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https://github.com/jwillia3/BASIC

An extensible BASIC interpreter in one header
https://github.com/jwillia3/BASIC

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An extensible BASIC interpreter in one header

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README 

        
			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