diff --git a/assembler.go b/assembler.go
index d800500..7efce84 100644
--- a/assembler.go
+++ b/assembler.go
@@ -48,7 +48,7 @@ func isWordHead(c byte) bool {
 	if c >= 'a' && c <= 'z' {
 		c -= 32
 	}
-	return c >= 'a' && c <= 'z' || c == '_'
+	return c >= 'A' && c <= 'Z' || c == '_'
 }
 
 func isWordTail(c byte) bool {
@@ -63,9 +63,12 @@ func (t *tokenizer) send(start location, kind int) {
 }
 
 // XXX: the handling could probably be simplified by extending the "byte"
-//   to also include a special value for io.EOF and other errors
+//   to also include a special out-of-band value for errors
 func (t *tokenizer) peek() (byte, error) {
 	buf, err := t.reader.Peek(1)
+	if err != nil {
+		return '?', err
+	}
 	return buf[0], err
 }
 
@@ -87,17 +90,14 @@ func (t *tokenizer) eat() (byte, error) {
 // -----------------------------------------------------------------------------
 
 func (t *tokenizer) step() error {
+	start := t.location
 	t.value = []byte{}
-	c, err := t.peek()
 
-	if err == io.EOF {
-		return nil
-	}
+	c, err := t.peek()
 	if err != nil {
 		return err
 	}
 
-	start := t.location
 	switch {
 	case isSpace(c):
 		c, err = t.eat()
@@ -107,10 +107,10 @@ func (t *tokenizer) step() error {
 
 		t.send(start, NEWLINE)
 	case isNumber(c):
-		c, err = t.eat()
-		t.value = append(t.value, c)
+		for isNumber(c) {
+			c, err = t.eat()
+			t.value = append(t.value, c)
 
-		for {
 			c, err = t.peek()
 			if err == io.EOF {
 				break
@@ -118,20 +118,13 @@ func (t *tokenizer) step() error {
 			if err != nil {
 				return err
 			}
-
-			if !isNumber(c) {
-				break
-			}
-
-			c, err = t.eat()
-			t.value = append(t.value, c)
 		}
 		t.send(start, NUMBER)
 	case isWordHead(c):
-		c, err = t.eat()
-		t.value = append(t.value, c)
+		for isWordTail(c) {
+			c, err = t.eat()
+			t.value = append(t.value, c)
 
-		for {
 			c, err = t.peek()
 			if err == io.EOF {
 				break
@@ -139,28 +132,23 @@ func (t *tokenizer) step() error {
 			if err != nil {
 				return err
 			}
-
-			if !isWordTail(c) {
-				break
-			}
-
-			c, err = t.eat()
-			t.value = append(t.value, c)
 		}
 		t.send(start, WORD)
 	case c == '/':
 		c, err = t.eat()
-		t.value = append(t.value, c)
-
 		c, err = t.peek()
+		if err == io.EOF {
+			return errors.New("unexpected EOF")
+		}
 		if err != nil {
 			return err
 		}
 
 		if c != '/' {
-			return errors.New("unrecognized input")
+			return errors.New(fmt.Sprintf("unrecognized input: '%c'", c))
 		}
-		for {
+		for c != '\n' {
+			c, err = t.eat()
 			c, err = t.peek()
 			if err == io.EOF {
 				break
@@ -168,14 +156,9 @@ func (t *tokenizer) step() error {
 			if err != nil {
 				return err
 			}
-
-			if c == '\n' {
-				break
-			}
-			t.eat()
 		}
 	default:
-		return errors.New("unrecognized input")
+		return errors.New(fmt.Sprintf("unrecognized input: '%c'", c))
 	}
 	return nil
 }
@@ -208,11 +191,12 @@ const (
 	ISUBTRACT
 	ISTORE
 	ILOAD
+	_
 	IBRANCH
 	IBRANCH_IF_ZERO
 	IBRANCH_IF_POSITIVE
 	IINPUT
-	IOUTUT
+	IOUTPUT
 	IDATA
 )
 
@@ -227,22 +211,22 @@ var instructions = map[string]int{
 	"BRZ": IBRANCH_IF_ZERO,
 	"BRP": IBRANCH_IF_POSITIVE,
 	"INP": IINPUT,
-	"OUT": IOUTUT,
+	"OUT": IOUTPUT,
 	"DAT": IDATA,
 }
 
 type instruction struct {
-	id     int
-	target string
-	number int
+	id     int    // What instruction this is
+	target string // Label name
+	number int    // Immediate value
 }
 
 // -----------------------------------------------------------------------------
 
 type assembler struct {
-	tokens chan token
-	output []instruction
-	labels map[string]int
+	tokens chan token     // Where tokens come from
+	output []instruction  // The assembled program
+	labels map[string]int // Addresses of labels
 }
 
 func (a *assembler) step() (bool, error) {
@@ -278,29 +262,42 @@ func (a *assembler) step() (bool, error) {
 			canonical = strings.ToUpper(token.value)
 			instr, found = instructions[canonical]
 		}
-
 		if !found {
 			return false, fmt.Errorf("Unknown instruction: %s", canonical)
 		}
 
 		instrHolder := instruction{id: instr}
-		token, ok := <-a.tokens
-		if !ok {
-			// This is fine, just assume zero
-			break
-		}
 
