diff --git a/doc/notes.txt b/doc/notes.txt
index 761af872..854dda8d 100644
--- a/doc/notes.txt
+++ b/doc/notes.txt
@@ -510,3 +510,19 @@ in devices:
 
 Should call trap() every cycle or insn of frame, or exit when returns true?
 
+
+BETTER DEBUGGING
+
+Need to mark start/end columns, not just line number
+Know if we are actively debugging or trap occurred
+isRunning() = stopped, running, waiting, debugging...
+Showing running PC may be distracting, maybe lines visited?
+Don't grab cursor focus when trap occurs (how do we know?)
+Use tick() and refresh(), not callbacks
+Show current datum when using READ
+Use https://codemirror.net/doc/manual.html#markText
+
+
+
+
+
diff --git a/presets/basic/skeleton.basic b/presets/basic/skeleton.basic
new file mode 100644
index 00000000..e86e9f18
--- /dev/null
+++ b/presets/basic/skeleton.basic
@@ -0,0 +1 @@
+10 PRINT "EXAMPLE BASIC PROGRAM"
diff --git a/src/codemirror/basic.js b/src/codemirror/basic.js
index eaf34014..0039acbf 100644
--- a/src/codemirror/basic.js
+++ b/src/codemirror/basic.js
@@ -26,14 +26,15 @@ CodeMirror.defineMode("basic", function(conf, parserConf) {
     var identifiers = new RegExp("^[_A-Za-z][_A-Za-z0-9]*");
 
     var openingKeywords = ['if','for'];
-    var middleKeywords = ['to'];
+    var middleKeywords = ['to','then'];
     var endKeywords = ['next','end'];
 
-    var operatorKeywords = ['and', 'or', 'not', 'xor', 'in'];
+    var operatorKeywords = ['and', 'or', 'not', 'xor', 'eqv', 'imp'];
     var wordOperators = wordRegexp(operatorKeywords);
     var commonKeywords = [
-        'let','print','go','goto','gosub','next','dim','input','data',
-        'read','restore','return','stop','on','def','option','then','step',
+        'BASE','DATA','DEF','DIM',
+        'GO','GOSUB','GOTO','INPUT','LET','ON','OPTION','PRINT',
+        'RANDOMIZE','READ','REM','RESTORE','RETURN','STEP','STOP','SUB'
     ];
     var commontypes = ['xxxxbyte','xxxxword'];
 
diff --git a/src/common/basic/compiler.ts b/src/common/basic/compiler.ts
index 543c07f4..2d20c451 100644
--- a/src/common/basic/compiler.ts
+++ b/src/common/basic/compiler.ts
@@ -13,7 +13,7 @@ class CompileError extends Error {
 
 // Lexer regular expression -- each (capture group) handles a different token type
 
-const re_toks = /([0-9.]+[E][+-]?\d+)|(\d*[.]\d*[E0-9]*)|(\d+)|(['].*)|(\bAND\b)|(\bOR\b)|(\w+[$]?)|(".*?")|([<>]?[=<>])|([-+*/^,;:()])|(\S+)/gi;
+const re_toks = /([0-9.]+[E][+-]?\d+)|(\d*[.]\d*[E0-9]*)|(\d+)|(['].*)|(\w+[$]?)|(".*?")|([<>]?[=<>])|([-+*/^,;:()])|(\S+)/gi;
 
 export enum TokenType {
     EOL = 0,
@@ -21,8 +21,6 @@ export enum TokenType {
     Float2,
     Int,
     Remark,
-    And,
-    Or,
     Ident,
     String,
     Relational,
@@ -35,7 +33,7 @@ export type ExprTypes = BinOp | UnOp | IndOp | Literal;
 
 export type Expr = ExprTypes & SourceLocated;
 
-export type Opcode = 'add' | 'sub' | 'mul' | 'div' | 'pow' | 'eq' | 'ne' | 'lt' | 'gt' | 'le' | 'ge' | 'land' | 'lor';
+export type Opcode = string;
 
 export type Value = string | number;
 
@@ -50,7 +48,7 @@ export interface BinOp {
 }
 
 export interface UnOp {
-    op: 'neg';
+    op: 'neg' | 'lnot';
     expr: Expr;
 }
 
@@ -152,6 +150,7 @@ export interface BASICLine {
 }
 
 export interface BASICProgram {
+    opts: BASICOptions;
     lines: BASICLine[];
 }
 
@@ -162,23 +161,25 @@ class Token {
 }
 
 const OPERATORS = {
-    'AND':  {f:'land',p:5},
-    'OR':   {f:'lor',p:5},
-    '=':    {f:'eq',p:10},
-    '<>':   {f:'ne',p:10},
-    '<':    {f:'lt',p:10},
-    '>':    {f:'gt',p:10},
-    '<=':   {f:'le',p:10},
-    '>=':   {f:'ge',p:10},
+    'OR':   {f:'bor',p:7},
+    'AND':  {f:'band',p:8},
+    '=':    {f:'eq',p:50},
+    '<>':   {f:'ne',p:50},
+    '<':    {f:'lt',p:50},
+    '>':    {f:'gt',p:50},
+    '<=':   {f:'le',p:50},
+    '>=':   {f:'ge',p:50},
     '+':    {f:'add',p:100},
     '-':    {f:'sub',p:100},
+    '%':    {f:'mod',p:140},
+    '\\':   {f:'idiv',p:150},
     '*':    {f:'mul',p:200},
     '/':    {f:'div',p:200},
     '^':    {f:'pow',p:300}
 };
 
