b c a c ) <= IF > IF ( b c -- ) TRUE ELSE FALSE THEN ELSE 2DROP ( b c -- ) FALSE THEN ; ( .S prints the contents of the stack. Very useful for debugging. ) : .S ( -- ) DSP@ ( get current stack pointer ) BEGIN DUP S0 @ < WHILE DUP @ . ( print the stack element ) 4+ ( move up ) REPEAT DROP ; ( DEPTH returns the depth of the stack. ) : DEPTH ( -- n ) S0 @ DSP@ - 4- ( adjust because S0 was on the stack when we pushed DSP ) ; ( S" string" is used in FORTH to define strings. It leaves the address of the string and its length on the stack, with the address at the top. The space following S" is the normal space between FORTH words and is not a part of the string. This is tricky to define because it has to do different things depending on whether we are compiling or in immediate mode. (Thus the word is marked IMMEDIATE so it can detect this and do different things). In compile mode we append LITSTRING to the current word. The primitive LITSTRING does the right thing when the current word is executed. In immediate mode there isn't a particularly good place to put the string, but in this case we put the string at HERE (but we _don't_ change HERE). This is meant as a temporary location, likely to be overwritten soon after. ) : S" IMMEDIATE ( -- len addr ) STATE @ IF ( compiling? ) ' LITSTRING , ( compile LITSTRING ) HERE @ ( save the address of the length word on the stack ) 0 , ( dummy length - we don't know what it is yet ) BEGIN KEY ( get next character of the string ) DUP '"' <> WHILE HERE @ !b ( store the character in the compiled image ) 1 HERE +! ( increment HERE pointer by 1 byte ) REPEAT DROP ( drop the double quote character at the end ) DUP ( get the saved address of the length word ) HERE @ SWAP - ( calculate the length ) 4- ( subtract 4 (because we measured from the start of the length word) ) SWAP ! ( and back-fill the length location ) HERE @ ( round up to next multiple of 4 bytes for the remaining code ) 3 + 3 INVERT AND HERE ! ELSE ( immediate mode ) HERE @ ( get the start address of the temporary space ) BEGIN KEY DUP '"' <> WHILE OVER !b ( save next character ) 1+ ( increment address ) REPEAT DROP ( drop the final " character ) HERE @ - ( calculate the length ) HERE @ ( push the start address ) THEN ; ( ." is the print string operator in FORTH. Example: ." Something to print" The space after the operator is the ordinary space required between words and is not a part of what is printed. In immediate mode we just keep reading characters and printing them until we get to the next double quote. In compile mode we use S" to store the string, then add EMITSTRING afterwards: LITSTRING EMITSTRING It may be interesting to note the use of [COMPILE] to turn the call to the immediate word S" into compilation of that word. It compiles it into the definition of .", not into the definition of the word being compiled when this is running (complicated enough for you?) ) : ." IMMEDIATE ( -- ) STATE @ IF ( compiling? ) [COMPILE] S" ( read the string, and compile LITSTRING, etc. ) ' EMITSTRING , ( compile the final EMITSTRING ) ELSE ( In immediate mode, just read characters and print them until we get to the ending double quote. ) BEGIN KEY DUP '"' = IF DROP ( drop the double quote character ) EXIT ( return from this function ) THEN EMIT AGAIN THEN ; ( In FORTH, global constants and variables are defined like this: 10 CONSTANT TEN when TEN is executed, it leaves the integer 10 on the stack VARIABLE VAR when VAR is executed, it leaves the address of VAR on the stack Constants can be read by not written, eg: TEN . CR prints 10 You can read a variable (in this example called VAR) by doing: VAR @ leaves the value of VAR on the stack VAR @ . CR prints the value of VAR and update the variable by doing: 20 VAR ! sets VAR to 20 Note that variables are uninitialised (but see VALUE later on which provides initialised variables with a slightly simpler syntax). How can we define the words CONSTANT and VARIABLE? The trick is to define a new word for the variable itself (eg. if the variable was called 'VAR' then we would define a new word called VAR). This is easy to do because we exposed dictionary entry creation through the CREATE word (part of the definition of : above). A call to CREATE TEN leaves the dictionary entry: +--- HERE | V +---------+---+---+---+---+ | LINK | 3 | T | E | N | +---------+---+---+---+---+ len For CONSTANT we can continue by appending DOCOL (the codeword), then LIT followed by the constant itself and then EXIT, forming a little word definition that returns the constant: +---------+---+---+---+---+------------+------------+------------+------------+ | LINK | 3 | T | E | N | DOCOL | LIT | 10 | EXIT | +---------+---+---+---+---+------------+------------+------------+------------+ len codeword Notice that this word definition is