\ A sometimes minimal FORTH compiler and tutorial for Linux / i386 systems. -*- asm -*-
\ By Richard W.M. Jones <rich@annexia.org> http://annexia.org/forth
\ This is PUBLIC DOMAIN (see public domain release statement below).
-\ $Id: jonesforth.f,v 1.5 2007-09-26 22:20:52 rich Exp $
+\ $Id: jonesforth.f,v 1.13 2007-10-07 11:07:15 rich Exp $
\
\ The first part of this tutorial is in jonesforth.S. Get if from http://annexia.org/forth
\
: MOD /MOD DROP ;
\ Define some character constants
-: '\n' 10 ;
-: 'SPACE' 32 ;
+: '\n' 10 ;
+: BL 32 ; \ BL (BLank) is a standard FORTH word for space.
\ 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 ! ;
+: SPACE BL EMIT ;
\ 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.
;
\ A few more character constants defined the same way as above.
+: ';' [ CHAR ; ] LITERAL ;
: '(' [ CHAR ( ] LITERAL ;
: ')' [ CHAR ) ] LITERAL ;
: '"' [ CHAR " ] LITERAL ;
, \ compile it
;
+\ CONTROL STRUCTURES ----------------------------------------------------------------------
+\
\ 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.
SWAP ! \ and back-fill it in the original location
;
+\ UNLESS is the same as IF but the test is reversed.
+\
+\ Note the use of [COMPILE]: Since IF is IMMEDIATE we don't want it to be executed while UNLESS
+\ is compiling, but while UNLESS is running (which happens to be when whatever word using UNLESS is
+\ being compiled -- whew!). So we use [COMPILE] to reverse the effect of marking IF as immediate.
+\ This trick is generally used when we want to write our own control words without having to
+\ implement them all in terms of the primitives 0BRANCH and BRANCH, but instead reusing simpler
+\ control words like (in this instance) IF.
+: UNLESS IMMEDIATE
+ ' NOT , \ compile NOT (to reverse the test)
+ [COMPILE] IF \ continue by calling the normal IF
+;
+
+\ COMMENTS ----------------------------------------------------------------------
+\
\ 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
(
From now on we can use ( ... ) for comments.
+ STACK NOTATION ----------------------------------------------------------------------
+
In FORTH style we can also use ( ... -- ... ) to show the effects that a word has on the
parameter stack. For example:
( -- ) means the word has no effect on the stack
)
+( Some more complicated stack examples, showing the stack notation. )
+: NIP ( x y -- y ) SWAP DROP ;
+: TUCK ( x y -- y x y ) DUP ROT ;
+: PICK ( x_u ... x_1 x_0 u -- x_u ... x_1 x_0 x_u )
+ 1+ ( add one because of 'u' on the stack )
+ 4 * ( multiply by the word size )
+ DSP@ + ( add to the stack pointer )
+ @ ( and fetch )
+;
+
( With the looping constructs, we can now write SPACES, which writes n spaces to stdout. )
: SPACES ( n -- )
BEGIN
: HEX ( -- ) 16 BASE ! ;
(
+ PRINTING NUMBERS ----------------------------------------------------------------------
+
The standard FORTH word . (DOT) is very important. It takes the number at the top
of the stack and prints it out. However first I'm going to implement some lower-level
FORTH words:
( This is the underlying recursive definition of U. )
: U. ( u -- )
BASE @ /MOD ( width rem quot )
- DUP 0<> IF ( if quotient <> 0 then )
+ ?DUP IF ( if quotient <> 0 then )
RECURSE ( print the quotient )
- ELSE
- DROP ( drop the zero quotient )
THEN
( print the remainder )
( This word returns the width (in characters) of an unsigned number in the current base )
: UWIDTH ( u -- width )
BASE @ / ( rem quot )
- DUP 0<> IF ( if quotient <> 0 then )
+ ?DUP IF ( if quotient <> 0 then )
RECURSE 1+ ( return 1+recursive call )
ELSE
- DROP ( drop the zero quotient )
1 ( return 1 )
THEN
;
: U. U. SPACE ;
( ? fetches the integer at an address and prints it. )
-: ? @ . ;
+: ? ( addr -- ) @ . ;
( c a b WITHIN returns true if a <= c and c < b )
: WITHIN
: ALIGN HERE @ ALIGNED HERE ! ;
(
+ STRINGS ----------------------------------------------------------------------
+
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
+ its length on the stack, (length at the top of stack). 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
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 )
+( C, appends a byte to the current compiled word. )
+: C,
+ HERE @ C! ( store the character in the compiled image )
+ 1 HERE +! ( increment HERE pointer by 1 byte )
+;
+
+: S" IMMEDIATE ( -- addr len )
STATE @ IF ( compiling? )
' LITSTRING , ( compile LITSTRING )
HERE @ ( save the address of the length word on the stack )
KEY ( get next character of the string )
DUP '"' <>
WHILE
- HERE @ C! ( store the character in the compiled image )
- 1 HERE +! ( increment HERE pointer by 1 byte )
