diff --git a/internal/alloc/alloc_unix.go b/internal/alloc/alloc_unix.go
index 157b544..203d847 100644
--- a/internal/alloc/alloc_unix.go
+++ b/internal/alloc/alloc_unix.go
@@ -9,24 +9,31 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-func NewMemory(_, max uint64) experimental.LinearMemory {
+func NewMemory(cap, max uint64) experimental.LinearMemory {
 	// Round up to the page size.
 	rnd := uint64(unix.Getpagesize() - 1)
-	max = (max + rnd) &^ rnd
+	res := (max + rnd) &^ rnd
 
-	if max > math.MaxInt {
-		// This ensures int(max) overflows to a negative value,
+	if res > math.MaxInt {
+		// This ensures int(res) overflows to a negative value,
 		// and unix.Mmap returns EINVAL.
-		max = math.MaxUint64
+		res = math.MaxUint64
 	}
 
-	// Reserve max bytes of address space, to ensure we won't need to move it.
+	com := res
+	prot := unix.PROT_READ | unix.PROT_WRITE
+	if cap < max { // Commit memory only if cap=max.
+		com = 0
+		prot = unix.PROT_NONE
+	}
+
+	// Reserve res bytes of address space, to ensure we won't need to move it.
 	// A protected, private, anonymous mapping should not commit memory.
-	b, err := unix.Mmap(-1, 0, int(max), unix.PROT_NONE, unix.MAP_PRIVATE|unix.MAP_ANON)
+	b, err := unix.Mmap(-1, 0, int(res), prot, unix.MAP_PRIVATE|unix.MAP_ANON)
 	if err != nil {
 		panic(err)
 	}
-	return &mmappedMemory{buf: b[:0]}
+	return &mmappedMemory{buf: b[:com]}
 }
 
 // The slice covers the entire mmapped memory:
@@ -52,8 +59,7 @@ func (m *mmappedMemory) Reallocate(size uint64) []byte {
 			return nil
 		}
 
-		// Update committed memory.
-		m.buf = m.buf[:new]
+		m.buf = m.buf[:new] // Update committed memory.
 	}
 	// Limit returned capacity because bytes beyond
 	// len(m.buf) have not yet been committed.
diff --git a/internal/alloc/alloc_windows.go b/internal/alloc/alloc_windows.go
index 2d753e2..ef83f01 100644
--- a/internal/alloc/alloc_windows.go
+++ b/internal/alloc/alloc_windows.go
@@ -9,20 +9,26 @@ import (
 	"golang.org/x/sys/windows"
 )
 
-func NewMemory(_, max uint64) experimental.LinearMemory {
+func NewMemory(cap, max uint64) experimental.LinearMemory {
 	// Round up to the page size.
 	rnd := uint64(windows.Getpagesize() - 1)
-	max = (max + rnd) &^ rnd
+	res := (max + rnd) &^ rnd
 
-	if max > math.MaxInt {
-		// This ensures uintptr(max) overflows to a large value,
+	if res > math.MaxInt {
+		// This ensures uintptr(res) overflows to a large value,
 		// and windows.VirtualAlloc returns an error.
-		max = math.MaxUint64
+		res = math.MaxUint64
 	}
 
-	// Reserve max bytes of address space, to ensure we won't need to move it.
-	// This does not commit memory.
-	r, err := windows.VirtualAlloc(0, uintptr(max), windows.MEM_RESERVE, windows.PAGE_READWRITE)
+	com := res
+	kind := windows.MEM_COMMIT
+	if cap < max { // Commit memory only if cap=max.
+		com = 0
+		kind = windows.MEM_RESERVE
+	}
+
+	// Reserve res bytes of address space, to ensure we won't need to move it.
+	r, err := windows.VirtualAlloc(0, uintptr(res), uint32(kind), windows.PAGE_READWRITE)
 	if err != nil {
 		panic(err)
 	}
@@ -30,8 +36,9 @@ func NewMemory(_, max uint64) experimental.LinearMemory {
 	mem := virtualMemory{addr: r}
 	// SliceHeader, although deprecated, avoids a go vet warning.
 	sh := (*reflect.SliceHeader)(unsafe.Pointer(&mem.buf))
-	sh.Cap = int(max)
 	sh.Data = r
+	sh.Len = int(com)
+	sh.Cap = int(res)
 	return &mem
 }
 
@@ -59,8 +66,7 @@ func (m *virtualMemory) Reallocate(size uint64) []byte {
 			return nil
 		}
 
-		// Update committed memory.
-		m.buf = m.buf[:new]
+		m.buf = m.buf[:new] // Update committed memory.
 	}
 	// Limit returned capacity because bytes beyond
 	// len(m.buf) have not yet been committed.