diff --git a/src/fat_boot_sector.ml b/src/fat_boot_sector.ml
index 7d75a22..05ff86b 100644
--- a/src/fat_boot_sector.ml
+++ b/src/fat_boot_sector.ml
@@ -72,8 +72,8 @@ let marshal (buf: Cstruct.t) t =
   set_t_total_sectors_small buf 0; (* means use total_sectors_large *)
   set_t_media_descriptor buf fat_id;
   set_t_sectors_per_fat buf t.sectors_per_fat;
-  set_t_sectors_per_track buf 0; (* not used *)
-  set_t_heads buf 0; (* not used *)
+  set_t_sectors_per_track buf 63;
+  set_t_heads buf 255;
   set_t_hidden_preceeding_sectors buf t.hidden_preceeding_sectors;
   set_t_total_sectors_large buf t.total_sectors;
   (* no boot code yet *)