diff --git a/lib/dos33/dos33.go b/lib/dos33/dos33.go
new file mode 100644
index 0000000..6239bb6
--- /dev/null
+++ b/lib/dos33/dos33.go
@@ -0,0 +1,228 @@
+// Copyright © 2016 Zellyn Hunter <zellyn@gmail.com>
+
+// Package dos33 contains routines for working with the on-disk structures of DOS 3.3.
+package dos33
+
+import (
+	"encoding/binary"
+	"fmt"
+)
+
+type FreeSectorMap [4]byte // Bit map of free sectors in a track
+
+// VTOC is the struct used to hold the DOS 3.3 VTOC structure.
+// See page 4-2 of Beneath Apple DOS.
+type VTOC struct {
+	Unused1       byte     // Not used
+	CatalogTrack  byte     // Track number of first catalog sector
+	CatalogSector byte     // Sector number of first catalog sector
+	DOSRelease    byte     // Release number of DOS used to INIT this diskette
+	Unused2       [2]byte  // Not used
+	Volume        byte     // Diskette volume number (1-254)
+	Unused3       [32]byte // Not used
+	// Maximum number of track/secotr pairs which will fit in one file
+	// track/sector list sector (122 for 256 byte sectors)
+	TrackSectorListMaxSize byte
+	Unused4                [8]byte // Not used
+	LastTrack              byte    // Last track where sectors were allocated
+	TrackDirection         int8    // Direction of track allocation (+1 or -1)
+	Unused5                [2]byte
+	NumTracks              byte   // Number of tracks per diskette (normally 35)
+	NumSectors             byte   // Number of sectors per track (13 or 16)
+	BytesPerSector         uint16 // Number of bytes per sector (LO/HI format)
+	FreeSectors            [50]FreeSectorMap
+}
+
+// MarshalBinary marshals the VTOC sector to bytes. Error is always nil.
+func (v VTOC) MarshalBinary() (data []byte, err error) {
+	buf := make([]byte, 256)
+	buf[0x00] = v.Unused1
+	buf[0x01] = v.CatalogTrack
+	buf[0x02] = v.CatalogSector
+	buf[0x03] = v.DOSRelease
+	copyBytes(buf[0x04:0x06], v.Unused2[:])
+	buf[0x06] = v.Volume
+	copyBytes(buf[0x07:0x27], v.Unused3[:])
+	buf[0x27] = v.TrackSectorListMaxSize
+	copyBytes(buf[0x28:0x30], v.Unused4[:])
+	buf[0x30] = v.LastTrack
+	buf[0x31] = byte(v.TrackDirection)
+	copyBytes(buf[0x32:0x34], v.Unused5[:])
+	buf[0x34] = v.NumTracks
+	buf[0x35] = v.NumSectors
+	binary.LittleEndian.PutUint16(buf[0x36:0x38], v.BytesPerSector)
+	for i, m := range v.FreeSectors {
+		copyBytes(buf[0x38+4*i:0x38+4*i+4], m[:])
+	}
+	return buf, nil
+}
+
+// copyBytes is just like the builtin copy, but just for byte slices,
+// and it checks that dst and src have the same length.
+func copyBytes(dst, src []byte) int {
+	if len(dst) != len(src) {
+		panic(fmt.Sprintf("copyBytes called with differing lengths %d and %d", len(dst), len(src)))
+	}
+	return copy(dst, src)
+}
+
+// UnmarshalBinary unmarshals the VTOC sector from bytes. Input is
+// expected to be exactly 256 bytes.