-function getOpcodeForOperator(op: string): Opcode {
-    return OPERATORS[op].f as Opcode;
+function getOperator(op: string) {
+    return OPERATORS[op];
 }
 
 function getPrecedence(tok: Token): number {
@@ -186,7 +187,7 @@ function getPrecedence(tok: Token): number {
         case TokenType.Operator:
         case TokenType.Relational:
         case TokenType.Ident:
-            let op = OPERATORS[tok.str]
+            let op = getOperator(tok.str);
             if (op) return op.p;
     }
     return -1;
@@ -202,7 +203,7 @@ function stripQuotes(s: string) {
     return s.substr(1, s.length-2);
 }
 
-// TODO
+// TODO: implement these
 export interface BASICOptions {
     uppercaseOnly : boolean;            // convert everything to uppercase?
     strictVarNames : boolean;           // only allow A0-9 for numerics, single letter for arrays/strings
@@ -222,21 +223,25 @@ export interface BASICOptions {
     printPrecision : number;            // print precision # of digits
     checkOverflow : boolean;            // check for overflow of numerics?
     defaultValues : boolean;            // initialize unset variables to default value? (0 or "")
+    multipleNextVars : boolean;         // NEXT Y,X
 }
 
 ///// BASIC PARSER
 
 export class BASICParser {
-    tokens: Token[];
+    opts : BASICOptions = ALTAIR_BASIC40;
     errors: WorkerError[];
+    listings: CodeListingMap;
     labels: { [label: string]: BASICLine };
     targets: { [targetlabel: string]: SourceLocation };
-    eol: Token;
+    decls: { [name: string]: SourceLocation }; // declared/set vars
+    refs: { [name: string]: SourceLocation }; // references
+
     lineno : number;
+    tokens: Token[];
+    eol: Token;
     curlabel: string;
-    listings: CodeListingMap;
     lasttoken: Token;
-    opts : BASICOptions = ALTAIR_BASIC40;
 
