jwillia3/BASIC

			BASIC INTERPRETER
			JERRY WILLIAMS JR

Thank you for you interest in my little BASIC interpreter.
Since a few people have asked, I've started including the license here in the repository.
I have enjoyed hearing from people who find a use for this, so please do send me an e-mail if you feel so inclined.
The license is the same as MIT with the paragraph removed:
	The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.


STATEMENTS:
	#	COMMENT ...
	>	EXPR
	DIM	VAR(LENGTH)
	LOCAL	[VAR [, VAR ...]]
	SUB	NAME [PARAM [, PARAM ...]] ... END SUB
	NAME	[ARG [, ARG ...]]
	RETURN	[EXPRESSION]	
	IF	CONDITION ... [ELSE IF CONDITION ...] [ELSE ...] END IF
	IF	CONDITION THEN STATEMENT
	WHILE	CONDITION ... END WHILE
	FOR	VAR=INIT TO FINAL ... END FOR
	FORMAT	STRING [, EXPRESSION ...]
	RESUME
	BREAK
	BYE
	
	* "%" IN "FORMAT" PRINTS THE NEXT ARGUMENT AS AN INTEGER
	* "SUB" MAY NOT BE NESTED
	* "LOCAL" MAY ONLY BE USED IN A SUBROUTINE

EXPRESSIONS:
	NUMBER, "", NAME, NAME(INDEX), NAME(ARG,...), UBOUND(VAR), (EXPRESSION)
	[EXPRESSION {*|/|\}] EXPRESSION
	[EXPRESSION {+|-}] EXPRESSION
	[EXPRESSION {=|<|>|<>|<=|=>}] EXPRESSION
	[EXPRESSION {AND|OR}] EXPRESSION
	
	* BOOLEAN VALUES ARE -1 FOR TRUE AND 0 FOR FALSE
	* "AND" AND "OR" ARE NOT SHORT-CIRCUITING
	* OPERATORS ARE LEFT-ASSOCIATIVE
	* STRINGS SHOULD ONLY BE USED FOR "FORMAT"

GENERAL NOTES:
	* VARIABLES ARE CASE-INSENSITIVE
	* ARRAY INDEXES START FROM 1
	


			IMPLEMENTATION

	* ALL BLOCK STRUCTURES LEAVE THEIR KEYWORD AT THE TOP
		OF THE C-STACK TO BE CHECKED AGAINST "END"
	* "TEMP" KEEPS TRACK OF HOW MANY CELLS ARE ON THE
		RUN-TIME STACK. "RETURN" DROPS THESE BEFORE
		EXITING THE FUNCTION.
	* "LOCAL" CANNOT CHANGE THE MODE OF A VARIABLE; A
		VARIABLE MUST BE NORMAL, AN ARRAY, OR SUBROUTINE
		IN ALL CONTEXTS.
	* "INITBASIC" IS NOT MEANT TO RESET THE INTERPRETER

COMPILING AND INTERPRETING:
	IF GIVEN A FILE, WE BEGIN IN COMPILED MODE
	IF NO FILE IS GIVEN, WE BEGIN IN INTERPRETED MODE
	"CPC" IS THE "PC" OF THE NEXT INSTRUCTION TO COMPILE
	"IPC" IS THE BEGINNING OF TEMPORARY INTERPRETED CODE
	"OPC" IS THE PC TO RETURN TO AFTER INTERPRETED CODE IS RUN
	A STATMENT IS COMPILED IN EITHER MODE
	AFTER A STATEMENT IS COMPILED, IT IS RUN IN I-MODE BY:
		SETTING "PC" TO "IPC"
		EMITTING A "STOP_"
		RUNNING THE DRIVER
		RESTORING THE "PC" AND "CPC"
	COMPLEX STATEMENTS LIKE IF SET ENTER COMPILED MODE
	* "OPC" IS NULL IF "BREAK" IS RUN FROM COMPILED CODE
	* "RESUME" WITH A VALUE FOR EXPRESSIONS NOT STATEMENTS

SUBROUTINES:
	C-STACK: {JMP}
	STACK: {LINK,SAVED...}
	SUBROUTINE DESCRIPTOR:
		TOTAL LOCALS
		PARAM COUNT
		INDEXES OF PARAMS
	VALUE OF A SUBROUTINE IS IT'S PC
	CALL TAKES VARIABLE INDEX
	* NO DISTINCTION BETWEEN FUNCTIONS AND SUBROUTINES
	* LOCALS MAY ONLY BE USED INSIDE OF A SUBROUTINE
	* TRYING TO USE THEM OUTSIDE WILL NOT REPORT AN ERROR

WHILE:	C-STACK: {TEST-FALSE-BRANCH, TOP}

FOR:
	C-STACK: {TEST-FALSE-BRANCH, I, TOP}
	STACK:	{HI}
	* "FOR_" AND "NEXT_" INSTRUCTIONS DROP THE HI VALUE
		FROM THE STACK ON FAILURE AND INCREMENT

DIMENSIONS:
	THE VALUE OF A DIM IS A POINTER
	THE LENGTH OF THE DIM IS STORED IN THE FIRST CELL