+ C, ( copy character )
REPEAT
DROP ( drop the double quote character at the end )
DUP ( get the saved address of the length word )
DROP ( drop the final " character )
HERE @ - ( calculate the length )
HERE @ ( push the start address )
+ SWAP ( addr len )
THEN
;
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 <string length> <string rounded up to 4 bytes> EMITSTRING
+ In compile mode we use S" to store the string, then add TELL afterwards:
+ LITSTRING <string length> <string rounded up to 4 bytes> TELL
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 .",
: ." IMMEDIATE ( -- )
STATE @ IF ( compiling? )
[COMPILE] S" ( read the string, and compile LITSTRING, etc. )
- ' EMITSTRING , ( compile the final EMITSTRING )
+ ' TELL , ( compile the final TELL )
ELSE
( In immediate mode, just read characters and print them until we get
to the ending double quote. )
;
(
+ CONSTANTS AND VARIABLES ----------------------------------------------------------------------
+
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
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:
+ A call to WORD [TEN] CREATE (where [TEN] means that "TEN" is the next word in the input)
+ leaves the dictionary entry:
+--- HERE
|
assembler part which returns the value of the assembler symbol of the same name.
)
: CONSTANT
- CREATE ( make the dictionary entry (the name follows CONSTANT) )
+ WORD ( get the name (the name follows CONSTANT) )
+ CREATE ( make the dictionary entry )
DOCOL , ( append DOCOL (the codeword field of this word) )
' LIT , ( append the codeword LIT )
, ( append the value on the top of the stack )
(
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:
+ nothing particularly special about the user memory (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:
+--------------------------------------------------------------+
| |
where <var> is the place to store the variable, and <addr var> 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.
+ arbitrary memory from the user memory.
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
)
: VARIABLE
1 CELLS ALLOT ( allocate 1 cell of memory, push the pointer to this memory )
- CREATE ( make the dictionary entry (the name follows VARIABLE) )
+ WORD 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 )
;
(
+ VALUES ----------------------------------------------------------------------
+
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.
way cannot be inlined).
)
: VALUE ( n -- )
- CREATE ( make the dictionary entry (the name follows VALUE) )
+ WORD CREATE ( make the dictionary entry (the name follows VALUE) )
DOCOL , ( append DOCOL )
' LIT , ( append the codeword LIT )
, ( append the initial value )
;
(
+ PRINTING THE DICTIONARY ----------------------------------------------------------------------
+
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.
: WORDS
LATEST @ ( start at LATEST dictionary entry )
BEGIN
- DUP 0<> ( while link pointer is not null )
+ ?DUP ( while link pointer is not null )
WHILE
DUP ?HIDDEN NOT IF ( ignore hidden words )
DUP ID. ( but if not hidden, print the word )
+ SPACE
THEN
- SPACE
@ ( dereference the link pointer - go to previous word )
REPEAT
- DROP
CR
;
(
+ FORGET ----------------------------------------------------------------------
+
So far we have only allocated words and memory. FORTH provides a rather primitive method
to deallocate.
;
(
+ DUMP ----------------------------------------------------------------------
+
DUMP is used to dump out the contents of memory, in the 'traditional' hexdump format.
+
+ Notice that the parameters to DUMP (address, length) are compatible with string words
+ such as WORD and S".
)
: DUMP ( addr len -- )
BASE @ ROT ( save the current BASE at the bottom of the stack )
BEGIN
DUP 0> ( while len > 0 )
WHILE
- OVER 8 .R ( print the address )
+ OVER 8 U.R ( print the address )
SPACE
( print up to 16 words on this line )
BASE ! ( restore saved BASE )
;
-( Finally print the welcome prompt. )
-." JONESFORTH VERSION " VERSION . CR
-." OK "
+(
+ CASE ----------------------------------------------------------------------
+
+ CASE...ENDCASE is how we do switch statements in FORTH. There is no generally
+ agreed syntax for this, so I've gone for the syntax mandated by the ISO standard
+ FORTH (ANS-FORTH).
+
+ ( some value on the stack )
+ CASE
+ test1 OF ... ENDOF
+ test2 OF ... ENDOF
+ testn OF ... ENDOF
+ ... ( default case )
+ ENDCASE
+
+ The CASE statement tests the value on the stack by comparing it for equality with
+ test1, test2, ..., testn and executes the matching piece of code within OF ... ENDOF.