     constructor() {
         this.labels = {};
@@ -245,6 +250,8 @@ export class BASICParser {
         this.lineno = 0;
         this.curlabel = null;
         this.listings = {};
+        this.decls = {};
+        this.refs = {};
     }
     compileError(msg: string, loc?: SourceLocation) {
         if (!loc) loc = this.peekToken().$loc;
@@ -253,7 +260,7 @@ export class BASICParser {
         throw new CompileError(`${msg} (line ${loc.line})`); // TODO: label too?
     }
     dialectError(what: string, loc?: SourceLocation) {
-        this.compileError(`The selected BASIC dialect doesn't support ${what}`, loc); // TODO
+        this.compileError(`The selected BASIC dialect doesn't support ${what}.`, loc); // TODO
     }
     consumeToken(): Token {
         var tok = this.lasttoken = (this.tokens.shift() || this.eol);
@@ -261,9 +268,9 @@ export class BASICParser {
     }
     expectToken(str: string) : Token {
         var tok = this.consumeToken();
-        var tokstr = tok.str.toUpperCase();
+        var tokstr = tok.str;
         if (str != tokstr) {
-            this.compileError(`I expected "${str}" here, but I saw "${tokstr}".`);
+            this.compileError(`There should be a "${str}" here.`);
         }
         return tok;
     }
@@ -278,7 +285,7 @@ export class BASICParser {
         let tok = this.consumeToken();
         switch (tok.type) {
             case TokenType.Int:
-                if (this.labels[tok.str] != null) this.compileError(`I saw a duplicated label "${tok.str}".`);
+                if (this.labels[tok.str] != null) this.compileError(`There's a duplicated label "${tok.str}".`);
                 this.labels[tok.str] = line;
                 line.label = tok.str;
                 this.curlabel = tok.str;
@@ -291,8 +298,8 @@ export class BASICParser {
     }
     parseFile(file: string, path: string) : BASICProgram {
         var pgmlines = file.split("\n").map((line) => this.parseLine(line));
-        this.checkLabels();
-        var program = { lines: pgmlines };
+        var program = { opts: this.opts, lines: pgmlines };
+        this.checkAll(program);
         this.listings[path] = this.generateListing(file, program);
         return program;
     }
@@ -308,11 +315,15 @@ export class BASICParser {
     tokenize(line: string) : void {
         this.lineno++;
         this.tokens = [];
-        var m;
+        var m : RegExpMatchArray;
         while (m = re_toks.exec(line)) {
             for (var i = 1; i < TokenType._LAST; i++) {
-                let s = m[i];
+                let s : string = m[i];
                 if (s != null) {
+                    // uppercase all identifiers, and maybe more
+                    if (i == TokenType.Ident || this.opts.uppercaseOnly)
+                        s = s.toUpperCase();
+                    // add token to list
                     this.tokens.push({
                         str: s,
                         type: i,
@@ -341,23 +352,27 @@ export class BASICParser {
     }
     parseStatement(): Statement | null {
         var cmdtok = this.consumeToken();
+        var cmd = cmdtok.str;
         var stmt;
         switch (cmdtok.type) {
             case TokenType.Remark:
                 if (!this.opts.tickComments) this.dialectError(`tick remarks`);
                 return null;
             case TokenType.Ident:
-                var cmd = cmdtok.str.toUpperCase();
-                // remark? ignore to eol
+                // remark? ignore all tokens to eol
                 if (cmd == 'REM') {
                     while (this.consumeToken().type != TokenType.EOL) { }
                     return null;
                 }
-                // look for "GO TO"
+                // look for "GO TO" and "GO SUB"
                 if (cmd == 'GO' && this.peekToken().str == 'TO') {
                     this.consumeToken();
                     cmd = 'GOTO';
+                } else if (cmd == 'GO' && this.peekToken().str == 'SUB') {
+                    this.consumeToken();
+                    cmd = 'GOSUB';
                 }
+                // lookup JS function for command
                 var fn = this['stmt__' + cmd];
                 if (fn) {
                     if (this.opts.validKeywords && this.opts.validKeywords.indexOf(cmd) < 0)
@@ -371,10 +386,8 @@ export class BASICParser {
                     break;
                 }
             case TokenType.EOL:
-                this.compileError(`I expected a command here`);
-                return null;
             default:
-                this.compileError(`Unknown command "${cmdtok.str}"`);
+                this.compileError(`There should be a command here.`);
                 return null;
         }
         if (stmt) stmt.$loc = { line: cmdtok.$loc.line, start: cmdtok.$loc.start, end: this.peekToken().$loc.start };
@@ -387,6 +400,7 @@ export class BASICParser {
         var tok = this.consumeToken();
         switch (tok.type) {
             case TokenType.Ident:
+                this.refs[tok.str] = tok.$loc;
                 let args = null;
                 if (this.peekToken().str == '(') {
                     this.expectToken('(');
@@ -410,8 +424,10 @@ export class BASICParser {
         this.pushbackToken(sep);
         return list;
     }
-    parseVarOrIndexedList(): IndOp[] {
-        return this.parseList(this.parseVarOrIndexed, ',');
+    parseLexprList(): IndOp[] {
+        var list = this.parseList(this.parseVarOrIndexed, ',');
+        list.forEach((lexpr) => this.decls[lexpr.name] = this.lasttoken.$loc);
+        return list;
     }
     parseExprList(): Expr[] {
         return this.parseList(this.parseExpr, ',');
@@ -427,7 +443,7 @@ export class BASICParser {
                 this.targets[label] = tok.$loc;
                 return {value:label};
             default:
-                this.compileError(`I expected a line number here`);
+                this.compileError(`There should be a line number here.`);
                 return;
         }
     }
@@ -437,12 +453,17 @@ export class BASICParser {
             case TokenType.Int:
             case TokenType.Float1:
             case TokenType.Float2:
-                return { value: parseFloat(tok.str), $loc: tok.$loc };
+                return { value: this.parseNumber(tok.str), $loc: tok.$loc };
             case TokenType.String:
                 return { value: stripQuotes(tok.str), $loc: tok.$loc };
             case TokenType.Ident:
-                this.pushbackToken(tok);
-                return this.parseVarOrIndexedOrFunc();
+                if (tok.str == 'NOT') {
+                    let expr = this.parsePrimary();
+                    return { op: 'lnot', expr: expr };
+                } else {
+                    this.pushbackToken(tok);
+                    return this.parseVarOrIndexedOrFunc();
+                }
             case TokenType.Operator:
                 if (tok.str == '(') {
                     let expr = this.parseExpr();
@@ -454,21 +475,33 @@ export class BASICParser {
                 } else if (tok.str == '+') {
                     return this.parsePrimary(); // TODO?
                 }
-            default:
-                this.compileError(`Unexpected "${tok.str}"`);
+            case TokenType.EOL:
+                this.compileError(`The expression is incomplete.`);
+                return;
         }
+        this.compileError(`There was an unexpected "${tok.str}" in this expression.`);
+    }
+    parseNumber(str: string) : number {
+        var n = parseFloat(str);
+        if (isNaN(n))
+            this.compileError(`The number ${str} is not a valid floating-point number.`);
+        if (this.opts.checkOverflow && !isFinite(n))
+            this.compileError(`The number ${str} is too big to fit into a floating-point value.`);
+        return n;
     }
     parseExpr1(left: Expr, minPred: number): Expr {
         let look = this.peekToken();
         while (getPrecedence(look) >= minPred) {
             let op = this.consumeToken();
+            if (this.opts.validOperators && this.opts.validOperators.indexOf(op.str) < 0)
+                this.dialectError(`the "${op.str}" operator`);
             let right: Expr = this.parsePrimary();
             look = this.peekToken();
             while (getPrecedence(look) > getPrecedence(op)) {
                 right = this.parseExpr1(right, getPrecedence(look));
                 look = this.peekToken();
             }
-            left = { op: getOpcodeForOperator(op.str), left: left, right: right };
+            left = { op: getOperator(op.str).f, left: left, right: right };
         }
         return left;
     }
@@ -481,6 +514,7 @@ export class BASICParser {
     stmt__LET(): LET_Statement {
         var lexpr = this.parseVarOrIndexed();
         this.expectToken("=");
+        this.decls[lexpr.name] = this.lasttoken.$loc;
         var right = this.parseExpr();
         return { command: "LET", lexpr: lexpr, right: right };
     }
@@ -547,7 +581,7 @@ export class BASICParser {
         return { command:'NEXT', lexpr:lexpr };
     }
     stmt__DIM() : DIM_Statement {
-        return { command:'DIM', args:this.parseVarOrIndexedList() };
+        return { command:'DIM', args:this.parseLexprList() };
     }
     stmt__INPUT() : INPUT_Statement {
         var prompt = this.consumeToken();
@@ -559,13 +593,13 @@ export class BASICParser {
             this.pushbackToken(prompt);
             promptstr = "";
         }
-        return { command:'INPUT', prompt:{ value: promptstr, $loc: prompt.$loc }, args:this.parseVarOrIndexedList() };
+        return { command:'INPUT', prompt:{ value: promptstr, $loc: prompt.$loc }, args:this.parseLexprList() };
     }
     stmt__DATA() : DATA_Statement {
         return { command:'DATA', datums:this.parseExprList() };
     }
     stmt__READ() {
-        return { command:'READ', args:this.parseVarOrIndexedList() };
+        return { command:'READ', args:this.parseLexprList() };
     }
     stmt__RESTORE() {
         return { command:'RESTORE' };
@@ -587,22 +621,43 @@ export class BASICParser {
     }
     stmt__DEF() : DEF_Statement {
         var lexpr = this.parseVarOrIndexed();
-        if (!lexpr.name.toUpperCase().startsWith('FN')) this.compileError(`Functions defined with DEF must begin with the letters "FN".`)
+        if (!lexpr.name.startsWith('FN')) this.compileError(`Functions defined with DEF must begin with the letters "FN".`)
         this.expectToken("=");
+        this.decls[lexpr.name] = this.lasttoken.$loc;
         var func = this.parseExpr();
         return { command:'DEF', lexpr:lexpr, def:func };
     }
     stmt__OPTION() : OPTION_Statement {
         var tokname = this.consumeToken();
-        if (tokname.type != TokenType.Ident) this.compileError(`I expected a name after the OPTION statement.`)
+        if (tokname.type != TokenType.Ident) this.compileError(`There should be a name after the OPTION statement.`)
         var list : string[] = [];
         var tok;
         do {
             tok = this.consumeToken();
             if (isEOS(tok)) break;
-            list.push(tok.str.toUpperCase());
+            list.push(tok.str);
         } while (true);
-        return { command:'OPTION', optname:tokname.str.toUpperCase(), optargs:list };
+        var stmt : OPTION_Statement = { command:'OPTION', optname:tokname.str, optargs:list };
+        this.parseOptions(stmt);
+        return stmt;
+    }
+    parseOptions(stmt: OPTION_Statement) {
+        switch (stmt.optname) {
+            case 'BASE': 
+                let base = parseInt(stmt.optargs[0]);
+                if (base == 0 || base == 1) this.opts.defaultArrayBase = base;
+                else this.compileError("OPTION BASE can only be 0 or 1.");
+                break;
+            case 'DIALECT':
+                let dname = stmt.optargs[0] || "";
+                let dialect = DIALECTS[dname];
+                if (dialect) this.opts = dialect;
+                else this.compileError(`The dialect named "${dname}" is not supported by this compiler.`);
+                break;
+            default:
+                this.compileError(`OPTION ${stmt.optname} is not supported by this compiler.`);
+                break;
+        }
     }
     
