diff --git a/build/cli.go b/build/cli.go
new file mode 100644
index 0000000..0e12753
--- /dev/null
+++ b/build/cli.go
@@ -0,0 +1,132 @@
+package build
+
+import (
+	"flag"
+	"io"
+	"log"
+	"os"
+
+	"github.com/mmcloughlin/avo/pass"
+	"github.com/mmcloughlin/avo/printer"
+)
+
+type Config struct {
+	ErrOut io.Writer
+	Passes []pass.Interface
+}
+
+func Main(cfg *Config, context *Context) int {
+	diag := log.New(cfg.ErrOut, "", 0)
+
+	f, errs := context.Result()
+	if errs != nil {
+		for _, err := range errs {
+			diag.Println(err)
+		}
+		return 1
+	}
+
+	p := pass.Concat(cfg.Passes...)
+	if err := p.Execute(f); err != nil {
+		diag.Println(err)
+		return 1
+	}
+
+	return 0
+}
+
+type Flags struct {
+	errout   *outputValue
+	printers []*printerValue
+}
+
+func NewFlags(fs *flag.FlagSet) *Flags {
+	f := &Flags{}
+
+	f.errout = newOutputValue(os.Stderr)
+	fs.Var(f.errout, "log", "diagnostics output")
+
+	goasm := newPrinterValue(printer.NewGoAsm, os.Stdout)
+	fs.Var(goasm, "out", "assembly output")
+	f.printers = append(f.printers, goasm)
+
+	stubs := newPrinterValue(printer.NewStubs, nil)
+	fs.Var(stubs, "stubs", "go stub file")
+	f.printers = append(f.printers, stubs)
+
+	return f
+}
+
+func (f *Flags) Config() *Config {
+	pc := printer.NewDefaultConfig()
+	passes := []pass.Interface{pass.Compile}
+	for _, pv := range f.printers {
+		p := pv.Build(pc)
+		if p != nil {
+			passes = append(passes, p)
+		}
+	}
+	return &Config{
+		ErrOut: f.errout.w,
+		Passes: passes,
+	}
+}
+
+type outputValue struct {
+	w        io.WriteCloser
+	filename string
+}
+
+func newOutputValue(dflt io.WriteCloser) *outputValue {
+	return &outputValue{w: dflt}
+}
+
+func (o *outputValue) String() string {
+	if o == nil {
+		return ""
+	}
+	return o.filename
+}
+
+func (o *outputValue) Set(s string) error {
+	o.filename = s
+	if s == "-" {
+		o.w = nopwritecloser{os.Stdout}
+		return nil
+	}
+	f, err := os.Create(s)
+	if err != nil {
+		return err
+	}
+	o.w = f
+	return nil
+}
+
+type printerValue struct {
+	*outputValue
+	Builder printer.Builder
+}
+
+func newPrinterValue(b printer.Builder, dflt io.WriteCloser) *printerValue {
+	return &printerValue{
+		outputValue: newOutputValue(dflt),
+		Builder:     b,
+	}
+}
+
+func (p *printerValue) Build(cfg printer.Config) pass.Interface {
+	if p.outputValue.w == nil {
+		return nil
+	}
+	return &pass.Output{
+		Writer:  p.outputValue.w,
+		Printer: p.Builder(cfg),
+	}
+}
+
+// nopwritecloser wraps a Writer and provides a null implementation of Close().
+type nopwritecloser struct {
+	io.Writer
+}
+
+func (nopwritecloser) Close() error { return nil }
diff --git a/build/context.go b/build/context.go
index 400bfdd..38ad6f8 100644
--- a/build/context.go
+++ b/build/context.go
@@ -2,13 +2,9 @@ package build
 
 import (
 	"errors"
-	"io"
-	"log"
-
-	"github.com/mmcloughlin/avo/gotypes"
-	"github.com/mmcloughlin/avo/pass"
 
 	"github.com/mmcloughlin/avo"
+	"github.com/mmcloughlin/avo/gotypes"
 	"github.com/mmcloughlin/avo/reg"
 )
 