+ If none of the test values match then the default case is executed. Inside the ... of
+ the default case, the value is still at the top of stack (it is implicitly DROP-ed
+ by ENDCASE). When ENDOF is executed it jumps after ENDCASE (ie. there is no "fall-through"
+ and no need for a break statement like in C).
+
+ The default case may be omitted. In fact the tests may also be omitted so that you
+ just have a default case, although this is probably not very useful.
+
+ An example (assuming that 'q', etc. are words which push the ASCII value of the letter
+ on the stack):
+
+ 0 VALUE QUIT
+ 0 VALUE SLEEP
+ KEY CASE
+ 'q' OF 1 TO QUIT ENDOF
+ 's' OF 1 TO SLEEP ENDOF
+ ( default case: )
+ ." Sorry, I didn't understand key <" DUP EMIT ." >, try again." CR
+ ENDCASE
+
+ (In some versions of FORTH, more advanced tests are supported, such as ranges, etc.
+ Other versions of FORTH need you to write OTHERWISE to indicate the default case.
+ As I said above, this FORTH tries to follow the ANS FORTH standard).
+
+ The implementation of CASE...ENDCASE is somewhat non-trivial. I'm following the
+ implementations from here:
+ http://www.uni-giessen.de/faq/archiv/forthfaq.case_endcase/msg00000.html
+
+ The general plan is to compile the code as a series of IF statements:
+
+ CASE (push 0 on the immediate-mode parameter stack)
+ test1 OF ... ENDOF test1 OVER = IF DROP ... ELSE
+ test2 OF ... ENDOF test2 OVER = IF DROP ... ELSE
+ testn OF ... ENDOF testn OVER = IF DROP ... ELSE
+ ... ( default case ) ...
+ ENDCASE DROP THEN [THEN [THEN ...]]
+
+ The CASE statement pushes 0 on the immediate-mode parameter stack, and that number
+ is used to count how many THEN statements we need when we get to ENDCASE so that each
+ IF has a matching THEN. The counting is done implicitly. If you recall from the
+ implementation above of IF, each IF pushes a code address on the immediate-mode stack,
+ and these addresses are non-zero, so by the time we get to ENDCASE the stack contains
+ some number of non-zeroes, followed by a zero. The number of non-zeroes is how many
+ times IF has been called, so how many times we need to match it with THEN.
+
+ This code uses [COMPILE] so that we compile calls to IF, ELSE, THEN instead of
+ actually calling them while we're compiling the words below.
+
+ As is the case with all of our control structures, they only work within word
+ definitions, not in immediate mode.
+)
+: CASE IMMEDIATE
+ 0 ( push 0 to mark the bottom of the stack )
+;
+
+: OF IMMEDIATE
+ ' OVER , ( compile OVER )
+ ' = , ( compile = )
+ [COMPILE] IF ( compile IF )
+ ' DROP , ( compile DROP )
+;
+
+: ENDOF IMMEDIATE
+ [COMPILE] ELSE ( ENDOF is the same as ELSE )
+;
+
+: ENDCASE IMMEDIATE
+ ' DROP , ( compile DROP )
+
+ ( keep compiling THEN until we get to our zero marker )
+ BEGIN
+ ?DUP
+ WHILE
+ [COMPILE] THEN
+ REPEAT
+;
+
+(
+ DECOMPILER ----------------------------------------------------------------------
+
+ CFA> is the opposite of >CFA. It takes a codeword and tries to find the matching
+ dictionary definition. (In truth, it works with any pointer into a word, not just
+ the codeword pointer, and this is needed to do stack traces).
+
+ In this FORTH this is not so easy. In fact we have to search through the dictionary
+ because we don't have a convenient back-pointer (as is often the case in other versions
+ of FORTH). Because of this search, CFA> should not be used when performance is critical,
+ so it is only used for debugging tools such as the decompiler and printing stack
+ traces.
+
+ This word returns 0 if it doesn't find a match.
+)
+: CFA>
+ LATEST @ ( start at LATEST dictionary entry )
+ BEGIN
+ ?DUP ( while link pointer is not null )
+ WHILE
+ 2DUP SWAP ( cfa curr curr cfa )
+ < IF ( current dictionary entry < cfa? )
+ NIP ( leave curr dictionary entry on the stack )
+ EXIT
+ THEN
+ @ ( follow link pointer back )
+ REPEAT
+ DROP ( restore stack )
+ 0 ( sorry, nothing found )
+;
+
+(
+ SEE decompiles a FORTH word.