exactly the same as you would have got if you had written : TEN 10 ; ) : CONSTANT CREATE ( make the dictionary entry (the name follows CONSTANT) ) DOCOL , ( append DOCOL (the codeword field of this word) ) ' LIT , ( append the codeword LIT ) , ( append the value on the top of the stack ) ' EXIT , ( append the codeword EXIT ) ; ( VARIABLE is a little bit harder because we need somewhere to put the variable. There is nothing particularly special about the 'user definitions area' (the area of memory pointed to by HERE where we have previously just stored new word definitions). We can slice off bits of this memory area to store anything we want, so one possible definition of VARIABLE might create this: +--------------------------------------------------------------+ | | V | +---------+---------+---+---+---+---+------------+------------+---|--------+------------+ | | LINK | 3 | V | A | R | DOCOL | LIT | | EXIT | +---------+---------+---+---+---+---+------------+------------+------------+------------+ len codeword where is the place to store the variable, and points back to it. To make this more general let's define a couple of words which we can use to allocate arbitrary memory from the user definitions area. First ALLOT, where n ALLOT allocates n bytes of memory. (Note when calling this that it's a very good idea to make sure that n is a multiple of 4, or at least that next time a word is compiled that HERE has been left as a multiple of 4). ) : ALLOT ( n -- addr ) HERE @ SWAP ( here n ) HERE +! ( adds n to HERE, after this the old value of HERE is still on the stack ) ; ( Second, CELLS. In FORTH the phrase 'n CELLS ALLOT' means allocate n integers of whatever size is the natural size for integers on this machine architecture. On this 32 bit machine therefore CELLS just multiplies the top of stack by 4. ) : CELLS ( n -- n ) 4 * ; ( So now we can define VARIABLE easily in much the same way as CONSTANT above. Refer to the diagram above to see what the word that this creates will look like. ) : VARIABLE 1 CELLS ALLOT ( allocate 1 cell of memory, push the pointer to this memory ) CREATE ( make the dictionary entry (the name follows VARIABLE) ) DOCOL , ( append DOCOL (the codeword field of this word) ) ' LIT , ( append the codeword LIT ) , ( append the pointer to the new memory ) ' EXIT , ( append the codeword EXIT ) ; ( VALUEs are like VARIABLEs but with a simpler syntax. You would generally use them when you want a variable which is read often, and written infrequently. 20 VALUE VAL creates VAL with initial value 20 VAL pushes the value directly on the stack 30 TO VAL updates VAL, setting it to 30 Notice that 'VAL' on its own doesn't return the address of the value, but the value itself, making values simpler and more obvious to use than variables (no indirection through '@'). The price is a more complicated implementation, although despite the complexity there is no performance penalty at runtime. A naive implementation of 'TO' would be quite slow, involving a dictionary search each time. But because this is FORTH we have complete control of the compiler so we can compile TO more efficiently, turning: TO VAL into: LIT ! and calculating (the address of the value) at compile time. Now this is the clever bit. We'll compile our value like this: +---------+---+---+---+---+------------+------------+------------+------------+ | LINK | 3 | V | A | L | DOCOL | LIT | | EXIT | +---------+---+---+---+---+------------+------------+------------+------------+ len codeword where is the actual value itself. Note that when VAL executes, it will push the value on the stack, which is what we want. But what will TO use for the address ? Why of course a pointer to that : code compiled - - - - --+------------+------------+------------+-- - - - - by TO VAL | LIT | | ! | - - - - --+------------+-----|------+------------+-- - - - - | V +---------+---+---+---+---+------------+------------+------------+------------+ | LINK | 3 | V | A | L | DOCOL | LIT | | EXIT | +---------+---+---+---+---+------------+------------+------------+------------+ len codeword In other words, this is a kind of self-modifying code. (Note to the people who want to modify this FORTH to add inlining: values defined this way cannot be inlined). ) : VALUE ( n -- ) CREATE ( make the dictionary entry (the name follows VALUE) ) DOCOL , ( append DOCOL ) ' LIT , ( append the codeword LIT ) , ( append the initial value ) ' EXIT , ( append the codeword EXIT ) ; : TO IMMEDIATE ( n -- ) WORD ( get the name of the value ) FIND ( look it up in the dictionary ) >DFA ( get a pointer to the first data field (the 'LIT') ) 4+ ( increment to point at the value ) STATE @ IF ( compiling? ) ' LIT , ( compile LIT ) , ( compile the address of the value ) ' ! , ( compile ! ) ELSE ( immediate mode ) ! ( update it