-		switch token.kind {
-		case WORD:
+		token, ok := <-a.tokens
+		eol := false
+		switch {
+		case token.kind == WORD:
 			instrHolder.target = strings.ToUpper(token.value)
-		case NEWLINE:
-			// This is fine, just assume zero
-		case NUMBER:
+		case token.kind == NUMBER:
 			instrHolder.number, _ = strconv.Atoi(token.value)
-		case ERROR:
+		case token.kind == ERROR:
 			return false, errors.New(token.value)
+		case !ok:
+			fallthrough
+		case token.kind == NEWLINE:
+			// This is fine, just assume zero
+			eol = true
 		}
 		a.output = append(a.output, instrHolder)
+
+		if !eol {
+			token, ok := <-a.tokens
+			switch {
+			case !ok:
+				break
+			case token.kind == NEWLINE:
+				break
+			case token.kind == ERROR:
+				return false, errors.New(token.value)
+			default:
+				return false, errors.New("Expected end of line")
+			}
+		}
 	case NEWLINE:
 		// Ignore empty lines
 	case NUMBER:
@@ -312,7 +309,7 @@ func (a *assembler) step() (bool, error) {
 }
 
 func Assemble(r io.Reader) (code []int16, err error) {
-	a := assembler{tokens: make(chan token)}
+	a := assembler{tokens: make(chan token), labels: make(map[string]int)}
 	go tokenize(r, a.tokens)
 
 	for {
@@ -325,18 +322,33 @@ func Assemble(r io.Reader) (code []int16, err error) {
 		}
 	}
 
-	for _, x := range a.output {
+	code = make([]int16, 100)
+	for i, x := range a.output {
+		if i >= len(code) {
+			return nil, errors.New("Program too long")
+		}
 		n := x.id * 100
-		if len(x.target) != 0 {
+		// XXX: this also stinks
+		if x.id == IDATA {
+			n = 0
+		}
+		// XXX: we should be able to handle the strange INP and OUT better
+		switch {
+		case x.id == IINPUT:
+			n = 901
+		case x.id == IOUTPUT:
+			n = 902
+		case len(x.target) != 0:
+			// Resolve targets to code locations
 			if resolved, ok := a.labels[x.target]; !ok {
 				return nil, errors.New("Unknown label")
 			} else {
 				n += resolved
 			}
-		} else {
+		default:
 			n += x.number
 		}
-		code = append(code, int16(n))
+		code[i] = int16(n)
 	}
 	return code, nil
 }
diff --git a/machine.go b/machine.go
index a4b188d..cca2aa2 100644
--- a/machine.go
+++ b/machine.go
@@ -1,11 +1,59 @@
 package main
 
-func Run(code []int16) {
-	// TODO: assert that the code is 100 boxes long
+import (
+	"errors"
+	"fmt"
+)
 
-	pc := 0
-	for pc < len(code) {
+func Run(code []int16) error {
+	if len(code) != 100 {
+		return errors.New("Code must be exactly 100 mailboxes long")
 	}
 
-	// TODO: throw an exception
+	//	for i, x := range code {
+	//		fmt.Printf("%d: %d\n", i, x)
+	//	}
+	//	fmt.Println()
+
+	var accumulator int16 = 0
+
+	pc := 0
+	for pc >= 0 && pc < len(code) {
+		i := code[pc]
+		pc++
+		switch i / 100 {
+		case IHALT:
+			return nil
+		case IADD:
+			accumulator += code[i%100]
+		case ISUBTRACT:
+			accumulator -= code[i%100]
+		case ISTORE:
+			code[i%100] = accumulator
+		case ILOAD:
+			accumulator = code[i%100]
+		case IBRANCH:
+			pc = int(i % 100)
+		case IBRANCH_IF_ZERO:
+			if accumulator == 0 {
+				pc = int(i % 100)
+			}
+		case IBRANCH_IF_POSITIVE:
+			if accumulator > 0 {
+				pc = int(i % 100)
+			}
+		default:
+			switch i {
+			case 901:
+				fmt.Printf("Input: ")
+				fmt.Scanf("%d\n", &accumulator)
+			case 902:
+				fmt.Printf("Output: %d\n", accumulator)
+			default:
+				e := fmt.Sprintf("Unsupported instruction %d at %d", i, pc)
+				return errors.New(e)
+			}
+		}
+	}
+	return errors.New("Program counter ran away")
 }
diff --git a/main.go b/main.go
index a891931..6fbd3f9 100644
--- a/main.go
+++ b/main.go
@@ -6,10 +6,23 @@ import (
 )
 
 func main() {
-	code, err := Assemble(os.Stdin)
+	if len(os.Args) != 2 {
+		fmt.Printf("usage: %s file", os.Args[0])
+		os.Exit(1)
+	}
+	file, err := os.Open(os.Args[1])
+	if err != nil {
+		fmt.Printf("Cannot open file: %s", err)
+		os.Exit(1)
+	}
+	code, err := Assemble(file)
 	if err != nil {
 		fmt.Printf("Assembly failed: %s", err)
 		os.Exit(1)
 	}
-	Run(code)
+	err = Run(code)
+	if err != nil {
+		fmt.Printf("Runtime error: %s", err)
+		os.Exit(1)
+	}
 }