+
+ We search for the dictionary entry of the word, then search again for the next
+ word (effectively, the end of the compiled word). This results in two pointers:
+
+ +---------+---+---+---+---+------------+------------+------------+------------+
+ | LINK | 3 | T | E | N | DOCOL | LIT | 10 | EXIT |
+ +---------+---+---+---+---+------------+------------+------------+------------+
+ ^ ^
+ | |
+ Start of word End of word
+
+ With this information we can have a go at decompiling the word. We need to
+ recognise "meta-words" like LIT, LITSTRING, BRANCH, etc. and treat those separately.
+)
+: SEE
+ WORD FIND ( find the dictionary entry to decompile )
+
+ ( Now we search again, looking for the next word in the dictionary. This gives us
+ the length of the word that we will be decompiling. (Well, mostly it does). )
+ HERE @ ( address of the end of the last compiled word )
+ LATEST @ ( word last curr )
+ BEGIN
+ 2 PICK ( word last curr word )
+ OVER ( word last curr word curr )
+ <> ( word last curr word<>curr? )
+ WHILE ( word last curr )
+ NIP ( word curr )
+ DUP @ ( word curr prev (which becomes: word last curr) )
+ REPEAT
+
+ DROP ( at this point, the stack is: start-of-word end-of-word )
+ SWAP ( end-of-word start-of-word )
+
+ ( begin the definition with : NAME [IMMEDIATE] )
+ ':' EMIT SPACE DUP ID. SPACE
+ DUP ?IMMEDIATE IF ." IMMEDIATE " THEN
+
+ >DFA ( get the data address, ie. points after DOCOL | end-of-word start-of-data )
+
+ ( now we start decompiling until we hit the end of the word )
+ BEGIN ( end start )
+ 2DUP >
+ WHILE
+ DUP @ ( end start codeword )
+
+ CASE
+ ' LIT OF ( is it LIT ? )
+ 4 + DUP @ ( get next word which is the integer constant )
+ . ( and print it )
+ ENDOF
+ ' LITSTRING OF ( is it LITSTRING ? )
+ [ CHAR S ] LITERAL EMIT '"' EMIT SPACE ( print S"<space> )
+ 4 + DUP @ ( get the length word )
+ SWAP 4 + SWAP ( end start+4 length )
+ 2DUP TELL ( print the string )
+ '"' EMIT SPACE ( finish the string with a final quote )
+ + ALIGNED ( end start+4+len, aligned )
+ 4 - ( because we're about to add 4 below )
+ ENDOF
+ ' 0BRANCH OF ( is it 0BRANCH ? )
+ ." 0BRANCH ( "
+ 4 + DUP @ ( print the offset )
+ .
+ ." ) "
+ ENDOF
+ ' BRANCH OF ( is it BRANCH ? )
+ ." BRANCH ( "
+ 4 + DUP @ ( print the offset )
+ .
+ ." ) "
+ ENDOF
+ ' ' OF ( is it ' (TICK) ? )
+ [ CHAR ' ] LITERAL EMIT SPACE
+ 4 + DUP @ ( get the next codeword )
+ CFA> ( and force it to be printed as a dictionary entry )
+ ID. SPACE
+ ENDOF
+ ' EXIT OF ( is it EXIT? )
+ ( We expect the last word to be EXIT, and if it is then we don't print it
+ because EXIT is normally implied by ;. EXIT can also appear in the middle
+ of words, and then it needs to be printed. )
+ 2DUP ( end start end start )
+ 4 + ( end start end start+4 )
+ <> IF ( end start | we're not at the end )
+ ." EXIT "
+ THEN
+ ENDOF
+ ( default case: )
+ DUP ( in the default case we always need to DUP before using )
+ CFA> ( look up the codeword to get the dictionary entry )
+ ID. SPACE ( and print it )
+ ENDCASE
+
+ 4 + ( end start+4 )
+ REPEAT
+
+ ';' EMIT CR
+
+ 2DROP ( restore stack )
+;
+
+(
+ EXECUTION TOKENS ----------------------------------------------------------------------
+
+ Standard FORTH defines a concept called an 'execution token' (or 'xt') which is very
+ similar to a function pointer in C. We map the execution token to a codeword address.
+
+ execution token of DOUBLE is the address of this codeword
+ |
+ V
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+ len pad codeword ^
+
+ There is one assembler primitive for execution tokens, EXECUTE ( xt -- ), which runs them.
+
+ You can make an execution token for an existing word the long way using >CFA,
+ ie: WORD [foo] FIND >CFA will push the xt for foo onto the stack where foo is the
+ next word in input. So a very slow way to run DOUBLE might be:
+
+ : DOUBLE DUP + ;
+ : SLOW WORD FIND >CFA EXECUTE ;
+ 5 SLOW DOUBLE . CR \ prints 10
+
+ We also offer a simpler and faster way to get the execution token of any word FOO:
+
+ ['] FOO
+
+ (Exercises for readers: (1) What is the difference between ['] FOO and ' FOO?