@@ -73,28 +69,3 @@ func (c *Context) AddErrorMessage(msg string) {
 func (c *Context) Result() (*avo.File, []error) {
 	return c.file, c.errs
 }
-
-func (c *Context) Main(wout, werr io.Writer) int {
-	diag := log.New(werr, "", 0)
-
-	f, errs := c.Result()
-	if errs != nil {
-		for _, err := range errs {
-			diag.Println(err)
-		}
-		return 1
-	}
-
-	if err := pass.Compile.Execute(f); err != nil {
-		diag.Println(err)
-		return 1
-	}
-
-	p := avo.NewGoPrinter(wout)
-	if err := p.Print(f); err != nil {
-		diag.Println(err)
-		return 1
-	}
-
-	return 0
-}
diff --git a/build/global.go b/build/global.go
index 1a2d808..7307328 100644
--- a/build/global.go
+++ b/build/global.go
@@ -2,8 +2,6 @@ package build
 
 import (
 	"flag"
-	"io"
-	"log"
 	"os"
 
 	"github.com/mmcloughlin/avo/gotypes"
@@ -23,26 +21,14 @@ func TEXT(name, signature string) {
 
 func LABEL(name string) { ctx.Label(avo.Label(name)) }
 
-var (
-	output = flag.String("output", "", "output filename (default stdout)")
-)
+var flags = NewFlags(flag.CommandLine)
 
-func EOF() {
+func Generate() {
 	if !flag.Parsed() {
 		flag.Parse()
 	}
-
-	var w io.Writer = os.Stdout
-	if *output != "" {
-		if f, err := os.Create(*output); err != nil {
-			log.Fatal(err)
-		} else {
-			defer f.Close()
-			w = f
-		}
-	}
-
-	os.Exit(ctx.Main(w, os.Stderr))
+	cfg := flags.Config()
+	os.Exit(Main(cfg, ctx))
 }
 
 func GP8v() reg.Virtual  { return ctx.GP8v() }
diff --git a/examples/add/add.s b/examples/add/add.s
new file mode 100644
index 0000000..82520ba
--- /dev/null
+++ b/examples/add/add.s
@@ -0,0 +1,10 @@
+
+#include "textflag.h"
+
+// func Add(x uint64, y uint64) uint64
+TEXT ·Add(SB),0,$0-24
+	MOVQ	x(FP), CX
+	MOVQ	y+8(FP), AX
+	ADDQ	CX, AX
+	MOVQ	CX, ret+16(FP)
+	RET	
diff --git a/examples/add/add_test.go b/examples/add/add_test.go
new file mode 100644
index 0000000..6252306
--- /dev/null
+++ b/examples/add/add_test.go
@@ -0,0 +1,12 @@
+package add
+
+import (
+	"testing"
+	"testing/quick"
+)
+
+//go:generate go run asm.go -out add.s -stubs stub.go
+
+func TestAdd(t *testing.T) {
+	quick.CheckEqual(Add, func(x, y uint64) uint64 { return x + y }, nil)
+}
diff --git a/examples/add/asm.go b/examples/add/asm.go
index 63d99ad..4505854 100644
--- a/examples/add/asm.go
+++ b/examples/add/asm.go
@@ -1,3 +1,5 @@
+// +build ignore
+
 package main
 
 import (
@@ -5,11 +7,11 @@ import (
 )
 
 func main() {
-	TEXT("add", "func(x, y uint64) uint64")
+	TEXT("Add", "func(x, y uint64) uint64")
 	x := Load(Param("x"), GP64v())
 	y := Load(Param("y"), GP64v())
 	ADDQ(x, y)
 	Store(x, ReturnIndex(0))
 	RET()
-	EOF()
+	Generate()
 }
diff --git a/examples/add/stub.go b/examples/add/stub.go
new file mode 100644
index 0000000..9e26631
--- /dev/null
+++ b/examples/add/stub.go
@@ -0,0 +1,3 @@
+package add
+
+func Add(x uint64, y uint64) uint64
diff --git a/internal/cmd/avogen/main.go b/internal/cmd/avogen/main.go
index 8a90852..70e4ffc 100644
--- a/internal/cmd/avogen/main.go
+++ b/internal/cmd/avogen/main.go
@@ -10,6 +10,7 @@ import (
 	"github.com/mmcloughlin/avo/internal/gen"
 	"github.com/mmcloughlin/avo/internal/inst"
 	"github.com/mmcloughlin/avo/internal/load"
+	"github.com/mmcloughlin/avo/printer"
 )
 
 var generators = map[string]gen.Builder{
@@ -45,9 +46,7 @@ func main() {
 		log.Fatalf("unknown generator type '%s'", t)
 	}
 