     // for workermain
@@ -620,13 +675,23 @@ export class BASICParser {
     }
 
     // LINT STUFF
+    checkAll(program : BASICProgram) {
+        this.checkLabels();
+        //this.checkUnsetVars();
+    }
     checkLabels() {
         for (let targ in this.targets) {
             if (this.labels[targ] == null) {
-                this.compileError(`I couldn't find line number ${targ}`, this.targets[targ]);
+                this.compileError(`There isn't a line number ${targ}.`, this.targets[targ]);
             }
         }
     }
+    checkUnsetVars() {
+        for (var ref in this.refs) {
+            if (this.decls[ref] == null)
+                this.compileError(`The variable "${ref}" was used but not set with a LET, DIM, READ, or INPUT statement.`);
+        }
+    }
 }
 
 ///// BASIC DIALECTS
@@ -643,18 +708,19 @@ export const ECMA55_MINIMAL : BASICOptions = {
     stringConcat : false,
     typeConvert : false,
     maxDimensions : 2,
-    maxArguments : Infinity,
+    maxArguments : 255,
     sparseArrays : false,
     tickComments : false,
     validKeywords : ['BASE','DATA','DEF','DIM','END',
         'FOR','GO','GOSUB','GOTO','IF','INPUT','LET','NEXT','ON','OPTION','PRINT',
         'RANDOMIZE','READ','REM','RESTORE','RETURN','STEP','STOP','SUB','THEN','TO'
     ],
-    validFunctions : ['ABS','ATN','COS','EXP','INT','LOG','RND','SGN','SIN','SQR','TAN'],
+    validFunctions : ['ABS','ATN','COS','EXP','INT','LOG','RND','SGN','SIN','SQR','TAB','TAN'],
     validOperators : ['=', '<>', '<', '>', '<=', '>=', '+', '-', '*', '/', '^'],
     printZoneLength : 15,
     printPrecision : 6,
     checkOverflow : true,
+    multipleNextVars : false,
 }
 
 export const ALTAIR_BASIC40 : BASICOptions = {
@@ -667,7 +733,7 @@ export const ALTAIR_BASIC40 : BASICOptions = {
     stringConcat : false,
     typeConvert : false,
     maxDimensions : 2,
-    maxArguments : Infinity,
+    maxArguments : 255,
     sparseArrays : false,
     tickComments : false,
     validKeywords : null, // all
@@ -676,4 +742,13 @@ export const ALTAIR_BASIC40 : BASICOptions = {
     printZoneLength : 15,
     printPrecision : 6,
     checkOverflow : true,
+    multipleNextVars : true,
 }
+
+export const DIALECTS = {
+    "DEFAULT":      ALTAIR_BASIC40,
+    "ALTAIR":       ALTAIR_BASIC40,
+    "ALTAIR40":     ALTAIR_BASIC40,
+    "ECMA55":       ECMA55_MINIMAL,
+    "MINIMAL":      ECMA55_MINIMAL,
+};
diff --git a/src/common/basic/run.ts b/src/common/basic/run.ts
index 6515c6a2..5e0267f4 100644
--- a/src/common/basic/run.ts
+++ b/src/common/basic/run.ts
@@ -17,9 +17,13 @@ var data = fs.readFileSync(filename, 'utf-8');
 try {
     var pgm = parser.parseFile(data, filename);
 } catch (e) {
-    console.log("@@@ " + e.message);
-    throw e;
+    if (parser.errors.length == 0)
+        console.log("@@@ " + e.msg);
+    else
+        console.log(e);
 }
+parser.errors.forEach((err) => console.log("@@@ " + err.msg));
+if (parser.errors.length) process.exit(2);
 