+ (2) What is the relationship between ', ['] and LIT?)
+
+ More useful is to define anonymous words and/or to assign xt's to variables.
+
+ To define an anonymous word (and push its xt on the stack) use :NONAME ... ; as in this
+ example:
+
+ :NONAME ." anon word was called" CR ; \ pushes xt on the stack
+ DUP EXECUTE EXECUTE \ executes the anon word twice
+
+ Stack parameters work as expected:
+
+ :NONAME ." called with parameter " . CR ;
+ DUP
+ 10 SWAP EXECUTE \ prints 'called with parameter 10'
+ 20 SWAP EXECUTE \ prints 'called with parameter 20'
+
+ Notice that the above code has a memory leak: the anonymous word is still compiled
+ into the data segment, so even if you lose track of the xt, the word continues to
+ occupy memory. A good way to keep track of the xt and thus avoid the memory leak is
+ to assign it to a CONSTANT, VARIABLE or VALUE:
+
+ 0 VALUE ANON
+ :NONAME ." anon word was called" CR ; TO ANON
+ ANON EXECUTE
+ ANON EXECUTE
+
+ Another use of :NONAME is to create an array of functions which can be called quickly
+ (think: fast switch statement). This example is adapted from the ANS FORTH standard:
+
+ 10 CELLS ALLOT CONSTANT CMD-TABLE
+ : SET-CMD CELLS CMD-TABLE + ! ;
+ : CALL-CMD CELLS CMD-TABLE + @ EXECUTE ;
+
+ :NONAME ." alternate 0 was called" CR ; 0 SET-CMD
+ :NONAME ." alternate 1 was called" CR ; 1 SET-CMD
+ \ etc...
+ :NONAME ." alternate 9 was called" CR ; 9 SET-CMD
+
+ 0 CALL-CMD
+ 1 CALL-CMD
+)
+
+: :NONAME
+ 0 0 CREATE ( create a word with no name - we need a dictionary header because ; expects it )
+ HERE @ ( current HERE value is the address of the codeword, ie. the xt )
+ DOCOL , ( compile DOCOL (the codeword) )
+ ] ( go into compile mode )
+;
+
+: ['] IMMEDIATE
+ ' LIT , ( compile LIT )
+;
+
+(
+ EXCEPTIONS ----------------------------------------------------------------------
+
+ Amazingly enough, exceptions can be implemented directly in FORTH, in fact rather easily.
+
+ The general usage is as follows:
+
+ : FOO ( n -- ) THROW ;
+
+ : TEST-EXCEPTIONS
+ 25 ['] FOO CATCH \ execute 25 FOO, catching any exception
+ ?DUP IF
+ ." called FOO and it threw exception number: "
+ . CR
+ DROP \ we have to drop the argument of FOO (25)
+ THEN
+ ;
+ \ prints: called FOO and it threw exception number: 25
+
+ CATCH runs an execution token and detects whether it throws any exception or not. The
+ stack signature of CATCH is rather complicated:
+
+ ( a_n-1 ... a_1 a_0 xt -- r_m-1 ... r_1 r_0 0 ) if xt did NOT throw an exception
+ ( a_n-1 ... a_1 a_0 xt -- ?_n-1 ... ?_1 ?_0 e ) if xt DID throw exception 'e'
+
+ where a_i and r_i are the (arbitrary number of) argument and return stack contents
+ before and after xt is EXECUTEd. Notice in particular the case where an exception
+ is thrown, the stack pointer is restored so that there are n of _something_ on the
+ stack in the positions where the arguments a_i used to be. We don't really guarantee
+ what is on the stack -- perhaps the original arguments, and perhaps other nonsense --
+ it largely depends on the implementation of the word that was executed.
+
+ THROW, ABORT and a few others throw exceptions.
+
+ Exception numbers are non-zero integers. By convention the positive numbers can be used
+ for app-specific exceptions and the negative numbers have certain meanings defined in
+ the ANS FORTH standard. (For example, -1 is the exception thrown by ABORT).
+
+ 0 THROW does nothing. This is the stack signature of THROW:
+
+ ( 0 -- )
+ ( * e -- ?_n-1 ... ?_1 ?_0 e ) the stack is restored to the state from the corresponding CATCH
+
+ The implementation hangs on the definitions of CATCH and THROW and the state shared
+ between them.
+
+ Up to this point, the return stack has consisted merely of a list of return addresses,
+ with the top of the return stack being the return address where we will resume executing
+ when the current word EXITs. However CATCH will push a more complicated 'exception stack
+ frame' on the return stack. The exception stack frame records some things about the
+ state of execution at the time that CATCH was called.
