diff --git a/lib/disk/disk.go b/lib/disk/disk.go
index fc4e3c9..18f0143 100644
--- a/lib/disk/disk.go
+++ b/lib/disk/disk.go
@@ -34,14 +34,28 @@ var Dos33PhysicalToLogicalSectorMap = []byte{
 	0x0B, 0x03, 0x0A, 0x02, 0x09, 0x01, 0x08, 0x0F,
 }
 
+// ProDOSLogicalToPhysicalSectorMap maps logical sector numbers to pysical ones.
+// See [UtA2e 9-43 - Sectors vs. Blocks].
+var ProDOSLogicalToPhysicalSectorMap = []byte{
+	0x00, 0x02, 0x04, 0x06, 0x08, 0x0A, 0x0C, 0x0E,
+	0x01, 0x03, 0x05, 0x07, 0x09, 0x0B, 0x0D, 0x0F,
+}
+
+// ProDosPhysicalToLogicalSectorMap maps physical sector numbers to logical ones.
+// See [UtA2e 9-43 - Sectors vs. Blocks].
+var ProDosPhysicalToLogicalSectorMap = []byte{
+	0x00, 0x08, 0x01, 0x09, 0x02, 0x0A, 0x03, 0x0B,
+	0x04, 0x0C, 0x05, 0x0D, 0x06, 0x0E, 0x07, 0x0F,
+}
+
 // TrackSector is a pair of track/sector bytes.
 type TrackSector struct {
 	Track  byte
 	Sector byte
 }
 
-// SectorDisk is the interface use to read and write disks by physical
-// (matches sector header) sector number.
+// SectorDisk is the interface used to read and write disks by
+// physical (matches sector header) sector number.
 type SectorDisk interface {
 	// ReadPhysicalSector reads a single physical sector from the disk. It
 	// always returns 256 byes.
diff --git a/lib/prodos/prodos.go b/lib/prodos/prodos.go
new file mode 100644
index 0000000..2d00cca
--- /dev/null
+++ b/lib/prodos/prodos.go
@@ -0,0 +1,175 @@
+// Copyright © 2017 Zellyn Hunter <zellyn@gmail.com>
+
+// Package prodos contains routines for working with the on-disk
+// structures of Apple ProDOS.
+package prodos
+
+import (
+	"fmt"
+	"io"
+)
+
+// A single ProDOS block.
+type Block [512]byte
+
+// BlockDevice is the interface used to read and write devices by
+// logical block number.
+type BlockDevice interface {
+	// ReadBlock reads a single block from the device. It always returns
+	// 512 byes.
+	ReadBlock(index uint16) (Block, error)
+	// WriteBlock writes a single block to a device. It expects exactly
+	// 512 bytes.
+	WriteBlock(index uint16, data Block) error
+	// Blocks returns the number of blocks on the device.
+	Blocks() uint16
+	// Write writes the device contents to the given Writer.
+	Write(io.Writer) (int, error)
+}
+
+type VolumeBitMap []Block
+
+func NewVolumeBitMap(blocks uint16) VolumeBitMap {
+	vbm := VolumeBitMap(make([]Block, (blocks+(512*8)-1)/(512*8)))
+	for b := 0; b < int(blocks); b++ {
+		vbm.MarkUnused(uint16(b))
+	}
+	return vbm
+}
+
+func (vbm VolumeBitMap) MarkUsed(block uint16) {
+	vbm.mark(block, false)
+}
+
+func (vbm VolumeBitMap) MarkUnused(block uint16) {
+	vbm.mark(block, true)
+}
+
+func (vbm VolumeBitMap) mark(block uint16, set bool) {
+	byteIndex := block >> 3
+	blockIndex := byteIndex / 512
+	blockByteIndex := byteIndex % 512
+	bit := byte(1 << (7 - (block & 7)))
+	if set {
+		vbm[blockIndex][blockByteIndex] |= bit
+	} else {
+		vbm[blockIndex][blockByteIndex] &^= bit
+	}
+}
+
+func (vbm VolumeBitMap) Free(block uint16) bool {
+	byteIndex := block >> 3
+	blockIndex := byteIndex / 512
+	blockByteIndex := byteIndex % 512
+	bit := byte(1 << (7 - (block & 7)))
+	return vbm[blockIndex][blockByteIndex]&bit > 0
+}
+
+// ReadVolumeBitMap
+func ReadVolumeBitMap(bd BlockDevice, startBlock uint16) (VolumeBitMap, error) {
+	blocks := bd.Blocks() / 4096
+	vbm := make([]Block, blocks)
+	for i := uint16(0); i < blocks; i++ {
+		block, err := bd.ReadBlock(startBlock + i)
+		if err != nil {
+			return nil, fmt.Errorf("cannot read block %d of Volume Bit Map: %v", err)
+		}
+		vbm[i] = block
+	}
+	return VolumeBitMap(vbm), nil
+}
+
+// Write writes the Volume Bit Map to a block device, starting at the
+// given block.
