\ 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 1.7 2007-09-28 18:55:10 rich Exp $
+\ $Id: jonesforth.f,v 1.17 2007-10-12 20:07:44 rich Exp $
\
\ The first part of this tutorial is in jonesforth.S. Get if from http://annexia.org/forth
\
\ 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.
\
: 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 ! ;
-
-\ 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 / ;
+: SPACE BL EMIT ;
\ NEGATE leaves the negative of a number on the stack.
: NEGATE 0 SWAP - ;
, \ 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:
: 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
;
: 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, (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.
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.
)
+( 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 )
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 )
;
(
+ 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
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* ;
+: CELLS ( n -- n ) 4 * ;
(
So now we can define VARIABLE easily in much the same way as CONSTANT above. Refer to the
)
: 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.
20 VALUE VAL creates VAL with initial value 20
- VAL pushes the value directly on the stack
+ VAL pushes the value (20) directly on the stack
30 TO VAL updates VAL, setting it to 30
+ VAL pushes the value (30) directly on the stack
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 '@').
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".
+
+ You can dump out the raw code for the last word you defined by doing something like:
+
+ LATEST @ 128 DUMP
)
: DUMP ( addr len -- )
BASE @ ROT ( save the current BASE at the bottom of the stack )
- HEX ( and switch the hexadecimal mode )
+ HEX ( and switch to hexadecimal mode )
BEGIN
- DUP 0> ( while len > 0 )
+ ?DUP ( 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 )
2DUP ( addr len addr len )
1- 15 AND 1+ ( addr len addr linelen )
BEGIN
- DUP 0> ( while linelen > 0 )
+ ?DUP ( while linelen > 0 )
WHILE
SWAP ( addr len linelen addr )
DUP C@ ( addr len linelen addr byte )
2 .R SPACE ( print the byte )
1+ SWAP 1- ( addr len linelen addr -- addr len addr+1 linelen-1 )
REPEAT
- 2DROP ( addr len )
+ DROP ( addr len )
( print the ASCII equivalents )
2DUP 1- 15 AND 1+ ( addr len addr linelen )
BEGIN
- DUP 0> ( while linelen > 0)
+ ?DUP ( while linelen > 0)
WHILE
SWAP ( addr len linelen addr )
DUP C@ ( addr len linelen addr byte )
THEN
1+ SWAP 1- ( addr len linelen addr -- addr len addr+1 linelen-1 )
REPEAT
- 2DROP ( addr len )
+ DROP ( addr len )
CR
DUP 1- 15 AND 1+ ( addr len linelen )
SWAP ( addr-linelen len-linelen )
REPEAT
- 2DROP ( restore stack )
+ DROP ( restore stack )
BASE ! ( restore saved BASE )
;
(
+ 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
+ ( 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.
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
+ 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.
;
(
+ DECOMPILER ----------------------------------------------------------------------
+
CFA> is the opposite of >CFA. It takes a codeword and tries to find the matching
- dictionary definition.
+ 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).
+ 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 0<> ( while link pointer is not null )
+ ?DUP ( while link pointer is not null )
WHILE
- DUP >CFA ( cfa curr curr-cfa )
- 2 PICK ( cfa curr curr-cfa cfa )
- = IF ( found a match? )
+ 2DUP SWAP ( cfa curr curr cfa )
+ < IF ( current dictionary entry < cfa? )
NIP ( leave curr dictionary entry on the stack )
- EXIT ( and return from the function )
+ EXIT
THEN
@ ( follow link pointer back )
REPEAT
- 2DROP ( restore stack )
+ DROP ( restore stack )
0 ( sorry, nothing found )
;
(
- SEE disassembles a FORTH word.
+ 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:
: SEE
WORD FIND ( find the dictionary entry to decompile )
- ( now we search again, looking for the next word )
+ ( 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
>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 )
- ( XXX we should ignore the final codeword if it is EXIT )
BEGIN ( end start )
2DUP >
WHILE
CASE
' LIT OF ( is it LIT ? )
- 4 + DUP @ ( get next word which is the integer constant )
- . ( and print it )
+ 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 )
+ 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 )
+ 4 + DUP @ ( print the offset )
.
- ')' EMIT SPACE
+ ." ) "
ENDOF
' BRANCH OF ( is it BRANCH ? )
." BRANCH ( "
- 4 + DUP @ ( print the offset )
+ 4 + DUP @ ( print the offset )
.
- ')' EMIT SPACE
+ ." ) "
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 )
+ 4 + DUP @ ( get the next codeword )
+ CFA> ( and force it to be printed as a dictionary entry )
ID. SPACE
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 )
+ ' 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 )
2DROP ( restore stack )
;
-( Finally print the welcome prompt. )
-." JONESFORTH VERSION " VERSION . CR
-." OK "
+(
+ 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
+;
+
+(
+ ASSEMBLER CODE ----------------------------------------------------------------------
+
+ This is just the outline of a simple assembler, allowing you to write FORTH primitives
+ in assembly language.