 var runtime = new BASICRuntime();
 runtime.trace = process.argv[3] == '-v';
@@ -47,7 +51,7 @@ runtime.resume = function() {
             }
         } catch (e) {
             console.log("### " + e.message);
-            throw e;
+            process.exit(1);
         }
     });
 }
diff --git a/src/common/basic/runtime.ts b/src/common/basic/runtime.ts
index a6337629..ff5684c4 100644
--- a/src/common/basic/runtime.ts
+++ b/src/common/basic/runtime.ts
@@ -39,16 +39,17 @@ export class BASICRuntime {
     dataptr : number;
     vars : {};
     arrays : {};
-    forLoops : {};
+    defs : {};
+    forLoops : {next:(name:string) => void}[];
     returnStack : number[];
     column : number;
-    abase : number; // array base
 
     running : boolean = false;
-    exited : boolean = false;
+    exited : boolean = true;
     trace : boolean = false;
 
     load(program: basic.BASICProgram) {
+        let prevlabel = this.label2pc && this.getLabelForPC(this.curpc);
         this.program = program;
         this.label2lineidx = {};
         this.label2pc = {};
@@ -59,6 +60,7 @@ export class BASICRuntime {
         // TODO: lines start @ 1?
         program.lines.forEach((line, idx) => {
             // make lookup tables
+            this.curpc = this.allstmts.length + 1; // set for error reporting
             if (line.label != null) this.label2lineidx[line.label] = idx;
             if (line.label != null) this.label2pc[line.label] = this.allstmts.length;
             this.line2pc.push(this.allstmts.length);
@@ -69,8 +71,6 @@ export class BASICRuntime {
             line.stmts.filter((stmt) => stmt.command == 'DATA').forEach((datastmt) => {
                 (datastmt as basic.DATA_Statement).datums.forEach(d => {
                     var functext = this.expr2js(d, {check:true, isconst:true});
-                    // TODO: catch exceptions
-                    // TODO: any value doing this ahead of time?
                     var value = new Function(`return ${functext};`).bind(this)();
                     this.datums.push({value:value});
                 });
@@ -78,6 +78,9 @@ export class BASICRuntime {
             // TODO: compile statements?
             //line.stmts.forEach((stmt) => this.compileStatement(stmt));
         });
+        // try to resume where we left off after loading
+        this.curpc = this.label2pc[prevlabel] || 0;
+        this.dataptr = Math.min(this.dataptr, this.datums.length);
     }
 
     reset() {
@@ -85,21 +88,29 @@ export class BASICRuntime {
         this.dataptr = 0;
         this.vars = {};
         this.arrays = {};
-        this.forLoops = {};
+        this.defs = this.getBuiltinFunctions();
+        this.forLoops = [];
         this.returnStack = [];
         this.column = 0;
-        this.abase = 1;
         this.running = true;
         this.exited = false;
     }
 
+    getBuiltinFunctions() {
+        var fnames = this.program && this.program.opts.validFunctions;
+        // if no valid function list, look for ABC...() functions in prototype
+        if (!fnames) fnames = Object.keys(BASICRuntime.prototype).filter((name) => /^[A-Z]{3,}[$]?$/.test(name));
+        var dict = {};
+        for (var fn of fnames) dict[fn] = this[fn].bind(this);
+        return dict;
+    }
+
     runtimeError(msg : string) {
         this.curpc--; // we did curpc++ before executing statement
         // TODO: pass source location to error
         throw new EmuHalt(`${msg} (line ${this.getLabelForPC(this.curpc)})`);
     }
 
-    // TODO: sometimes on next line
     getLineForPC(pc:number) {
         var line;
         do {
@@ -131,7 +142,7 @@ export class BASICRuntime {
         if (this.trace) console.log(this.curpc, stmt, this.vars, Object.keys(this.arrays));
         // skip to next statment
         this.curpc++;
-        // compile statement to JS?
+        // compile (unless cached) and execute statement
         this.compileStatement(stmt);
         this.executeStatement(stmt);
         return this.running;
@@ -171,7 +182,7 @@ export class BASICRuntime {
     }
 
     gosubLabel(label) {
-        this.returnStack.push(this.curpc + 1);
+        this.returnStack.push(this.curpc);
         this.gotoLabel(label);
     }
 
@@ -240,9 +251,9 @@ export class BASICRuntime {
             } else {
                 if (opts.isconst) this.runtimeError(`I expected a constant value here`);
                 var s = '';
-                if (expr.args && this[expr.name]) { // is it a function?
-                    s += `this.${expr.name}(`;
-                    s += expr.args.map((arg) => this.expr2js(arg, opts)).join(', ');
+                if (this.defs[expr.name]) { // is it a function?
+                    s += `this.defs.${expr.name}(`;
+                    if (expr.args) s += expr.args.map((arg) => this.expr2js(arg, opts)).join(', ');
                     s += ')';
                 } else if (expr.args) { // is it a subscript?
                     s += `this.getArray(${JSON.stringify(expr.name)}, ${expr.args.length})`;
@@ -252,7 +263,7 @@ export class BASICRuntime {
                     s = `this.vars.${expr.name}`;
                 }
                 if (opts.check)
-                    return `this.checkValue(${s}, ${JSON.stringify(expr.name)})`; // TODO: better error
+                    return `this.checkValue(${s}, ${JSON.stringify(expr.name)})`;
                 else
                     return s;
             }
@@ -260,64 +271,85 @@ export class BASICRuntime {
             var left = this.expr2js(expr.left, opts);
             var right = this.expr2js(expr.right, opts);
             return `this.${expr.op}(${left}, ${right})`;
-        } else if (isUnOp(expr) && expr.op == 'neg') {
+        } else if (isUnOp(expr)) {
             var e = this.expr2js(expr.expr, opts);
-            return `-(${e})`; // TODO: other ops?
+            return `this.${expr.op}(${e})`;
         }
     }
 