-	g := builder(gen.Config{
-		Argv: os.Args,
-	})
+	g := builder(printer.NewDefaultConfig())
 
 	// Determine output writer.
 	w := os.Stdout
diff --git a/internal/gen/asmtest.go b/internal/gen/asmtest.go
index f97818b..ee43565 100644
--- a/internal/gen/asmtest.go
+++ b/internal/gen/asmtest.go
@@ -7,17 +7,19 @@ import (
 	"strings"
 
 	"github.com/mmcloughlin/avo/internal/inst"
+	"github.com/mmcloughlin/avo/internal/prnt"
+	"github.com/mmcloughlin/avo/printer"
 )
 
 type asmtest struct {
-	cfg   Config
+	cfg   printer.Config
 	sym   string // reference to the test function symbol
 	rel8  string // label to be used for near jumps
 	rel32 string // label for far jumps
-	generator
+	prnt.Generator
 }
 
-func NewAsmTest(cfg Config) Interface {
+func NewAsmTest(cfg printer.Config) Interface {
 	return &asmtest{cfg: cfg}
 }
 
diff --git a/internal/gen/build.go b/internal/gen/build.go
index d77786c..0b6c4b4 100644
--- a/internal/gen/build.go
+++ b/internal/gen/build.go
@@ -1,13 +1,17 @@
 package gen
 
-import "github.com/mmcloughlin/avo/internal/inst"
+import (
+	"github.com/mmcloughlin/avo/internal/inst"
+	"github.com/mmcloughlin/avo/internal/prnt"
+	"github.com/mmcloughlin/avo/printer"
+)
 
 type build struct {
-	cfg Config
-	generator
+	cfg printer.Config
+	prnt.Generator
 }
 
-func NewBuild(cfg Config) Interface {
+func NewBuild(cfg printer.Config) Interface {
 	return GoFmt(&build{cfg: cfg})
 }
 
diff --git a/internal/gen/ctors.go b/internal/gen/ctors.go
index 4af5304..af50351 100644
--- a/internal/gen/ctors.go
+++ b/internal/gen/ctors.go
@@ -6,14 +6,16 @@ import (
 	"strings"
 
 	"github.com/mmcloughlin/avo/internal/inst"
+	"github.com/mmcloughlin/avo/internal/prnt"
+	"github.com/mmcloughlin/avo/printer"
 )
 
 type ctors struct {
-	cfg Config
-	generator
+	cfg printer.Config
+	prnt.Generator
 }
 
-func NewCtors(cfg Config) Interface {
+func NewCtors(cfg printer.Config) Interface {
 	return GoFmt(&ctors{cfg: cfg})
 }
 
diff --git a/internal/gen/ctorstest.go b/internal/gen/ctorstest.go
index 9bc653e..d117e8e 100644
--- a/internal/gen/ctorstest.go
+++ b/internal/gen/ctorstest.go
@@ -3,15 +3,18 @@ package gen
 import (
 	"strings"
 
+	"github.com/mmcloughlin/avo/internal/prnt"
+	"github.com/mmcloughlin/avo/printer"
+
 	"github.com/mmcloughlin/avo/internal/inst"
 )
 
 type ctorstest struct {
-	cfg Config
-	generator
+	cfg printer.Config
+	prnt.Generator
 }
 
-func NewCtorsTest(cfg Config) Interface {
+func NewCtorsTest(cfg printer.Config) Interface {
 	return GoFmt(&ctorstest{cfg: cfg})
 }
 
diff --git a/internal/gen/gen.go b/internal/gen/gen.go
index 76fd2a1..c202b83 100644
--- a/internal/gen/gen.go
+++ b/internal/gen/gen.go
@@ -1,12 +1,10 @@
 package gen
 
 import (
-	"bytes"
-	"fmt"
 	"go/format"
-	"strings"
 
