revise startup code with comments and new definitions

startup.4th

@@ -1,30 +1,93 @@
+\ Read the next word and return the first character
+( "<spaces>name" -- c )
 : CHAR WORD DROP C@ ;
 
-: TAB 9 EMIT ;
+\ Some common non-word characters
-: LF 10 EMIT ;
+: HT  9 ;  \ Horizontal Tab
-: SPACE 32 EMIT ;
+: LF 10 ;  \ Line Feed (newline)
+: CR 13 ;  \ Carriage Return
+: BL 32 ;  \ BLank (space)
 
-: BANNER "JumpForth version " .S VERSION . ", by Jesse McDonald" .S LF ;
+\ Emit an implementation-dependent End-of-Line sequence
+: .EOL LF EMIT ;
 
+\ Switch from compiling to interpreting, or vice-versa
 : [ FALSE STATE ! ; IMMEDIATE
 : ] TRUE STATE ! ; IMMEDIATE
 
-: POSTPONE WORD FINDERR , ; IMMEDIATE
+\ COMPILE, appends the effect of the execution token on the top of the stack
-
+\ to the current definition. In this implementation it's equivalent to , since
+\ definitions are just arrays of execution tokens.
 : COMPILE, , ;
 
-: ['] [ ' LIT , ' LIT , ] , ' , ; IMMEDIATE
+\ Read a word and append it to the current definition, ignoring the IMMED flag.
+\ This is used to "call" an immediate word as a normal function.
+\ POSTPONE has no effect (as a modifier) if the IMMED flag is not set.
+: POSTPONE WORD FINDERR COMPILE, ; IMMEDIATE
 
-: LITERAL ['] LIT , , ; IMMEDIATE
+\ Append the LIT xt and the topmost word on the stack to the current definition.
+\ If [COMPILE] were already defined then this could simply be written as:
+\
+\   : LITERAL [COMPILE] LIT , ; IMMEDIATE
+\
+\ ... but since it isn't we must manually insert the LIT sequence to obtain the
+\ address of LIT as a literal to be compiled into the current definition.
+: LITERAL [ ' LIT COMPILE, ' LIT , ] COMPILE, , ; IMMEDIATE
 
-: IF ['] 0BRANCH , HERE 0 , ; IMMEDIATE
+\ Like CHAR, but generates a literal at compile-time.
+: [CHAR] CHAR POSTPONE LITERAL ; IMMEDIATE
+
+\ Like ', but generates a literal at compile-time.
+: ['] ' POSTPONE LITERAL ; IMMEDIATE
+
+\ "[COMPILE] <word>" is equivalent to "['] <word> COMPILE,".
+\ Definition has been manually expanded (vs. [COMPILE] COMPILE,) to avoid recursion.
+: [COMPILE] POSTPONE ['] ['] COMPILE, COMPILE, ; IMMEDIATE
+
+\ Our first control-flow primitive: <cond> IF <true> {ELSE <false>} THEN
+\ IF consumes <cond> and executes <true> if <cond> was non-zero or else
+\ <false> if <cond> was zero. In either case execution continues after THEN.
+\ IF and ELSE leave the address of the latest branch "hole" on the data stack.
+\ ELSE and THEN fill in the previous "hole" with the relative branch offset.
+\ The {ELSE <false>} clause is optional.
+: IF [COMPILE] 0BRANCH HERE 0 , ; IMMEDIATE
 : THEN HERE OVER - SWAP ! ; IMMEDIATE
-: ELSE POSTPONE THEN ['] BRANCH , HERE 0 , ; IMMEDIATE
+: ELSE [COMPILE] BRANCH HERE 0 , SWAP POSTPONE THEN ; IMMEDIATE
 
+\ Unbounded loop: BEGIN <code> AGAIN
+\ BEGIN places the offset of the start of <code> on the stack.
+\ AGAIN creates a relative branch back to the start of <code>.
 : BEGIN HERE ; IMMEDIATE
-: AGAIN ['] BRANCH , HERE - , ; IMMEDIATE
+: AGAIN [COMPILE] BRANCH HERE - , ; IMMEDIATE
 
-CREATE VAR 123 , DOES> @ ;
+\ Define a named constant.
-: VAR! [ ' VAR >BODY ] LITERAL ! ;
+\   Execution: ( value "<spaces>name" -- )
+\   name Execution: ( -- value )
+: CONSTANT : POSTPONE LITERAL POSTPONE ; ;
+
+\ Define a single-cell named variable which returns its data address when executed.
+\ The initial value is formally undefined. This implementation sets it to zero.
+\   Execution: ( "<spaces>name" -- )
+\   name Execution: ( -- a-addr )
+: VARIABLE CREATE 0 , ;
+
+\ Define a single-cell named value which returns its data (not address) when executed.
+\ Named values defined with VALUE can be modified with TO.
+\   Execution: ( x "<spaces>name" -- )
+\   name execution: ( -- value )
+: VALUE CREATE , POSTPONE DOES> [COMPILE] @ POSTPONE ; ;
+
+\ When compiling, append code to store to the data field area of the named value.
+\ When interpreting, store to the data field directly.
+\ An ambiguous condition exists if the name was not created with VALUE.
+( x "<spaces>name" -- )
+: TO ' >BODY STATE @ IF POSTPONE LITERAL [COMPILE] ! ELSE ! THEN ; IMMEDIATE
+
+: / /MOD NIP ;
+: MOD /MOD DROP ;
+
+: DEPTH SP@ S0 SWAP - CELL / ;
+
+: BANNER "JumpForth version " .S VERSION . ", by Jesse McDonald" .S .EOL ;
 
 BANNER