diff --git a/mkimg/bin/dune b/mkimg/bin/dune
new file mode 100644
index 0000000..45431b5
--- /dev/null
+++ b/mkimg/bin/dune
@@ -0,0 +1,3 @@
+(executable
+ (name mkimg)
+ (libraries unix cstruct gptar gpt uuidm cmdliner))
diff --git a/mkimg/bin/mkimg.ml b/mkimg/bin/mkimg.ml
new file mode 100644
index 0000000..4592518
--- /dev/null
+++ b/mkimg/bin/mkimg.ml
@@ -0,0 +1,148 @@
+(* I just made these ones up... *)
+let tar_guid = Uuidm.of_string "53cd6812-46cc-474e-a141-30b3aed85f53" |> Option.get
+let cache_guid = Uuidm.of_string "22ab9cf5-6e51-45c2-998a-862e23aab264" |> Option.get
+let git_guid = Uuidm.of_string "30faa50a-4c9d-47ff-a1a5-ecfb3401c027" |> Option.get
+
+(* GPT uses a 72 byte utf16be encoded string for partition names *)
+let gptutf16be_of_ascii s =
+  String.init 72
+    (fun i ->
+       if i mod 2 = 0 && i / 2 < String.length s then
+         s.[i/2]
+       else
+         '\000')
+
+let jump dest sector_size size_sectors sectors_cache sectors_git =
+  let ( let* ) = Result.bind in
+  let* () =
+    if sector_size < 0 then Error "negative sector size"
+    else if size_sectors < 0L then Error "negative size"
+    else if sectors_cache < 0L then Error "negative cache size"
+    else if sectors_git < 0L then Error "negative git dump size"
+    else Ok ()
+  in
+  let* () =
+    if sector_size >= 512 && sector_size land (pred sector_size) == 0 then
+      Ok ()
+    else Error "sector size must be a power of two greater than or equal 512"
+  in
+  (* ocaml-gpt uses a fixed size partition entries table. Create an empty GPT
+     to figure out the first usable LBA *)
+  let empty =
+    Gpt.make ~sector_size ~disk_sectors:size_sectors []
+    |> Result.get_ok
+  in
+  let* () =
+    let ( + ) = Int64.add in
+    if size_sectors <
+       (* protective MBR + GPT header + GPT table *)
+       empty.first_usable_lba +
+       min 1L (Int64.of_int (2 * Tar.Header.length / sector_size)) + sectors_cache + sectors_cache + sectors_git
+       + 1L (* backup GPT header *) then
+      Error "too small size"
+    else Ok ()
+  in
+  (* TODO: handle exceptions *)
+  let fd = Unix.openfile dest Unix.[ O_WRONLY; O_CREAT ] 0o664 in
+  Unix.ftruncate fd (sector_size * Int64.to_int size_sectors);
+  let gpt =
+    let partitions =
+      (* Current implementation of [Gpt.Partition.make] only returns [Ok _] or
+         raises [Invalid_argument _] :/ *)
+      let attributes = 1L in
+      let sha512s =
+        Gpt.Partition.make
+          ~name:(gptutf16be_of_ascii "sha512s")
+          ~type_guid:cache_guid
+          ~attributes
+          Int64.(succ (sub empty.last_usable_lba sectors_cache))
+          empty.last_usable_lba
+        |> Result.get_ok
+      in
+      let md5s =
+        Gpt.Partition.make
+          ~name:(gptutf16be_of_ascii "md5s")
+          ~type_guid:cache_guid
+          ~attributes
+          (Int64.sub sha512s.starting_lba sectors_cache)
+          (Int64.pred sha512s.starting_lba)
+        |> Result.get_ok
+      in
+      let git_dump =
+        Gpt.Partition.make
+          ~name:(gptutf16be_of_ascii "git_dump")
+          ~type_guid:git_guid
+          ~attributes
+          (Int64.sub md5s.starting_lba sectors_git)
+          (Int64.pred md5s.starting_lba)
+        |> Result.get_ok
+      in
+      let tar =
+        Gpt.Partition.make
+          ~name:(gptutf16be_of_ascii "tar")
+          ~type_guid:tar_guid
+          ~attributes
+          empty.first_usable_lba
+          (Int64.pred git_dump.starting_lba)
+        |> Result.get_ok
+      in
+      [ tar; git_dump; md5s; sha512s ]
+    in
+    Gpt.make ~sector_size ~disk_sectors:size_sectors partitions
+    |> Result.get_ok
+  in
+  let buf =
+    Cstruct.create (sector_size * (Int64.to_int gpt.first_usable_lba + 2 * Tar.Header.length))
+  in
+  Gptar.marshal_header ~sector_size buf gpt;
+  Gpt.marshal_partition_table ~sector_size
+    (Cstruct.shift buf (sector_size * Int64.to_int gpt.partition_entry_lba))
+    gpt;
+  let s = Cstruct.to_string buf in
+  ignore (Unix.write_substring fd s 0 (String.length s));
+  ignore (Unix.lseek fd (Int64.to_int gpt.backup_lba * sector_size) Unix.SEEK_SET);
+  (* Let's reuse the buffer *)
+  let buf = Cstruct.sub buf 0 sector_size in
+  Cstruct.memset buf 0;
+  Gpt.marshal_header ~sector_size ~primary:false buf gpt;
+  let s = Cstruct.to_string buf in
+  ignore (Unix.write_substring fd s 0 (String.length s));
+  Unix.close fd;
+  Ok ()
+
+open Cmdliner
+
+let dest =
+  Arg.(required & pos 0 (some string) None &
+       info ~docv:"DEST" [])
+
+let sector_size =
+  let doc = "Sector size or block size to use" in
+  (* TODO: should be a power of two >= 512 *)
+  Arg.(value & opt int 512 &
+       info ~doc ~docv:"SECTOR-SIZE" ["sector-size"])
+
+let size_sectors =
+  let doc = "Size of disk image in terms of sectors" in
+  Arg.(value & opt int64 (Int64.mul 1024L 2048L) &
+       info ~doc ~docv:"SIZE-SECTORS" ["size-sectors"])
+
+let sectors_cache =
+  let doc = "Number of sectors reserved for each checksum cache (md5, sha512)." in
+  Arg.(value & opt int64 (Int64.mul 4L 2048L) &
+       info ~doc ~docv:"SECTORS-CACHE" ["sectors-cache"])
+
+let sectors_git =
+  let doc = "Number of sectors reserved for git dump." in
+  Arg.(value & opt int64 (Int64.mul 40L 2048L) &
+       info ~doc ~docv:"SECTORS-GIT" ["sectors-git"])
+
+let command =
+  let info =
+    Cmd.info "mkimg"
+  in
+  Cmd.v info
+    Term.(const jump $ dest $ sector_size $ size_sectors $ sectors_cache $ sectors_git)
+
+let () =
+  exit (Cmdliner.Cmd.eval_result command)
diff --git a/mkimg/dune-project b/mkimg/dune-project
new file mode 100644
index 0000000..d6ad5c6
--- /dev/null
+++ b/mkimg/dune-project
@@ -0,0 +1,2 @@
+(lang dune 3.14)
+(name mkimg)