diff --git a/osbase/build/mkimage/osimage/BUILD.bazel b/osbase/build/mkimage/osimage/BUILD.bazel
new file mode 100644
index 0000000..cfcf096
--- /dev/null
+++ b/osbase/build/mkimage/osimage/BUILD.bazel
@@ -0,0 +1,15 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "osimage",
+    srcs = ["osimage.go"],
+    importpath = "source.monogon.dev/osbase/build/mkimage/osimage",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//osbase/blockdev",
+        "//osbase/efivarfs",
+        "//osbase/fat32",
+        "//osbase/gpt",
+        "@com_github_google_uuid//:uuid",
+    ],
+)
diff --git a/osbase/build/mkimage/osimage/osimage.go b/osbase/build/mkimage/osimage/osimage.go
new file mode 100644
index 0000000..d139d6b
--- /dev/null
+++ b/osbase/build/mkimage/osimage/osimage.go
@@ -0,0 +1,237 @@
+// Copyright 2020 The Monogon Project Authors.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// This package provides self-contained implementation used to generate
+// Metropolis disk images.
+package osimage
+
+import (
+	"fmt"
+	"io"
+	"strings"
+
+	"github.com/google/uuid"
+
+	"source.monogon.dev/osbase/blockdev"
+	"source.monogon.dev/osbase/efivarfs"
+	"source.monogon.dev/osbase/fat32"
+	"source.monogon.dev/osbase/gpt"
+)
+
+var (
+	SystemAType = uuid.MustParse("ee96054b-f6d0-4267-aaaa-724b2afea74c")
+	SystemBType = uuid.MustParse("ee96054b-f6d0-4267-bbbb-724b2afea74c")
+
+	DataType = uuid.MustParse("9eeec464-6885-414a-b278-4305c51f7966")
+)
+
+const (
+	SystemALabel = "METROPOLIS-SYSTEM-A"
+	SystemBLabel = "METROPOLIS-SYSTEM-B"
+	DataLabel    = "METROPOLIS-NODE-DATA"
+	ESPLabel     = "ESP"
+
+	EFIPayloadPath = "/EFI/BOOT/BOOTx64.EFI"
+	EFIBootAPath   = "/EFI/metropolis/boot-a.efi"
+	EFIBootBPath   = "/EFI/metropolis/boot-b.efi"
+	nodeParamsPath = "metropolis/parameters.pb"
+)
+
+// PartitionSizeInfo contains parameters used during partition table
+// initialization and, in case of image files, space allocation.
+type PartitionSizeInfo struct {
+	// Size of the EFI System Partition (ESP), in mebibytes. The size must
+	// not be zero.
+	ESP int64
+	// Size of the Metropolis system partition, in mebibytes. The partition
+	// won't be created if the size is zero.
+	System int64
+	// Size of the Metropolis data partition, in mebibytes. The partition
+	// won't be created if the size is zero. If the image is output to a
+	// block device, the partition will be extended to fill the remaining
+	// space.
+	Data int64
+}
+
+// Params contains parameters used by Plan or Write to build a Metropolis OS
+// image.
+type Params struct {
+	// Output is the block device to which the OS image is written.
+	Output blockdev.BlockDev
+	// ABLoader provides the A/B loader which then loads the EFI loader for the
+	// correct slot.
+	ABLoader fat32.SizedReader
+	// EFIPayload provides contents of the EFI payload file. It must not be
+	// nil. This gets put into boot slot A.
+	EFIPayload fat32.SizedReader
+	// SystemImage provides contents of the Metropolis system partition.
+	// If nil, no contents will be copied into the partition.
+	SystemImage io.Reader
+	// NodeParameters provides contents of the node parameters file. If nil,
+	// the node parameters file won't be created in the target ESP
+	// filesystem.
+	NodeParameters fat32.SizedReader
+	// DiskGUID is a unique identifier of the image and a part of Table
+	// header. It's optional and can be left blank if the identifier is
+	// to be randomly generated. Setting it to a predetermined value can
+	// help in implementing reproducible builds.
+	DiskGUID uuid.UUID
+	// PartitionSize specifies a size for the ESP, Metropolis System and
+	// Metropolis data partition.
+	PartitionSize PartitionSizeInfo
+}
+
+type plan struct {
+	*Params
+	rootInode        fat32.Inode
+	tbl              *gpt.Table
+	efiPartition     *gpt.Partition
+	systemPartitionA *gpt.Partition
+	systemPartitionB *gpt.Partition
+	dataPartition    *gpt.Partition
+}
+
+// Apply actually writes the planned installation to the blockdevice.
+func (i *plan) Apply() (*efivarfs.LoadOption, error) {
+	// Discard the entire device, we're going to write new data over it.
+	// Ignore errors, this is only advisory.