 	"github.com/mmcloughlin/avo/internal/inst"
+	"github.com/mmcloughlin/avo/printer"
 )
 
 type Interface interface {
@@ -19,23 +17,7 @@ func (f Func) Generate(is []inst.Instruction) ([]byte, error) {
 	return f(is)
 }
 
-type Config struct {
-	Name string
-	Argv []string
-}
-
-func (c Config) GeneratedBy() string {
-	if c.Argv == nil {
-		return c.Name
-	}
-	return fmt.Sprintf("command: %s", strings.Join(c.Argv, " "))
-}
-
-func (c Config) GeneratedWarning() string {
-	return fmt.Sprintf("Code generated by %s. DO NOT EDIT.", c.GeneratedBy())
-}
-
-type Builder func(Config) Interface
+type Builder func(printer.Config) Interface
 
 // GoFmt formats Go code produced from the given generator.
 func GoFmt(i Interface) Interface {
@@ -47,27 +29,3 @@ func GoFmt(i Interface) Interface {
 		return format.Source(b)
 	})
 }
-
-type generator struct {
-	buf bytes.Buffer
-	err error
-}
-
-func (g *generator) Printf(format string, args ...interface{}) {
-	if g.err != nil {
-		return
-	}
-	if _, err := fmt.Fprintf(&g.buf, format, args...); err != nil {
-		g.AddError(err)
-	}
-}
-
-func (g *generator) AddError(err error) {
-	if err != nil && g.err == nil {
-		g.err = err
-	}
-}
-
-func (g *generator) Result() ([]byte, error) {
-	return g.buf.Bytes(), g.err
-}
diff --git a/internal/gen/godata.go b/internal/gen/godata.go
index ba083a8..221c374 100644
--- a/internal/gen/godata.go
+++ b/internal/gen/godata.go
@@ -2,14 +2,16 @@ package gen
 
 import (
 	"github.com/mmcloughlin/avo/internal/inst"
+	"github.com/mmcloughlin/avo/internal/prnt"
+	"github.com/mmcloughlin/avo/printer"
 )
 
 type godata struct {
-	cfg Config
-	generator
+	cfg printer.Config
+	prnt.Generator
 }
 
-func NewGoData(cfg Config) Interface {
+func NewGoData(cfg printer.Config) Interface {
 	return GoFmt(&godata{cfg: cfg})
 }
 
@@ -65,11 +67,11 @@ func (g *godata) Generate(is []inst.Instruction) ([]byte, error) {
 }
 
 type godatatest struct {
-	cfg Config
-	generator
+	cfg printer.Config
+	prnt.Generator
 }
 
-func NewGoDataTest(cfg Config) Interface {
+func NewGoDataTest(cfg printer.Config) Interface {
 	return GoFmt(&godatatest{cfg: cfg})
 }
 
diff --git a/internal/gen/mov.go b/internal/gen/mov.go
index 26b64fa..3e0f56d 100644
--- a/internal/gen/mov.go
+++ b/internal/gen/mov.go
@@ -7,14 +7,16 @@ import (
 	"strings"
 
 	"github.com/mmcloughlin/avo/internal/inst"
+	"github.com/mmcloughlin/avo/internal/prnt"
+	"github.com/mmcloughlin/avo/printer"
 )
 
 type mov struct {
-	cfg Config
-	generator
+	cfg printer.Config
+	prnt.Generator
 }
 
-func NewMOV(cfg Config) Interface {
+func NewMOV(cfg printer.Config) Interface {
 	return GoFmt(&mov{cfg: cfg})
 }
 