+
+ When called, THROW walks up the return stack (the process is called 'unwinding') until
+ it finds the exception stack frame. It then uses the data in the exception stack frame
+ to restore the state allowing execution to continue after the matching CATCH. (If it
+ unwinds the stack and doesn't find the exception stack frame then it prints a message
+ and drops back to the prompt, which is also normal behaviour for so-called 'uncaught
+ exceptions').
+
+ This is what the exception stack frame looks like. (As is conventional, the return stack
+ is shown growing downwards from higher to lower memory addresses).
+
+ +------------------------------+
+ | return address from CATCH | Notice this is already on the
+ | | return stack when CATCH is called.
+ +------------------------------+
+ | original parameter stack |
+ | pointer |
+ +------------------------------+ ^
+ | exception stack marker | |
+ | (EXCEPTION-MARKER) | | Direction of stack
+ +------------------------------+ | unwinding by THROW.
+ |
+ |
+
+ The EXCEPTION-MARKER marks the entry as being an exception stack frame rather than an
+ ordinary return address, and it is this which THROW "notices" as it is unwinding the
+ stack. (If you want to implement more advanced exceptions such as TRY...WITH then
+ you'll need to use a different value of marker if you want the old and new exception stack
+ frame layouts to coexist).
+
+ What happens if the executed word doesn't throw an exception? It will eventually
+ return and call EXCEPTION-MARKER, so EXCEPTION-MARKER had better do something sensible
+ without us needing to modify EXIT. This nicely gives us a suitable definition of
+ EXCEPTION-MARKER, namely a function that just drops the stack frame and itself
+ returns (thus "returning" from the original CATCH).
+
+ One thing to take from this is that exceptions are a relatively lightweight mechanism
+ in FORTH.
+)
+
+: EXCEPTION-MARKER
+ RDROP ( drop the original parameter stack pointer )
+ 0 ( there was no exception, this is the normal return path )
+;
+
+: CATCH ( xt -- exn? )
+ DSP@ 4+ >R ( save parameter stack pointer (+4 because of xt) on the return stack )
+ ' EXCEPTION-MARKER 4+ ( push the address of the RDROP inside EXCEPTION-MARKER ... )
+ >R ( ... on to the return stack so it acts like a return address )
+ EXECUTE ( execute the nested function )
+;
+
+: THROW ( n -- )
+ ?DUP IF ( only act if the exception code <> 0 )
+ RSP@ ( get return stack pointer )
+ BEGIN
+ DUP R0 4- < ( RSP < R0 )
+ WHILE
+ DUP @ ( get the return stack entry )
+ ' EXCEPTION-MARKER 4+ = IF ( found the EXCEPTION-MARKER on the return stack )
+ 4+ ( skip the EXCEPTION-MARKER on the return stack )
+ RSP! ( restore the return stack pointer )
+
+ ( Restore the parameter stack. )
+ DUP DUP DUP ( reserve some working space so the stack for this word
+ doesn't coincide with the part of the stack being restored )
+ R> ( get the saved parameter stack pointer | n dsp )
+ 4- ( reserve space on the stack to store n )
+ SWAP OVER ( dsp n dsp )
+ ! ( write n on the stack )
+ DSP! EXIT ( restore the parameter stack pointer, immediately exit )
+ THEN
+ 4+
+ REPEAT
+
+ ( No matching catch - print a message and restart the INTERPRETer. )
+ DROP
+
+ CASE
+ 0 1- OF ( ABORT )
+ ." ABORTED" CR
+ ENDOF
+ ( default case )
+ ." UNCAUGHT THROW "
+ DUP . CR
+ ENDCASE
+ QUIT
+ THEN
+;
+
+: ABORT ( -- )
+ 0 1- THROW
+;
+
+( Print a stack trace by walking up the return stack. )
+: PRINT-STACK-TRACE
+ RSP@ ( start at caller of this function )
+ BEGIN
+ DUP R0 4- < ( RSP < R0 )
+ WHILE
+ DUP @ ( get the return stack entry )
+ CASE
+ ' EXCEPTION-MARKER 4+ OF ( is it the exception stack frame? )
+ ." CATCH ( DSP="
+ 4+ DUP @ U. ( print saved stack pointer )
+ ." ) "
+ ENDOF
+ ( default case )
+ DUP
+ CFA> ( look up the codeword to get the dictionary entry )
+ ?DUP IF ( and print it )
+ 2DUP ( dea addr dea )
+ ID. ( print word from dictionary entry )
+ [ CHAR + ] LITERAL EMIT
+ SWAP >DFA 4+ - . ( print offset )
+ THEN
+ ENDCASE
+ 4+ ( move up the stack )
+ REPEAT
+ DROP
+ CR
+;
+
+(
+ C STRINGS ----------------------------------------------------------------------
+
+ FORTH strings are represented by a start address and length kept on the stack or in memory.