+func (v *VTOC) UnmarshalBinary(data []byte) error {
+	if len(data) != 256 {
+		return fmt.Errorf("VTOC.UnmarshalBinary expects exactly 256 bytes; got %d", len(data))
+	}
+
+	v.Unused1 = data[0x00]
+	v.CatalogTrack = data[0x01]
+	v.CatalogSector = data[0x02]
+	v.DOSRelease = data[0x03]
+	copyBytes(v.Unused2[:], data[0x04:0x06])
+	v.Volume = data[0x06]
+	copyBytes(v.Unused3[:], data[0x07:0x27])
+	v.TrackSectorListMaxSize = data[0x27]
+	copyBytes(v.Unused4[:], data[0x28:0x30])
+	v.LastTrack = data[0x30]
+	v.TrackDirection = int8(data[0x31])
+	copyBytes(v.Unused5[:], data[0x32:0x34])
+	v.NumTracks = data[0x34]
+	v.NumSectors = data[0x35]
+	v.BytesPerSector = binary.LittleEndian.Uint16(data[0x36:0x38])
+	for i := range v.FreeSectors {
+		copyBytes(v.FreeSectors[i][:], data[0x38+4*i:0x38+4*i+4])
+	}
+
+	return nil
+}
+
+func DefaultVTOC() VTOC {
+	v := VTOC{
+		CatalogTrack:           0x11,
+		CatalogSector:          0x0f,
+		DOSRelease:             0x03,
+		Volume:                 0x01,
+		TrackSectorListMaxSize: 122,
+		LastTrack:              0x00, // TODO(zellyn): what should this be?
+		TrackDirection:         1,
+		NumTracks:              0x23,
+		NumSectors:             0x10,
+		BytesPerSector:         0x100,
+	}
+	for i := range v.FreeSectors {
+		v.FreeSectors[i] = FreeSectorMap{}
+		if i < 35 {
+			v.FreeSectors[i] = FreeSectorMap([4]byte{0xff, 0xff, 0x00, 0x00})
+		}
+	}
+	return v
+}
+
+// CatalogSector is the struct used to hold the DOS 3.3 Catalog
+// sector.
+type CatalogSector struct {
+	Unused1    byte        // Not used
+	NextTrack  byte        // Track number of next catalog sector (usually 11 hex)
+	NextSector byte        // Sector number of next catalog sector
+	Unused2    [8]byte     // Not used
+	FileDescs  [7]FileDesc // File descriptive entries
+}
+
+// MarshalBinary marshals the CatalogSector to bytes. Error is always nil.
+func (cs CatalogSector) MarshalBinary() (data []byte, err error) {
+	buf := make([]byte, 256)
+	buf[0x00] = cs.Unused1
+	buf[0x01] = cs.NextTrack
+	buf[0x02] = cs.NextSector
+	copyBytes(buf[0x03:0x0b], cs.Unused2[:])
+	for i, fd := range cs.FileDescs {
+		fdBytes, _ := fd.MarshalBinary()
+		copyBytes(buf[0x0b+35*i:0x0b+35*(i+1)], fdBytes)
+	}
+	return buf, nil
+}
+
+// UnmarshalBinary unmarshals the CatalogSector from bytes. Input is
+// expected to be exactly 256 bytes.
+func (cs *CatalogSector) UnmarshalBinary(data []byte) error {
+	if len(data) != 256 {
+		return fmt.Errorf("CatalogSector.UnmarshalBinary expects exactly 256 bytes; got %d", len(data))
+	}
+
+	cs.Unused1 = data[0x00]
+	cs.NextTrack = data[0x01]
+	cs.NextSector = data[0x02]
+	copyBytes(cs.Unused2[:], data[0x03:0x0b])
+
+	for i := range cs.FileDescs {
+		if err := cs.FileDescs[i].UnmarshalBinary(data[0x0b+35*i : 0x0b+35*(i+1)]); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+type Filetype byte
+
+const (
+	// Hex 80+file type - file is locked
+	// Hex 00+file type - file is not locked
+	FiletypeLocked Filetype = 0x80
+
+	FileTypeText        Filetype = 0x00 // Text file
+	FileTypeInteger     Filetype = 0x01 // INTEGER BASIC file
+	FileTypeApplesoft   Filetype = 0x02 // APPLESOFT BASIC file
+	FileTypeBinary      Filetype = 0x04 // BINARY file
+	FileTypeS           Filetype = 0x08 // S type file
+	FileTypeRelocatable Filetype = 0x10 // RELOCATABLE object module file
+	FileTypeA           Filetype = 0x20 // A type file
+	FileTypeB           Filetype = 0x40 // B type file
+)
+
+// FileDesc is the struct used to represent the DOS 3.3 File
+// Descriptive entry.