diff --git a/internal/prnt/printer.go b/internal/prnt/printer.go
new file mode 100644
index 0000000..33a9a2d
--- /dev/null
+++ b/internal/prnt/printer.go
@@ -0,0 +1,40 @@
+package prnt
+
+import (
+	"bytes"
+	"fmt"
+)
+
+type Generator struct {
+	buf bytes.Buffer
+	err error
+}
+
+func (g *Generator) Printf(format string, args ...interface{}) {
+	if g.err != nil {
+		return
+	}
+	if _, err := fmt.Fprintf(&g.buf, format, args...); err != nil {
+		g.AddError(err)
+	}
+}
+
+func (g *Generator) NL() {
+	g.Printf("\n")
+}
+
+func (g *Generator) Comment(lines ...string) {
+	for _, line := range lines {
+		g.Printf("// %s\n", line)
+	}
+}
+
+func (g *Generator) AddError(err error) {
+	if err != nil && g.err == nil {
+		g.err = err
+	}
+}
+
+func (g *Generator) Result() ([]byte, error) {
+	return g.buf.Bytes(), g.err
+}
diff --git a/pass/pass.go b/pass/pass.go
index e60df46..b9e2a9f 100644
--- a/pass/pass.go
+++ b/pass/pass.go
@@ -1,6 +1,11 @@
 package pass
 
-import "github.com/mmcloughlin/avo"
+import (
+	"io"
+
+	"github.com/mmcloughlin/avo"
+	"github.com/mmcloughlin/avo/printer"
+)
 
