diff --git a/src/cmd/asm/main.go b/src/cmd/asm/main.go
index 40e1d9c4a9..c612583e6b 100644
--- a/src/cmd/asm/main.go
+++ b/src/cmd/asm/main.go
@@ -44,12 +44,15 @@ func main() {
 	defer ctxt.Bso.Flush()
 
 	// Create object file, write header.
-	output, err := bio.Create(*flags.OutputFile)
+	out, err := os.Create(*flags.OutputFile)
 	if err != nil {
 		log.Fatal(err)
 	}
-	fmt.Fprintf(output, "go object %s %s %s\n", obj.Getgoos(), obj.Getgoarch(), obj.Getgoversion())
-	fmt.Fprintf(output, "!\n")
+	defer bio.MustClose(out)
+	buf := bufio.NewWriter(bio.MustWriter(out))
+
+	fmt.Fprintf(buf, "go object %s %s %s\n", obj.Getgoos(), obj.Getgoarch(), obj.Getgoversion())
+	fmt.Fprintf(buf, "!\n")
 
 	lexer := lex.NewLexer(flag.Arg(0), ctxt)
 	parser := asm.NewParser(ctxt, architecture, lexer)
@@ -63,12 +66,12 @@ func main() {
 	pList.Firstpc, ok = parser.Parse()
 	if ok {
 		// reports errors to parser.Errorf
-		obj.Writeobjdirect(ctxt, output)
+		obj.Writeobjdirect(ctxt, buf)
 	}
 	if !ok || diag {
 		log.Printf("assembly of %s failed", flag.Arg(0))
 		os.Remove(*flags.OutputFile)
 		os.Exit(1)
 	}
-	output.Flush()
+	buf.Flush()
 }
diff --git a/src/cmd/compile/internal/gc/obj.go b/src/cmd/compile/internal/gc/obj.go
index 59ce0547c8..b60f78f638 100644
--- a/src/cmd/compile/internal/gc/obj.go
+++ b/src/cmd/compile/internal/gc/obj.go
@@ -88,7 +88,7 @@ func dumpobj() {
 	externdcl = tmp
 
 	dumpdata()
-	obj.Writeobjdirect(Ctxt, bout)
+	obj.Writeobjdirect(Ctxt, bout.Writer)
 
 	if writearchive {
 		bout.Flush()
diff --git a/src/cmd/internal/bio/buf.go b/src/cmd/internal/bio/buf.go
index 7a077041c2..54ce3c7681 100644
--- a/src/cmd/internal/bio/buf.go
+++ b/src/cmd/internal/bio/buf.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package bio implements seekable buffered I/O.
+// Package bio implements common I/O abstractions used within the Go toolchain.
 package bio
 
 import (
diff --git a/src/cmd/internal/bio/must.go b/src/cmd/internal/bio/must.go
new file mode 100644
index 0000000000..3604b29175
--- /dev/null
+++ b/src/cmd/internal/bio/must.go
@@ -0,0 +1,43 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bio
+
+import (
+	"io"
+	"log"
+)
+
+// MustClose closes Closer c and calls log.Fatal if it returns a non-nil error.
+func MustClose(c io.Closer) {
+	if err := c.Close(); err != nil {
+		log.Fatal(err)
+	}
+}
+
+// MustWriter returns a Writer that wraps the provided Writer,
+// except that it calls log.Fatal instead of returning a non-nil error.
+func MustWriter(w io.Writer) io.Writer {
+	return mustWriter{w}
+}
+
+type mustWriter struct {
+	w io.Writer
+}
+
+func (w mustWriter) Write(b []byte) (int, error) {
+	n, err := w.w.Write(b)
+	if err != nil {
+		log.Fatal(err)
+	}
+	return n, nil
+}
+
+func (w mustWriter) WriteString(s string) (int, error) {
+	n, err := io.WriteString(w.w, s)
+	if err != nil {
+		log.Fatal(err)
+	}
+	return n, nil
+}
diff --git a/src/cmd/internal/obj/objfile.go b/src/cmd/internal/obj/objfile.go
index 7d88db2bcc..60505dfbb5 100644
--- a/src/cmd/internal/obj/objfile.go
+++ b/src/cmd/internal/obj/objfile.go
@@ -109,7 +109,6 @@ package obj
 
 import (
 	"bufio"
-	"cmd/internal/bio"
 	"cmd/internal/sys"
 	"fmt"
 	"log"
@@ -120,7 +119,7 @@ import (
 // The Go and C compilers, and the assembler, call writeobj to write
 // out a Go object file. The linker does not call this; the linker
 // does not write out object files.
-func Writeobjdirect(ctxt *Link, b *bio.Writer) {
+func Writeobjdirect(ctxt *Link, b *bufio.Writer) {
 	Flushplist(ctxt)
 	WriteObjFile(ctxt, b)
 }
@@ -187,16 +186,16 @@ func (w *objWriter) writeLengths() {
 	w.writeInt(int64(w.nFile))
 }
 
-func newObjWriter(ctxt *Link, b *bio.Writer) *objWriter {
+func newObjWriter(ctxt *Link, b *bufio.Writer) *objWriter {
 	return &objWriter{
 		ctxt:    ctxt,
-		wr:      b.Writer,
+		wr:      b,
 		vrefIdx: make(map[string]int),
 		refIdx:  make(map[string]int),
 	}
 }
 
-func WriteObjFile(ctxt *Link, b *bio.Writer) {
+func WriteObjFile(ctxt *Link, b *bufio.Writer) {
 	w := newObjWriter(ctxt, b)
 
 	// Magic header