-    startForLoop(name, init, targ, step) {
-        // TODO: check for loop params
+    startForLoop(forname, init, targ, step) {
+        // TODO: support 0-iteration loops
         var pc = this.curpc;
         if (!step) step = 1;
-        this.vars[name] = init;
-        if (this.trace) console.log(`FOR ${name} = ${this.vars[name]} TO ${targ} STEP ${step}`);
-        this.forLoops[name] = {
-            next: () => {
-                var done = step >= 0 ? this.vars[name] >= targ : this.vars[name] <= targ;
+        this.vars[forname] = init;
+        if (this.trace) console.log(`FOR ${forname} = ${init} TO ${targ} STEP ${step}`);
+        this.forLoops.unshift({
+            next: (nextname:string) => {
+                if (nextname && forname != nextname)
+                    this.runtimeError(`I executed NEXT "${nextname}", but the last FOR was for "${forname}".`)
+                this.vars[forname] += step;
+                var done = step >= 0 ? this.vars[forname] > targ : this.vars[forname] < targ;
                 if (done) {
-                    delete this.forLoops[name];
+                    this.forLoops.shift(); // pop FOR off the stack and continue
                 } else {
-                    this.vars[name] += step;
-                    this.curpc = pc;
+                    this.curpc = pc; // go back to FOR location
                 }
-                if (this.trace) console.log(`NEXT ${name}: ${this.vars[name]} TO ${targ} STEP ${step} DONE=${done}`);
+                if (this.trace) console.log(`NEXT ${forname}: ${this.vars[forname]} TO ${targ} STEP ${step} DONE=${done}`);
             }
-        };
+        });
     }
 
     nextForLoop(name) {
-        // TODO: check for for loop
-        var fl = this.forLoops[name];
-        if (!fl) this.runtimeError(`I couldn't find a matching FOR for this NEXT.`)
-        this.forLoops[name].next();
+        var fl = this.forLoops[0];
+        if (!fl) this.runtimeError(`I couldn't find a FOR for this NEXT.`)
+        else fl.next(name);
     }
 
     // converts a variable to string/number based on var name
+    assign(name: string, right: number|string) : number|string {
+        if (this.program.opts.typeConvert)
+            return this.convert(name, right);
+        // TODO: use options
+        if (name.endsWith("$")) {
+            return this.convertToString(right, name);
+        } else {
+            return this.convertToNumber(right, name);
+        }
+    }
+
     convert(name: string, right: number|string) : number|string {
-        // TODO: error check?
-        if (name.endsWith("$"))
+        if (name.endsWith("$")) {
             return right+"";
-        else if (typeof right === 'string')
-            return parseFloat(right);
-        else if (typeof right === 'number')
+        } else if (typeof right === 'number') {
             return right;
-        else
-            return this.checkValue(right, name);
+        } else {
+            return parseFloat(right+"");
+        }
+    }
+
+    convertToString(right: number|string, name?: string) {
+        if (typeof right !== 'string') this.runtimeError(`I can't convert ${right} to a string.`);
+        else return right;
+    }
+
+    convertToNumber(right: number|string, name?: string) {
+        if (typeof right !== 'number') this.runtimeError(`I can't convert ${right} to a number.`);
+        else return this.checkNum(right);
     }
 
     // dimension array
     dimArray(name: string, ...dims:number[]) {
+        if (this.arrays[name]) this.runtimeError(`I already dimensioned this array (${name}) earlier.`)
         var isstring = name.endsWith('$');
-        // TODO: option for undefined float array elements?
-        var arrcons = isstring ? Array : Float64Array;
-        var ab = this.abase;
+        // if defaultValues is true, we use Float64Array which inits to 0
+        var arrcons = isstring || !this.program.opts.defaultValues ? Array : Float64Array;
+        // TODO? var ab = this.program.opts.defaultArrayBase;
         if (dims.length == 1) {
-            this.arrays[name] = new arrcons(dims[0]+ab);
+            this.arrays[name] = new arrcons(dims[0]+1);
         } else if (dims.length == 2) {
-            this.arrays[name] = new Array(dims[0]+ab);
-            for (var i=ab; i<dims[0]+ab; i++)
-                this.arrays[name][i] = new arrcons(dims[1]+ab);
+            this.arrays[name] = new Array(dims[0]+1);
+            for (var i=0; i<dims[0]+1; i++)
+                this.arrays[name][i] = new arrcons(dims[1]+1);
         } else {
             this.runtimeError(`I only support arrays of one or two dimensions.`)
         }
@@ -364,15 +396,22 @@ export class BASICRuntime {
         var setvals = '';
         stmt.args.forEach((arg, index) => {
             var lexpr = this.expr2js(arg, {check:false});
-            setvals += `${lexpr} = this.convert(${JSON.stringify(arg.name)}, vals[${index}]);`
+            setvals += `valid &= this.isValid(${lexpr} = this.convert(${JSON.stringify(arg.name)}, vals[${index}]));`
         });
-        return `this.running=false; this.input(${prompt}, ${stmt.args.length}).then((vals) => {${setvals}; this.running=true; this.resume();})`;
+        return `this.running=false;
+                this.input(${prompt}, ${stmt.args.length}).then((vals) => {
+                    let valid = 1;
+                    ${setvals}
+                    if (!valid) this.curpc--;
+                    this.running=true;
+                    this.resume();
+                })`;
     }
 
     do__LET(stmt : basic.LET_Statement) {
         var lexpr = this.expr2js(stmt.lexpr, {check:false});
         var right = this.expr2js(stmt.right, {check:true});
-        return `${lexpr} = this.convert(${JSON.stringify(stmt.lexpr.name)}, ${right});`;
+        return `${lexpr} = this.assign(${JSON.stringify(stmt.lexpr.name)}, ${right});`;
     }
 
     do__FOR(stmt : basic.FOR_Statement) {
@@ -384,7 +423,7 @@ export class BASICRuntime {
     }
 
     do__NEXT(stmt : basic.NEXT_Statement) {
-        var name = JSON.stringify(stmt.lexpr.name); // TODO: args? lexpr == null?
+        var name = stmt.lexpr && JSON.stringify(stmt.lexpr.name); // TODO: args? lexpr == null?
         return `this.nextForLoop(${name})`;
     }
 