 var Compile = Concat(
 	FunctionPass(LabelTarget),
@@ -42,3 +47,19 @@ func Concat(passes ...Interface) Interface {
 		return nil
 	})
 }
+
+type Output struct {
+	Writer  io.WriteCloser
+	Printer printer.Printer
+}
+
+func (o *Output) Execute(f *avo.File) error {
+	b, err := o.Printer.Print(f)
+	if err != nil {
+		return err
+	}
+	if _, err = o.Writer.Write(b); err != nil {
+		return err
+	}
+	return o.Writer.Close()
+}
diff --git a/printer.go b/printer.go
deleted file mode 100644
index 3ba5743..0000000
--- a/printer.go
+++ /dev/null
@@ -1,102 +0,0 @@
-package avo
-
-import (
-	"fmt"
-	"io"
-	"strings"
-
-	"github.com/mmcloughlin/avo/operand"
-)
-
-// dot is the pesky unicode dot used in Go assembly.
-const dot = "\u00b7"
-
-type Printer interface {
-	Print(*File) error
-}
-
-type GoPrinter struct {
-	w   io.Writer
-	by  string // generated by
-	err error
-}
-
-func NewGoPrinter(w io.Writer) *GoPrinter {
-	return &GoPrinter{
-		w:  w,
-		by: "avo",
-	}
-}
-
-func (p *GoPrinter) SetGeneratedBy(by string) {
-	p.by = by
-}
-
-func (p *GoPrinter) Print(f *File) error {
-	p.header()
-
-	for _, fn := range f.Functions {
-		p.function(fn)
-	}
-
-	return p.err
-}
-
-func (p *GoPrinter) header() {
-	p.generated()
-	p.nl()
-	p.incl("textflag.h")
-	p.nl()
-}
-
-func (p *GoPrinter) generated() {
-	p.comment(fmt.Sprintf("Code generated by %s. DO NOT EDIT.", p.by))
-}
-
-func (p *GoPrinter) incl(path string) {
-	p.printf("#include \"%s\"\n", path)
-}
-
-func (p *GoPrinter) comment(line string) {
-	p.multicomment([]string{line})
-}
-
-func (p *GoPrinter) multicomment(lines []string) {
-	for _, line := range lines {
-		p.printf("// %s\n", line)
-	}
-}
-
-func (p *GoPrinter) function(f *Function) {
-	p.printf("// %s\n", f.Stub())
-	p.printf("TEXT %s%s(SB),0,$%d-%d\n", dot, f.Name, f.FrameBytes(), f.ArgumentBytes())
-
-	for _, node := range f.Nodes {
-		switch n := node.(type) {
-		case *Instruction:
-			p.printf("\t%s\t%s\n", n.Opcode, joinOperands(n.Operands))
-		case Label:
-			p.printf("%s:\n", n)
-		default:
-			panic("unexpected node type")
-		}
-	}
-}
-
-func (p *GoPrinter) nl() {
-	p.printf("\n")
-}
-
-func (p *GoPrinter) printf(format string, args ...interface{}) {
-	if _, err := fmt.Fprintf(p.w, format, args...); err != nil {
-		p.err = err
-	}
-}
-
-func joinOperands(operands []operand.Op) string {
-	asm := make([]string, len(operands))
-	for i, op := range operands {
-		asm[i] = op.Asm()
-	}
-	return strings.Join(asm, ", ")
-}
diff --git a/printer/goasm.go b/printer/goasm.go
new file mode 100644
index 0000000..8c615cb
--- /dev/null
+++ b/printer/goasm.go
@@ -0,0 +1,63 @@
+package printer
+
+import (
+	"strings"
+
+	"github.com/mmcloughlin/avo"
+	"github.com/mmcloughlin/avo/internal/prnt"
+	"github.com/mmcloughlin/avo/operand"
+)
+
+// dot is the pesky unicode dot used in Go assembly.
+const dot = "\u00b7"
+
+type goasm struct {
+	cfg Config
+	prnt.Generator
+}
+
+func NewGoAsm(cfg Config) Printer {
+	return &goasm{cfg: cfg}
+}
+
+func (p *goasm) Print(f *avo.File) ([]byte, error) {
+	p.header()
+	for _, fn := range f.Functions {
+		p.function(fn)
+	}
+	return p.Result()
+}
+
+func (p *goasm) header() {
+	p.NL()
+	p.include("textflag.h")
+	p.NL()
+}
+
+func (p *goasm) include(path string) {
+	p.Printf("#include \"%s\"\n", path)
+}
+
+func (p *goasm) function(f *avo.Function) {
+	p.Comment(f.Stub())
+	p.Printf("TEXT %s%s(SB),0,$%d-%d\n", dot, f.Name, f.FrameBytes(), f.ArgumentBytes())
+
+	for _, node := range f.Nodes {
+		switch n := node.(type) {
+		case *avo.Instruction:
+			p.Printf("\t%s\t%s\n", n.Opcode, joinOperands(n.Operands))
+		case avo.Label:
+			p.Printf("%s:\n", n)
+		default:
+			panic("unexpected node type")
+		}
+	}
+}
+
+func joinOperands(operands []operand.Op) string {
+	asm := make([]string, len(operands))
+	for i, op := range operands {
+		asm[i] = op.Asm()
+	}
+	return strings.Join(asm, ", ")
+}
diff --git a/printer/printer.go b/printer/printer.go
new file mode 100644
index 0000000..7b1ee9b
--- /dev/null
+++ b/printer/printer.go
@@ -0,0 +1,43 @@
+package printer
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/mmcloughlin/avo"
+)
+
+type Printer interface {
+	Print(*avo.File) ([]byte, error)
+}
+
+type Builder func(Config) Printer
+
+type Config struct {
+	Name string
+	Argv []string
+	Pkg  string
+}
+
+func NewDefaultConfig() Config {
+	cfg := Config{
+		Argv: os.Args,
+	}
+	if cwd, err := os.Getwd(); err == nil {
+		cfg.Pkg = filepath.Base(cwd)
+	}
+	return cfg
+}
+
+func (c Config) GeneratedBy() string {
+	if c.Argv == nil {
+		return c.Name
+	}
+	return fmt.Sprintf("command: %s", strings.Join(c.Argv, " "))
+}
+
+func (c Config) GeneratedWarning() string {
+	return fmt.Sprintf("Code generated by %s. DO NOT EDIT.", c.GeneratedBy())
+}
diff --git a/printer/stubs.go b/printer/stubs.go
new file mode 100644
index 0000000..9e7b518
--- /dev/null
+++ b/printer/stubs.go
@@ -0,0 +1,23 @@
+package printer
+
+import (
+	"github.com/mmcloughlin/avo"
+	"github.com/mmcloughlin/avo/internal/prnt"
+)
+
+type stubs struct {
+	cfg Config
+	prnt.Generator
+}
+
+func NewStubs(cfg Config) Printer {
+	return &stubs{cfg: cfg}
+}
+
+func (s *stubs) Print(f *avo.File) ([]byte, error) {
+	s.Printf("package %s\n\n", s.cfg.Pkg)
+	for _, fn := range f.Functions {
+		s.Printf("%s\n", fn.Stub())
+	}
+	return s.Result()
+}