+
+ Assembly primitives begin ': NAME' in the normal way, but are ended with ;CODE. ;CODE
+ updates the header so that the codeword isn't DOCOL, but points instead to the assembled
+ code (in the DFA part of the word).
+
+ We provide a convenience macro NEXT (you guessed what it does). However you don't need to
+ use it because ;CODE will put a NEXT at the end of your word.
+
+ The rest consists of some immediate words which expand into machine code appended to the
+ definition of the word. Only a very tiny part of the i386 assembly space is covered, just
+ enough to write a few assembler primitives below.
+)
+
+HEX
+
+( Equivalent to the NEXT macro )
+: NEXT IMMEDIATE AD C, FF C, 20 C, ;
+
+: ;CODE IMMEDIATE
+ [COMPILE] NEXT ( end the word with NEXT macro )
+ ALIGN ( machine code is assembled in bytes so isn't necessarily aligned at the end )
+ LATEST @ DUP
+ HIDDEN ( unhide the word )
+ DUP >DFA SWAP >CFA ! ( change the codeword to point to the data area )
+ [COMPILE] [ ( go back to immediate mode )
+;
+
+( The i386 registers )
+: EAX IMMEDIATE 0 ;
+: ECX IMMEDIATE 1 ;
+: EDX IMMEDIATE 2 ;
+: EBX IMMEDIATE 3 ;
+: ESP IMMEDIATE 4 ;
+: EBP IMMEDIATE 5 ;
+: ESI IMMEDIATE 6 ;
+: EDI IMMEDIATE 7 ;
+
+( i386 stack instructions )
+: PUSH IMMEDIATE 50 + C, ;
+: POP IMMEDIATE 58 + C, ;
+
+( RDTSC instruction )
+: RDTSC IMMEDIATE 0F C, 31 C, ;
+
+DECIMAL
+
+(
+ RDTSC is an assembler primitive which reads the Pentium timestamp counter (a very fine-
+ grained counter which counts processor clock cycles). Because the TSC is 64 bits wide
+ we have to push it onto the stack in two slots.
+)
+: RDTSC ( -- lsb msb )
+ RDTSC ( writes the result in %edx:%eax )
+ EAX PUSH ( push lsb )
+ EDX PUSH ( push msb )
+;CODE
+
+(
+ INLINE can be used to inline an assembler primitive into the current (assembler)
+ word.
+
+ For example:
+
+ : 2DROP INLINE DROP INLINE DROP ;CODE
+
+ will build an efficient assembler word 2DROP which contains the inline assembly code
+ for DROP followed by DROP (eg. two 'pop %eax' instructions in this case).
+
+ Another example. Consider this ordinary FORTH definition:
+
+ : C@++ ( addr -- addr+1 byte ) DUP 1+ SWAP C@ ;
+
+ (it is equivalent to the C operation '*p++' where p is a pointer to char). If we
+ notice that all of the words used to define C@++ are in fact assembler primitives,
+ then we can write a faster (but equivalent) definition like this:
+
+ : C@++ INLINE DUP INLINE 1+ INLINE SWAP INLINE C@ ;CODE
+
+ One interesting point to note is that this "concatenative" style of programming
+ allows you to write assembler words portably. The above definition would work
+ for any CPU architecture.
+
+ There are several conditions that must be met for INLINE to be used successfully:
+
+ (1) You must be currently defining an assembler word (ie. : ... ;CODE).
+
+ (2) The word that you are inlining must be known to be an assembler word. If you try
+ to inline a FORTH word, you'll get an error message.
+
+ (3) The assembler primitive must be position-independent code and must end with a
+ single NEXT macro.
+
+ Exercises for the reader: (a) Generalise INLINE so that it can inline FORTH words when
+ building FORTH words. (b) Further generalise INLINE so that it does something sensible
+ when you try to inline FORTH into assembler and vice versa.
+
+ The implementation of INLINE is pretty simple. We find the word in the dictionary,
+ check it's an assembler word, then copy it into the current definition, byte by byte,
+ until we reach the NEXT macro (which is not copied).
+)
+HEX
+: =NEXT ( addr -- next? )
+ DUP C@ AD <> IF DROP FALSE EXIT THEN
+ 1+ DUP C@ FF <> IF DROP FALSE EXIT THEN
+ 1+ C@ 20 <> IF FALSE EXIT THEN
+ TRUE
+;
+DECIMAL
+
+( (INLINE) is the lowlevel inline function. )
+: (INLINE) ( cfa -- )
+ @ ( remember codeword points to the code )
+ BEGIN ( copy bytes until we hit NEXT macro )
+ DUP =NEXT NOT
+ WHILE
+ DUP C@ C,
+ 1+
+ REPEAT
+ DROP
+;
+
+: INLINE IMMEDIATE
+ WORD FIND ( find the word in the dictionary )
+ >CFA ( codeword )
+
+ DUP @ DOCOL = IF ( check codeword <> DOCOL (ie. not a FORTH word) )
+ ." Cannot INLINE FORTH words" CR ABORT
+ THEN
+
+ (INLINE)
+;
+
+HIDE =NEXT
+
+(
+ 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
git.annexia.org / jonesforth.git / blobdiff