+func (vbm VolumeBitMap) Write(bd BlockDevice, startBlock uint16) error {
+	for i, block := range vbm {
+		if err := bd.WriteBlock(startBlock+uint16(i), block); err != nil {
+			return fmt.Errorf("cannot write block %d of Volume Bit Map: %v", err)
+		}
+	}
+	return nil
+}
+
+type DateTime struct {
+	YMD [2]byte
+	HM  [2]byte
+}
+
+// VolumeDirectoryKeyBlock is the struct used to hold the ProDOS Volume Directory Key
+// Block structure.  See page 4-4 of Beneath Apple ProDOS.
+type VolumeDirectoryKeyBlock struct {
+	Prev        uint16 // Pointer to previous block (always zero: the KeyBlock is the first Volume Directory block
+	Next        uint16 // Pointer to next block in the Volume Directory
+	Header      VolumeDirectoryHeader
+	Descriptors [12]FileDescriptor
+}
+
+// VolumeDirectoryBlock is a normal (non-key) segment in the Volume Directory Header.
+type VolumeDirectoryBlock struct {
+	Prev        uint16 // Pointer to previous block in the Volume Directory.
+	Next        uint16 // Pointer to next block in the Volume Directory.
+	Descriptors [13]FileDescriptor
+}
+
+type VolumeDirectoryHeader struct {
+	TypeAndNameLength byte     // Storage type (top four bits) and volume name length (lower four).
+	VolumeName        [15]byte // Volume name (actual length defined in TypeAndNameLength)
+	Unused1           [8]byte
+	Creation          DateTime // Date and time volume was formatted
+	Version           byte
+	MinVersion        byte
+	Access            byte
+	EntryLength       byte   // Length of each entry in the Volume Directory: usually $27
+	EntriesPerBlock   byte   // Usually $0D
+	FileCount         uint16 // Number of active entries in the Volume Directory, not counting the Volume Directory Header
+	BitMapPointer     uint16 // Block number of start of VolumeBitMap. Usually 6
+	TotalBlocks       uint16 // Total number of blocks on the device. $118 (280) for a 35-track diskette.
+}
+
+type Access byte
+
+const (
+	AccessReadable           Access = 0x01
+	AccessWritable           Access = 0x02
+	AccessChangedSinceBackup Access = 0x20
+	AccessRenamable          Access = 0x40
+	AccessDestroyable        Access = 0x80
+)
+
+// FileDescriptor is the entry in the volume directory for a file or
+// subdirectory.
+type FileDescriptor struct {
+	TypeAndNameLength byte     // Storage type (top four bits) and volume name length (lower four)
+	FileName          [15]byte // Filename (actual length defined in TypeAndNameLength)
+	FileType          byte     // ProDOS / SOS filetype
+	KeyPointer        uint16   // block number of key block for file
+	BlocksUsed        uint16   // Total number of blocks used including index blocks and data blocks. For a subdirectory, the number of directory blocks
+	Eof               [3]byte  // 3-byte offset of EOF from first byte. For sequential files, just the length
+	Creation          DateTime // Date and time of of file creation
+	Version           byte
+	MinVersion        byte
+	Access            Access
+	// For TXT files, random access record length (L from OPEN)
+	// For BIN files, load address for binary image (A from BSAVE)
+	// For BAS files, load address for program image (when SAVEd)
+	// For VAR files, address of compressed variables image (when STOREd)
+	//  For SYS files, load address for system program (usually $2000)
+	AuxType       uint16
+	LastMod       DateTime
+	HeaderPointer uint16 // Block  number of the key block for the directory which describes this file.
+}
+
+// An index block contains 256 16-bit block numbers, pointing to other
+// blocks. The LSBs are stored in the first half, MSBs in the second.
+type IndexBlock Block
+
+// Get the blockNum'th block number from an index block.
+func (i IndexBlock) Get(blockNum byte) uint16 {
+	return uint16(i[blockNum]) + uint16(i[256+int(blockNum)])<<8
+}
+
+// Set the blockNum'th block number in an index block.
+func (i IndexBlock) Set(blockNum byte, block uint16) {
+	i[blockNum] = byte(block)
+	i[256+int(blockNum)] = byte(block >> 8)
+}
diff --git a/lib/prodos/prodos_test.go b/lib/prodos/prodos_test.go
new file mode 100644
index 0000000..c660cf8
--- /dev/null
+++ b/lib/prodos/prodos_test.go
@@ -0,0 +1 @@
+package prodos
diff --git a/lib/prodos/testdata/ProDOS_2_4_1.dsk b/lib/prodos/testdata/ProDOS_2_4_1.dsk
new file mode 100644
index 0000000..ef4e8b1
Binary files /dev/null and b/lib/prodos/testdata/ProDOS_2_4_1.dsk differ