+	i.Output.Discard(0, i.Output.BlockCount()*i.Output.BlockSize())
+
+	if err := fat32.WriteFS(blockdev.NewRWS(i.efiPartition), i.rootInode, fat32.Options{
+		BlockSize:  uint16(i.efiPartition.BlockSize()),
+		BlockCount: uint32(i.efiPartition.BlockCount()),
+		Label:      "MNGN_BOOT",
+	}); err != nil {
+		return nil, fmt.Errorf("failed to write FAT32: %w", err)
+	}
+
+	if _, err := io.Copy(blockdev.NewRWS(i.systemPartitionA), i.SystemImage); err != nil {
+		return nil, fmt.Errorf("failed to write system partition A: %w", err)
+	}
+
+	if err := i.tbl.Write(); err != nil {
+		return nil, fmt.Errorf("failed to write Table: %w", err)
+	}
+
+	// Build an EFI boot entry pointing to the image's ESP.
+	return &efivarfs.LoadOption{
+		Description: "Metropolis",
+		FilePath: efivarfs.DevicePath{
+			&efivarfs.HardDrivePath{
+				PartitionNumber:     1,
+				PartitionStartBlock: i.efiPartition.FirstBlock,
+				PartitionSizeBlocks: i.efiPartition.SizeBlocks(),
+				PartitionMatch: efivarfs.PartitionGPT{
+					PartitionUUID: i.efiPartition.ID,
+				},
+			},
+			efivarfs.FilePath(EFIPayloadPath),
+		},
+	}, nil
+}
+
+// Plan allows to prepare an installation without modifying any data on the
+// system. To apply the planned installation, call Apply on the returned plan.
+func Plan(p *Params) (*plan, error) {
+	params := &plan{Params: p}
+
+	var err error
+	params.tbl, err = gpt.New(params.Output)
+	if err != nil {
+		return nil, fmt.Errorf("invalid block device: %w", err)
+	}
+
+	params.tbl.ID = params.DiskGUID
+	params.efiPartition = &gpt.Partition{
+		Type: gpt.PartitionTypeEFISystem,
+		Name: ESPLabel,
+	}
+
+	if err := params.tbl.AddPartition(params.efiPartition, params.PartitionSize.ESP*Mi); err != nil {
+		return nil, fmt.Errorf("failed to allocate ESP: %w", err)
+	}
+
+	params.rootInode = fat32.Inode{
+		Attrs: fat32.AttrDirectory,
+	}
+	if err := params.rootInode.PlaceFile(strings.TrimPrefix(EFIBootAPath, "/"), params.EFIPayload); err != nil {
+		return nil, err
+	}
+	// Place the A/B loader at the EFI bootloader autodiscovery path.
+	if err := params.rootInode.PlaceFile(strings.TrimPrefix(EFIPayloadPath, "/"), params.ABLoader); err != nil {
+		return nil, err
+	}
+	if params.NodeParameters != nil {
+		if err := params.rootInode.PlaceFile(nodeParamsPath, params.NodeParameters); err != nil {
+			return nil, err
+		}
+	}
+
+	// Try to layout the fat32 partition. If it detects that the disk is too
+	// small, an error will be returned.
+	if _, err := fat32.SizeFS(params.rootInode, fat32.Options{
+		BlockSize:  uint16(params.efiPartition.BlockSize()),
+		BlockCount: uint32(params.efiPartition.BlockCount()),
+		Label:      "MNGN_BOOT",
+	}); err != nil {
+		return nil, fmt.Errorf("failed to calculate size of FAT32: %w", err)
+	}
+
+	// Create the system partition only if its size is specified.
+	if params.PartitionSize.System != 0 && params.SystemImage != nil {
+		params.systemPartitionA = &gpt.Partition{
+			Type: SystemAType,
+			Name: SystemALabel,
+		}
+		if err := params.tbl.AddPartition(params.systemPartitionA, params.PartitionSize.System*Mi); err != nil {
+			return nil, fmt.Errorf("failed to allocate system partition A: %w", err)
+		}
+		params.systemPartitionB = &gpt.Partition{
+			Type: SystemBType,
+			Name: SystemBLabel,
+		}
+		if err := params.tbl.AddPartition(params.systemPartitionB, params.PartitionSize.System*Mi); err != nil {
+			return nil, fmt.Errorf("failed to allocate system partition B: %w", err)
+		}
+	} else if params.PartitionSize.System == 0 && params.SystemImage != nil {
+		// Safeguard against contradicting parameters.
+		return nil, fmt.Errorf("the system image parameter was passed while the associated partition size is zero")
+	}
+	// Create the data partition only if its size is specified.
+	if params.PartitionSize.Data != 0 {
+		params.dataPartition = &gpt.Partition{
+			Type: DataType,
+			Name: DataLabel,
+		}
+		if err := params.tbl.AddPartition(params.dataPartition, -1); err != nil {
+			return nil, fmt.Errorf("failed to allocate data partition: %w", err)
+		}
+	}
+
+	return params, nil
+}
+
+const Mi = 1024 * 1024
+
+// Write writes a Metropolis OS image to a block device.
+func Write(params *Params) (*efivarfs.LoadOption, error) {
+	p, err := Plan(params)
+	if err != nil {
+		return nil, err
+	}
+
+	return p.Apply()
+}