straightaway ) THEN ; ( x +TO VAL adds x to VAL ) : +TO IMMEDIATE WORD ( get the name of the value ) FIND ( look it up in the dictionary ) >DFA ( get a pointer to the first data field (the 'LIT') ) 4+ ( increment to point at the value ) STATE @ IF ( compiling? ) ' LIT , ( compile LIT ) , ( compile the address of the value ) ' +! , ( compile +! ) ELSE ( immediate mode ) +! ( update it straightaway ) THEN ; ( ID. takes an address of a dictionary entry and prints the word's name. For example: LATEST @ ID. would print the name of the last word that was defined. ) : ID. 4+ ( skip over the link pointer ) DUP @b ( get the flags/length byte ) F_LENMASK AND ( mask out the flags - just want the length ) BEGIN DUP 0> ( length > 0? ) WHILE SWAP 1+ ( addr len -- len addr+1 ) DUP @b ( len addr -- len addr char | get the next character) EMIT ( len addr char -- len addr | and print it) SWAP 1- ( len addr -- addr len-1 | subtract one from length ) REPEAT 2DROP ( len addr -- ) ; ( 'WORD word FIND ?HIDDEN' returns true if 'word' is flagged as hidden. 'WORD word FIND ?IMMEDIATE' returns true if 'word' is flagged as immediate. ) : ?HIDDEN 4+ ( skip over the link pointer ) @b ( get the flags/length byte ) F_HIDDEN AND ( mask the F_HIDDEN flag and return it (as a truth value) ) ; : ?IMMEDIATE 4+ ( skip over the link pointer ) @b ( get the flags/length byte ) F_IMMED AND ( mask the F_IMMED flag and return it (as a truth value) ) ; ( WORDS prints all the words defined in the dictionary, starting with the word defined most recently. However it doesn't print hidden words. The implementation simply iterates backwards from LATEST using the link pointers. ) : WORDS LATEST @ ( start at LATEST dictionary entry ) BEGIN DUP 0<> ( while link pointer is not null ) WHILE DUP ?HIDDEN NOT IF ( ignore hidden words ) DUP ID. ( but if not hidden, print the word ) THEN SPACE @ ( dereference the link pointer - go to previous word ) REPEAT DROP CR ; ( So far we have only allocated words and memory. FORTH provides a rather primitive method to deallocate. 'FORGET word' deletes the definition of 'word' from the dictionary and everything defined after it, including any variables and other memory allocated after. The implementation is very simple - we look up the word (which returns the dictionary entry address). Then we set HERE to point to that address, so in effect all future allocations and definitions will overwrite memory starting at the word. We also need to set LATEST to point to the previous word. Note that you cannot FORGET built-in words (well, you can try but it will probably cause a segfault). XXX: Because we wrote VARIABLE to store the variable in memory allocated before the word, in the current implementation VARIABLE FOO FORGET FOO will leak 1 cell of memory. ) : FORGET WORD FIND ( find the word, gets the dictionary entry address ) DUP @ LATEST ! ( set LATEST to point to the previous word ) HERE ! ( and store HERE with the dictionary address ) ; ( RECURSE makes a recursive call to the current word that is being compiled. Normally while a word is being compiled, it is marked HIDDEN so that references to the same word within are calls to the previous definition of the word. However we still have access to the word which we are currently compiling through the LATEST pointer so we can use that to compile a recursive call. ) : RECURSE IMMEDIATE LATEST @ >CFA ( LATEST points to the word being compiled at the moment ) , ( compile it ) ; ( DUMP is used to dump out the contents of memory, in the 'traditional' hexdump format. ) : DUMP ( addr len -- ) BASE @ ROT ( save the current BASE at the bottom of the stack ) HEX ( and switch the hexadecimal mode ) BEGIN DUP 0> ( while len > 0 ) WHILE OVER . ( print the address ) SPACE ( print up to 16 words on this line ) 2DUP ( addr len addr len ) 1- 15 AND 1+ ( addr len addr linelen ) BEGIN DUP 0> ( while linelen > 0 ) WHILE SWAP ( addr len linelen addr ) DUP @b ( addr len linelen addr byte ) . SPACE ( print the byte ) 1+ SWAP 1- ( addr len linelen addr -- addr len addr+1 linelen-1 ) REPEAT 2DROP ( addr len ) ( print the ASCII equivalents ) 2DUP 1- 15 AND 1+ ( addr len addr linelen ) BEGIN DUP 0> ( while linelen > 0) WHILE SWAP ( addr len linelen addr ) DUP @b ( addr len linelen addr byte ) DUP 32 128 WITHIN IF ( 32 <= c < 128? ) EMIT ELSE DROP [ CHAR ? ] LITERAL EMIT THEN 1+ SWAP 1- ( addr len linelen addr -- addr len addr+1 linelen-1 ) REPEAT 2DROP ( addr len ) CR DUP 1- 15 AND 1+ ( addr len linelen ) DUP ( addr len linelen linelen ) ROT ( addr linelen len linelen ) - ( addr linelen len-linelen ) ROT ( len-linelen addr linelen ) + ( len-linelen addr+linelen ) SWAP ( addr-linelen len-linelen ) REPEAT 2DROP ( restore stack ) BASE ! ( restore saved BASE ) ; ( Finally print the welcome prompt. ) ." JONESFORTH VERSION " VERSION . CR ." OK "