+
+ Most FORTHs don't handle C strings, but we need them in order to access the process arguments
+ and environment left on the stack by the Linux kernel, and to make some system calls.
+
+ Operation Input Output FORTH word Notes
+ ----------------------------------------------------------------------
+
+ Create FORTH string addr len S" ..."
+
+ Create C string c-addr Z" ..."
+
+ C -> FORTH c-addr addr len DUP STRLEN
+
+ FORTH -> C addr len c-addr CSTRING Allocated in a temporary buffer, so
+ should be consumed / copied immediately.
+ FORTH string should not contain NULs.
+
+ For example, DUP STRLEN TELL prints a C string.
+)
+
+(
+ Z" .." is like S" ..." except that the string is terminated by an ASCII NUL character.
+
+ To make it more like a C string, at runtime Z" just leaves the address of the string
+ on the stack (not address & length as with S"). To implement this we need to add the
+ extra NUL to the string and also a DROP instruction afterwards. Apart from that the
+ implementation just a modified S".
+)
+: Z" IMMEDIATE
+ 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 @ C! ( store the character in the compiled image )
+ 1 HERE +! ( increment HERE pointer by 1 byte )
+ REPEAT
+ 0 HERE @ C! ( add the ASCII NUL byte )
+ 1 HERE +!
+ 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 )
+ ALIGN ( round up to next multiple of 4 bytes for the remaining code )
+ ' DROP , ( compile DROP (to drop the length) )
+ ELSE ( immediate mode )
+ HERE @ ( get the start address of the temporary space )
+ BEGIN
+ KEY
+ DUP '"' <>
+ WHILE
+ OVER C! ( save next character )
+ 1+ ( increment address )
+ REPEAT
+ DROP ( drop the final " character )
+ 0 SWAP C! ( store final ASCII NUL )
+ HERE @ ( push the start address )
+ THEN
+;
+
+: STRLEN ( str -- len )
+ DUP ( save start address )
+ BEGIN
+ DUP C@ 0<> ( zero byte found? )
+ WHILE
+ 1+
+ REPEAT
+
+ SWAP - ( calculate the length )
+;
+
+: CSTRING ( addr len -- c-addr )
+ SWAP OVER ( len saddr len )
+ HERE @ SWAP ( len saddr daddr len )
+ CMOVE ( len )
+
+ HERE @ + ( daddr+len )
+ 0 SWAP C! ( store terminating NUL char )
+
+ HERE @ ( push start address )
+;
+
+(
+ THE ENVIRONMENT ----------------------------------------------------------------------
+
+ Linux makes the process arguments and environment available to us on the stack.
+
+ The top of stack pointer is saved by the early assembler code when we start up in the FORTH
+ variable S0, and starting at this pointer we can read out the command line arguments and the
+ environment.
+
+ Starting at S0, S0 itself points to argc (the number of command line arguments).
+
+ S0+4 points to argv[0], S0+8 points to argv[1] etc up to argv[argc-1].
+
+ argv[argc] is a NULL pointer.
+
+ After that the stack contains environment variables, a set of pointers to strings of the
+ form NAME=VALUE and on until we get to another NULL pointer.
+
+ The first word that we define, ARGC, pushes the number of command line arguments (note that
+ as with C argc, this includes the name of the command).
+)
+: ARGC
+ S0 @ @
+;
+
+(
+ n ARGV gets the nth command line argument.
+
+ For example to print the command name you would do:
+ 0 ARGV TELL CR
+)
+: ARGV ( n -- str u )
+ 1+ CELLS S0 @ + ( get the address of argv[n] entry )
+ @ ( get the address of the string )
+ DUP STRLEN ( and get its length / turn it into a FORTH string )
+;
+
+(
+ ENVIRON returns the address of the first environment string. The list of strings ends
+ with a NULL pointer.
+
+ For example to print the first string in the environment you could do:
+ ENVIRON @ DUP STRLEN TELL
+)
+: ENVIRON ( -- addr )
+ ARGC ( number of command line parameters on the stack to skip )
+ 2 + ( skip command line count and NULL pointer after the command line args )
+ CELLS ( convert to an offset )
+ S0 @ + ( add to base stack address )
+;
+
+(
+ SYSTEM CALLS AND FILES ----------------------------------------------------------------------
+
+ Miscellaneous words related to system calls, and standard access to files.