@@ -394,9 +433,9 @@ export class BASICRuntime {
     }
 
     do__DEF(stmt : basic.DEF_Statement) {
-        var lexpr = `this.${stmt.lexpr.name}`;
+        var lexpr = `this.defs.${stmt.lexpr.name}`;
         var args = [];
-        for (var arg of stmt.lexpr.args) {
+        for (var arg of stmt.lexpr.args || []) {
             if (isLookup(arg)) {
                 args.push(arg.name);
             } else {
@@ -404,14 +443,13 @@ export class BASICRuntime {
             }
         }
         var functext = this.expr2js(stmt.def, {check:true, locals:args});
-        // TODO: use stmt.args to add function params
-        return `${lexpr} = function(${args.join(',')}) { return ${functext}; }`;
+        return `${lexpr} = function(${args.join(',')}) { return ${functext}; }.bind(this)`;
     }
 
     _DIM(dim : basic.IndOp) {
         var argsstr = '';
         for (var arg of dim.args) {
-            // TODO: check for float
+            // TODO: check for float (or at compile time)
             argsstr += ', ' + this.expr2js(arg, {check:true});
         }
         return `this.dimArray(${JSON.stringify(dim.name)}${argsstr});`;
@@ -450,13 +488,13 @@ export class BASICRuntime {
     do__READ(stmt : basic.READ_Statement) {
         var s = '';
         stmt.args.forEach((arg) => {
-            s += `${this.expr2js(arg, {check:false})} = this.convert(${JSON.stringify(arg.name)}, this.nextDatum());`;
+            s += `${this.expr2js(arg, {check:false})} = this.assign(${JSON.stringify(arg.name)}, this.nextDatum());`;
         });
         return s;
     }
 
     do__RESTORE() {
-        this.dataptr = 0; // TODO: line number?
+        this.dataptr = 0;
     }
 
     do__END() {
@@ -468,26 +506,30 @@ export class BASICRuntime {
     }
 
     do__OPTION(stmt: basic.OPTION_Statement) {
-        switch (stmt.optname) {
-            case 'BASE': 
-                let base = parseInt(stmt.optargs[0]);
-                if (base == 0 || base == 1) this.abase = base;
-                else this.runtimeError("OPTION BASE can only be 0 or 1.");
-                break;
-            default:
-                this.runtimeError(`OPTION ${stmt.optname} is not supported by this compiler.`);
-                break;
-        }
+        // already parsed in compiler
     }
 
-    // TODO: "SUBSCRIPT ERROR"
+    // TODO: "SUBSCRIPT ERROR" (range check)
     // TODO: gosubs nested too deeply
     // TODO: memory quota
 
     // FUNCTIONS
 
-    checkValue(obj:number|string, exprname:string) {
+    isValid(obj:number|string) : boolean {
+        if (typeof obj === 'number' && !isNaN(obj))
+            return true;
+        else if (typeof obj === 'string')
+            return true;
+        else
+            return false;
+    }
+    checkValue(obj:number|string, exprname:string) : number|string {
+        // check for unreferenced value
         if (typeof obj !== 'number' && typeof obj !== 'string') {
+            // assign default value?
+            if (obj == null && this.program.opts.defaultValues) {
+                return exprname.endsWith("$") ? "" : 0;
+            }
             if (exprname != null && obj == null) {
                 this.runtimeError(`I didn't find a value for ${exprname}`);
             } else if (exprname != null) {
@@ -527,6 +569,12 @@ export class BASICRuntime {
         if (b == 0) this.runtimeError(`I can't divide by zero.`);
         return this.checkNum(a / b);
     }
+    idiv(a:number, b:number) : number {
+        return this.div(Math.floor(a), Math.floor(b));
+    }
+    mod(a:number, b:number) : number {
+        return this.checkNum(a % b);
+    }
     pow(a:number, b:number) : number {
         if (a == 0 && b < 0) this.runtimeError(`I can't raise zero to a negative power.`);
         return this.checkNum(Math.pow(a, b));
@@ -537,6 +585,18 @@ export class BASICRuntime {
     lor(a:number, b:number) : number {
         return a || b;
     }
+    lnot(a:number) : number {
+        return a ? 0 : 1;
+    }
+    neg(a:number) : number {
+        return -a;
+    }
+    band(a:number, b:number) : number {
+        return a & b;
+    }
+    bor(a:number, b:number) : number {
+        return a | b;
+    }
     eq(a:number, b:number) : boolean {
         return a == b;
     }
@@ -604,7 +664,7 @@ export class BASICRuntime {
         return this.checkNum(Math.log(arg));
     }
     MID$(arg : string, start : number, count : number) : string {
-        if (start < 1) this.runtimeError(`The second parameter to MID$ must be between 1 and the length of the string in the first parameter.`)
+        if (start < 1) this.runtimeError(`I tried to compute MID$ but the second parameter is less than zero (${start}).`)
         return arg.substr(start-1, count);
     }
     RIGHT$(arg : string, count : number) : string {
@@ -614,7 +674,7 @@ export class BASICRuntime {
         return Math.random(); // argument ignored
     }
     ROUND(arg : number) : number {
-        return this.checkNum(Math.round(arg)); // TODO?
+        return this.checkNum(Math.round(arg));
     }
     SGN(arg : number) : number {
         return (arg < 0) ? -1 : (arg > 0) ? 1 : 0;
@@ -623,24 +683,25 @@ export class BASICRuntime {
         return this.checkNum(Math.sin(arg));
     }
     SPACE$(arg : number) : string {
-        return ' '.repeat(this.checkNum(arg));
+        return (arg > 0) ? ' '.repeat(this.checkNum(arg)) : '';
     }
     SQR(arg : number) : number {
         if (arg < 0) this.runtimeError(`I can't take the square root of a negative number (${arg}).`)
         return this.checkNum(Math.sqrt(arg));
     }
-    STR$(arg) : string {
-        return this.valueToString(arg);
+    STR$(arg : number) : string {
+        return this.valueToString(this.checkNum(arg));
     }
     TAB(arg : number) : string {
-        if (arg < 0) this.runtimeError(`I got a negative value for the TAB() function.`);
-        var spaces = this.ROUND(arg) - this.column;
+        if (arg < 1) { arg = 1; } // TODO: SYSTEM MESSAGE IDENTIFYING THE EXCEPTION
+        var spaces = this.ROUND(arg) - 1 - this.column;
         return (spaces > 0) ? ' '.repeat(spaces) : '';
     }
     TAN(arg : number) : number {
         return this.checkNum(Math.tan(arg));
     }
-    VAL(arg) : number {
-        return parseFloat(arg+"");
+    VAL(arg : string) : number {
+        var n = parseFloat(this.checkString(arg));
+        return isNaN(n) ? 0 : n; // TODO? altair works this way
     }
 }
diff --git a/src/common/emu.ts b/src/common/emu.ts
index ed05ca06..6869f0c1 100644
--- a/src/common/emu.ts
+++ b/src/common/emu.ts
@@ -225,6 +225,10 @@ export class RAM {
 }
 