+type FileDesc struct {
+	// Track of first track/sector list sector. If this is a deleted
+	// file, this byte contains a hex FF and the original track number
+	// is copied to the last byte of the file name field (BYTE 20). If
+	// this byte contains a hex 00, the entry is assumed to never have
+	// been used and is available for use. (This means track 0 can never
+	// be used for data even if the DOS image is "wiped" from the
+	// diskette.)
+	TrackSectorListTrack  byte
+	TrackSectorListSector byte     // Sector of first track/sector list sector
+	Filetype              Filetype // File type and flags
+	Filename              [30]byte // File name (30 characters) Length of file in
+	// sectors (LO/HI format). The CATALOG command will only format the
+	// LO byte of this length giving 1-255 but a full 65,535 may be
+	// stored here.
+	SectorCount uint16
+}
+
+// MarshalBinary marshals the FileDesc to bytes. Error is always nil.
+func (fd FileDesc) MarshalBinary() (data []byte, err error) {
+	buf := make([]byte, 35)
+	buf[0x00] = fd.TrackSectorListTrack
+	buf[0x01] = fd.TrackSectorListSector
+	buf[0x02] = byte(fd.Filetype)
+	copyBytes(buf[0x03:0x21], fd.Filename[:])
+	binary.LittleEndian.PutUint16(buf[0x21:0x23], fd.SectorCount)
+
+	return buf, nil
+}
+
+// UnmarshalBinary unmarshals the FileDesc from bytes. Input is
+// expected to be exactly 35 bytes.
+func (fd *FileDesc) UnmarshalBinary(data []byte) error {
+	if len(data) != 35 {
+		return fmt.Errorf("FileDesc.UnmarshalBinary expects exactly 35 bytes; got %d", len(data))
+	}
+
+	fd.TrackSectorListTrack = data[0x00]
+	fd.TrackSectorListSector = data[0x01]
+	fd.Filetype = Filetype(data[0x02])
+	copyBytes(data[0x03:0x21], fd.Filename[:])
+	fd.SectorCount = binary.LittleEndian.Uint16(data[0x21:0x23])
+
+	return nil
+}
diff --git a/lib/dos33/dos33_test.go b/lib/dos33/dos33_test.go
new file mode 100644
index 0000000..6d6c2fa
--- /dev/null
+++ b/lib/dos33/dos33_test.go
@@ -0,0 +1,49 @@
+package dos33
+
+import (
+	"crypto/rand"
+	"reflect"
+	"testing"
+)
+
+// TestVTOCMarshalRoundtrip checks a simple roundtrip of VTOC data.
+func TestVTOCMarshalRoundtrip(t *testing.T) {
+	buf := make([]byte, 256)
+	rand.Read(buf)
+	vtoc1 := &VTOC{}
+	if err := vtoc1.UnmarshalBinary(buf); err != nil {
+		t.Fatal(err)
+	}
+	buf2, _ := vtoc1.MarshalBinary()
+	if !reflect.DeepEqual(buf, buf2) {
+		t.Errorf("Buffers differ: %v != %v", buf, buf2)
+	}
+	vtoc2 := &VTOC{}
+	if err := vtoc2.UnmarshalBinary(buf2); err != nil {
+		t.Fatal(err)
+	}
+	if *vtoc1 != *vtoc2 {
+		t.Errorf("Structs differ: %v != %v", vtoc1, vtoc2)
+	}
+}
+
+// TestCatalogSectorMarshalRoundtrip checks a simple roundtrip of CatalogSector data.
+func TestCatalogSectorMarshalRoundtrip(t *testing.T) {
+	buf := make([]byte, 256)
+	rand.Read(buf)
+	cs1 := &CatalogSector{}
+	if err := cs1.UnmarshalBinary(buf); err != nil {
+		t.Fatal(err)
+	}
+	buf2, _ := cs1.MarshalBinary()
+	if !reflect.DeepEqual(buf, buf2) {
+		t.Errorf("Buffers differ: %v != %v", buf, buf2)
+	}
+	cs2 := &CatalogSector{}
+	if err := cs2.UnmarshalBinary(buf2); err != nil {
+		t.Fatal(err)
+	}
+	if *cs1 != *cs2 {
+		t.Errorf("Structs differ: %v != %v", cs1, cs2)
+	}
+}