+(
+ 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
+;
+
+(
+ CFA> is the opposite of >CFA. It takes a codeword and tries to find the matching
+ dictionary definition.
+
+ 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).
+
+ 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 )
+ WHILE
+ DUP >CFA ( cfa curr curr-cfa )
+ 2 PICK ( cfa curr curr-cfa cfa )
+ = IF ( found a match? )
+ NIP ( leave curr dictionary entry on the stack )
+ EXIT ( and return from the function )
+ THEN
+ @ ( follow link pointer back )
+ REPEAT
+ 2DROP ( restore stack )
+ 0 ( sorry, nothing found )
+;
+
+(
+ SEE disassembles 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 )
+ 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 )
+ .
+ ')' EMIT SPACE
+ ENDOF
+ ' BRANCH OF ( is it BRANCH ? )
+ ." BRANCH ( "
+ 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 )
+ 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 )
+;
+
git.annexia.org / jonesforth.git / blobdiff