diff --git a/examples/sum/asm.go b/examples/sum/asm.go
new file mode 100644
index 0000000..d0eb561
--- /dev/null
+++ b/examples/sum/asm.go
@@ -0,0 +1,27 @@
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+	"github.com/mmcloughlin/avo/operand"
+)
+
+func main() {
+	TEXT("Sum", "func(xs []uint64) uint64")
+	ptr := Load(Param("xs").Base(), GP64v())
+	n := Load(Param("xs").Len(), GP64v())
+	s := GP64v()
+	XORQ(s, s)
+	LABEL("loop")
+	CMPQ(n, operand.Imm(0))
+	JE(operand.LabelRef("done"))
+	ADDQ(operand.Mem{Base: ptr}, s)
+	ADDQ(operand.Imm(8), ptr)
+	DECQ(n)
+	JMP(operand.LabelRef("loop"))
+	LABEL("done")
+	Store(s, ReturnIndex(0))
+	RET()
+	Generate()
+}
diff --git a/examples/sum/stub.go b/examples/sum/stub.go
new file mode 100644
index 0000000..859a440
--- /dev/null
+++ b/examples/sum/stub.go
@@ -0,0 +1,3 @@
+package sum
+
+func Sum(xs []uint64) uint64
diff --git a/examples/sum/sum.s b/examples/sum/sum.s
new file mode 100644
index 0000000..7bbe334
--- /dev/null
+++ b/examples/sum/sum.s
@@ -0,0 +1,18 @@
+
+#include "textflag.h"
+
+// func Sum(xs []uint64) uint64
+TEXT ·Sum(SB),0,$0-32
+	MOVQ	xs_base(FP), DX
+	MOVQ	xs_len+8(FP), CX
+	XORQ	AX, AX
+loop:
+	CMPQ	CX, $0x0
+	JE	done
+	ADDQ	(DX), AX
+	ADDQ	$0x8, DX
+	DECQ	CX
+	JMP	loop
+done:
+	MOVQ	AX, ret+24(FP)
+	RET	
diff --git a/examples/sum/sum_test.go b/examples/sum/sum_test.go
new file mode 100644
index 0000000..6178198
--- /dev/null
+++ b/examples/sum/sum_test.go
@@ -0,0 +1,20 @@
+package sum
+
+import (
+	"testing"
+	"testing/quick"
+)
+
+//go:generate go run asm.go -out sum.s -stubs stub.go
+
+func expect(xs []uint64) uint64 {
+	var s uint64
+	for _, x := range xs {
+		s += x
+	}
+	return s
+}
+
+func TestSum(t *testing.T) {
+	quick.CheckEqual(Sum, expect, nil)
+}
diff --git a/gotypes/components.go b/gotypes/components.go
index 0220f4b..6362134 100644
--- a/gotypes/components.go
+++ b/gotypes/components.go
@@ -126,12 +126,11 @@ func (c *component) Index(i int) Component {
 }
 
 func (c *component) sub(suffix string, offset int, t types.Type) *component {
-	var s *component
-	*s = *c
+	s := *c
 	s.name += suffix
 	s.offset += offset
 	s.typ = t
-	return s
+	return &s
 }
 
 // TODO(mbm): gotypes.Component handling for structs
diff --git a/operand/types.go b/operand/types.go
index 74f0b03..4e012a8 100644
--- a/operand/types.go
+++ b/operand/types.go
@@ -65,7 +65,7 @@ func (m Mem) Asm() string {
 type Imm uint64
 
 func (i Imm) Asm() string {
-	return fmt.Sprintf("%#x", i)
+	return fmt.Sprintf("$%#x", i)
 }
 
 // Rel is an offset relative to the instruction pointer.