+)
+
+( BYE exits by calling the Linux exit(2) syscall. )
+: BYE ( -- )
+ 0 ( return code (0) )
+ SYS_EXIT ( system call number )
+ SYSCALL1
+;
+
+(
+ UNUSED returns the number of cells remaining in the user memory (data segment).
+
+ For our implementation we will use Linux brk(2) system call to find out the end
+ of the data segment and subtract HERE from it.
+)
+: GET-BRK ( -- brkpoint )
+ 0 SYS_BRK SYSCALL1 ( call brk(0) )
+;
+
+: UNUSED ( -- n )
+ GET-BRK ( get end of data segment according to the kernel )
+ HERE @ ( get current position in data segment )
+ -
+ 4 / ( returns number of cells )
+;
+
+(
+ MORECORE increases the data segment by the specified number of (4 byte) cells.
+
+ NB. The number of cells requested should normally be a multiple of 1024. The
+ reason is that Linux can't extend the data segment by less than a single page
+ (4096 bytes or 1024 cells).
+
+ This FORTH doesn't automatically increase the size of the data segment "on demand"
+ (ie. when , (COMMA), ALLOT, CREATE, and so on are used). Instead the programmer
+ needs to be aware of how much space a large allocation will take, check UNUSED, and
+ call MORECORE if necessary. A simple programming exercise is to change the
+ implementation of the data segment so that MORECORE is called automatically if
+ the program needs more memory.
+)
+: BRK ( brkpoint -- )
+ SYS_BRK SYSCALL1
+;
+
+: MORECORE ( cells -- )
+ CELLS GET-BRK + BRK
+;
+
+(
+ Standard FORTH provides some simple file access primitives which we model on
+ top of Linux syscalls.
+
+ The main complication is converting FORTH strings (address & length) into C
+ strings for the Linux kernel.
+
+ Notice there is no buffering in this implementation.
+)
+
+: R/O ( -- fam ) O_RDONLY ;
+: R/W ( -- fam ) O_RDWR ;
+
+: OPEN-FILE ( addr u fam -- fd 0 (if successful) | c-addr u fam -- fd errno (if there was an error) )
+ ROT ( fam addr u )
+ CSTRING ( fam cstring )
+ SYS_OPEN SYSCALL2 ( open (filename, flags) )
+ DUP ( fd fd )
+ DUP 0< IF ( errno? )
+ NEGATE ( fd errno )
+ ELSE
+ DROP 0 ( fd 0 )
+ THEN
+;
+
+: CREATE-FILE ( addr u fam -- fd 0 (if successful) | c-addr u fam -- fd errno (if there was an error) )
+ O_CREAT OR
+ O_TRUNC OR
+ ROT ( fam addr u )
+ CSTRING ( fam cstring )
+ 420 ROT ( 0644 fam cstring )
+ SYS_OPEN SYSCALL3 ( open (filename, flags|O_TRUNC|O_CREAT, 0644) )
+ DUP ( fd fd )
+ DUP 0< IF ( errno? )
+ NEGATE ( fd errno )
+ ELSE
+ DROP 0 ( fd 0 )
+ THEN
+;
+
+: CLOSE-FILE ( fd -- 0 (if successful) | fd -- errno (if there was an error) )
+ SYS_CLOSE SYSCALL1
+ NEGATE
+;
+
+: READ-FILE ( addr u fd -- u2 0 (if successful) | addr u fd -- 0 0 (if EOF) | addr u fd -- u2 errno (if error) )
+ ROT SWAP -ROT ( u addr fd )
+ SYS_READ SYSCALL3
+
+ DUP ( u2 u2 )
+ DUP 0< IF ( errno? )
+ NEGATE ( u2 errno )
+ ELSE
+ DROP 0 ( u2 0 )
+ THEN
+;
+
+(
+ PERROR prints a message for an errno, similar to C's perror(3) but we don't have the extensive
+ list of strerror strings available, so all we can do is print the errno.
+)
+: PERROR ( errno addr u -- )
+ TELL
+ ':' EMIT SPACE
+ ." ERRNO="
+ . CR
+;
+
+(
+ NOTES ----------------------------------------------------------------------
+
+ DOES> isn't possible to implement with this FORTH because we don't have a separate
+ data pointer.
+)
+
+(
+ WELCOME MESSAGE ----------------------------------------------------------------------
+
+ Print the version and OK prompt.
+)
+
+: WELCOME
+ S" TEST-MODE" FIND NOT IF
+ ." JONESFORTH VERSION " VERSION . CR
+ UNUSED . ." CELLS REMAINING" CR
+ ." OK "
+ THEN
+;
+
+WELCOME
+HIDE WELCOME
projects / rrq / jonesforth.git / blobdiff