 export class EmuHalt extends Error {
+  constructor(msg:string) {
+    super(msg);
+    Object.setPrototypeOf(this, EmuHalt.prototype);
+  }
 }
 
 export class AnimationTimer {
diff --git a/src/ide/views.ts b/src/ide/views.ts
index 924e3f6d..88d146ca 100644
--- a/src/ide/views.ts
+++ b/src/ide/views.ts
@@ -98,7 +98,7 @@ export class SourceEditor implements ProjectView {
     this.newEditor(div, asmOverride);
     if (text) {
       this.setText(text); // TODO: this calls setCode() and builds... it shouldn't
-      this.editor.setSelection({line:0,ch:0}, {line:0,ch:0}, {scroll:true});
+      this.editor.setSelection({line:0,ch:0}, {line:0,ch:0}, {scroll:true}); // move cursor to start
     }
     this.setupEditor();
     return div;
diff --git a/src/platform/basic.ts b/src/platform/basic.ts
index 8b024154..da609da6 100644
--- a/src/platform/basic.ts
+++ b/src/platform/basic.ts
@@ -1,6 +1,6 @@
 
 import { Platform, BreakpointCallback } from "../common/baseplatform";
-import { PLATFORMS, AnimationTimer } from "../common/emu";
+import { PLATFORMS, AnimationTimer, EmuHalt } from "../common/emu";
 import { loadScript } from "../ide/ui";
 import { BASICRuntime } from "../common/basic/runtime";
 import { BASICProgram } from "../common/basic/compiler";
@@ -278,7 +278,7 @@ class BASICPlatform implements Platform {
         //var printhead = $('<div id="printhead" class="transcript-print-head"/>').appendTo(parent);
         this.tty = new TeleTypeWithKeyboard(windowport[0], true, inputline[0] as HTMLInputElement, this);
         this.tty.scrolldiv = parent;
-        this.timer = new AnimationTimer(60, this.advance1_60.bind(this));
+        this.timer = new AnimationTimer(60, this.animate.bind(this));
         this.resize = () => {
             // set font size proportional to window width
             var charwidth = $(gameport).width() * 1.6 / 80;
@@ -286,11 +286,15 @@ class BASICPlatform implements Platform {
             this.tty.scrollToBottom();
         }
         this.resize();
-        this.runtime.print = this.tty.print.bind(this.tty);
+        this.runtime.print = (s:string) => {
+            // TODO: why null sometimes?
+            this.clock = 0; // exit advance loop when printing
+            this.tty.print(s);
+        }
         this.runtime.resume = this.resume.bind(this);
     }
 
-    advance1_60() {
+    animate() {
         if (this.tty.isBusy()) return;
         this.clock += this.ips/60;
         while (!this.runtime.exited && this.clock-- > 0) {
@@ -298,33 +302,36 @@ class BASICPlatform implements Platform {
         }
     }
 
+    // should not depend on tty state
     advance(novideo?: boolean) : number {
         if (this.runtime.running) {
-            try {
-                var more = this.runtime.step();
-                if (!more) {
-                    this.pause();
-                    if (this.runtime.exited) {
-                        this.tty.print("\n\n");
-                        this.tty.addtext("*** END OF PROGRAM ***", 1);
-                    }
+            var more = this.runtime.step();
+            if (!more) {
+                this.pause();
+                if (this.runtime.exited) {
+                    this.exitmsg();
                 }
-            } catch (e) {
-                this.break();
-                throw e;
             }
+            // TODO: break() when EmuHalt at location?
             return 1;
         } else {
             return 0;
         }
     }
+    
+    exitmsg() {
+        this.tty.print("\n\n");
+        this.tty.addtext("*** END OF PROGRAM ***", 1);
+    }
 
     resize: () => void;
 
     loadROM(title, data) {
-        this.reset();
+        var didExit = this.runtime.exited;
         this.program = data;
         this.runtime.load(data);
+        // only reset if we exited, otherwise we try to resume
+        if (didExit) this.reset();
     }
 
     getROMExtension() {
@@ -403,8 +410,9 @@ class BASICPlatform implements Platform {
         this.break();
     }
     break() {
+        // TODO: don't highlight text in editor
         if (this.onBreakpointHit) {
-            this.onBreakpointHit(this.saveState());
+            //TODO: this.onBreakpointHit(this.saveState());
         }
     }
 }