<pre>
$ go build -o x.exe x.go
$ go tool objdump -s main.main x.exe
TEXT main.main(SB) /tmp/x.go
  x.go:3		0x10501c0		65488b0c2530000000	MOVQ GS:0x30, CX
  x.go:3		0x10501c9		483b6110		CMPQ 0x10(CX), SP
  x.go:3		0x10501cd		7634			JBE 0x1050203
  x.go:3		0x10501cf		4883ec10		SUBQ $0x10, SP
  x.go:3		0x10501d3		48896c2408		MOVQ BP, 0x8(SP)
  x.go:3		0x10501d8		488d6c2408		LEAQ 0x8(SP), BP

			prefix = p.parseString(ustar.prefix())

			// For Format detection, check if block is properly formatted since
			// the parser is more liberal than what USTAR actually permits.
			notASCII := func(r rune) bool { return r >= 0x80 }
			if bytes.IndexFunc(tr.blk[:], notASCII) >= 0 {
				hdr.Format = FormatUnknown // Non-ASCII characters in block.
			}
			nul := func(b []byte) bool { return int(b[len(b)-1]) == 0 }

	LDAXRB (R19), R16                          // 70fe5f08
	LDAXRH (R5), R8                            // a8fc5f48
	//TODO LDNP 0xcc(RSP), ZR, R12             // ecff5928
	//TODO LDNP 0x40(R28), R9, R5              // 852744a8
	//TODO LDPSW -0xd0(R2), R0, R12            // 4c00e668
	//TODO LDPSW 0x5c(R4), R8, R5              // 85a0cb69
	//TODO LDPSW 0x6c(R12), R2, R27            // 9b894d69

			Devminor: 0,
			Format:   FormatGNU,
		}, {
			Name:     "sparse-posix-0.0",
			Mode:     420,
			Uid:      1000,
			Gid:      1000,
			Size:     200,
			ModTime:  time.Unix(1392342187, 0),
			Typeflag: 0x30,
			Linkname: "",
			Uname:    "david",
			Gname:    "david",
			Devmajor: 0,
			Devminor: 0,
			PAXRecords: map[string]string{
				"GNU.sparse.size":      "200",
				"GNU.sparse.numblocks": "95",

	s_IFLNK  = 0xa000
	s_IFREG  = 0x8000
	s_IFBLK  = 0x6000
	s_IFDIR  = 0x4000
	s_IFCHR  = 0x2000
	s_IFIFO  = 0x1000
	s_ISUID  = 0x800
	s_ISGID  = 0x400
	s_ISVTX  = 0x200

	msdosDir      = 0x10
	msdosReadOnly = 0x01
)

// Mode returns the permission and mode bits for the [FileHeader].
func (h *FileHeader) Mode() (mode fs.FileMode) {
	switch h.CreatorVersion >> 8 {
	case creatorUnix, creatorMacOSX:

			// Argsize set below.
		},
	}
	nameAddr.Sym.Func().Text = prog
	prog.To.Val = int32(argSize)
	p.append(prog, "", true)
}

// asmData assembles a DATA pseudo-op.
// DATA masks<>+0x00(SB)/4, $0x00000000
func (p *Parser) asmData(operands [][]lex.Token) {
	if len(operands) != 2 {
		p.errorf("expect two operands for DATA")
		return
	}

	// Operand 0 has the general form foo<>+0x04(SB)/4.

		tw := NewWriter(new(bytes.Buffer))
		hdr := &Header{Name: "dir/", Typeflag: TypeDir}
		if err := tw.WriteHeader(hdr); err != nil {
			t.Fatalf("WriteHeader() = %v, want nil", err)
		}
		if _, err := tw.Write([]byte{0x00}); err != ErrWriteTooLong {
			t.Fatalf("Write() = %v, want %v", err, ErrWriteTooLong)
		}
	})

	t.Run("NegativeSize", func(t *testing.T) {
		tw := NewWriter(new(bytes.Buffer))

		// subset of CP-437, which itself is ASCII-like.
		//
		// Forbid 0x7e and 0x5c since EUC-KR and Shift-JIS replace those
		// characters with localized currency and overline characters.
		if r < 0x20 || r > 0x7d || r == 0x5c {
			if !utf8.ValidRune(r) || (r == utf8.RuneError && size == 1) {
				return false, false
			}
			require = true
		}
	}
	return true, require
}