diff --git a/osbase/build/mkimage/main.go b/osbase/build/mkimage/main.go
index 37691a9..5e2b082 100644
--- a/osbase/build/mkimage/main.go
+++ b/osbase/build/mkimage/main.go
@@ -40,17 +40,19 @@
 func main() {
 	// Fill in the image parameters based on flags.
 	var (
-		efiPayload      string
-		systemImage     string
-		abLoaderPayload string
-		nodeParams      string
-		outputPath      string
-		diskUUID        string
-		cfg             osimage.Params
+		efiPayload          string
+		systemImage         string
+		abLoaderPayload     string
+		biosBootCodePayload string
+		nodeParams          string
+		outputPath          string
+		diskUUID            string
+		cfg                 osimage.Params
 	)
 	flag.StringVar(&efiPayload, "efi", "", "Path to the UEFI payload used")
 	flag.StringVar(&systemImage, "system", "", "Path to the system partition image used")
 	flag.StringVar(&abLoaderPayload, "abloader", "", "Path to the abloader payload used")
+	flag.StringVar(&biosBootCodePayload, "bios_bootcode", "", "Optional path to the BIOS bootcode which gets placed at the start of the first block of the image. Limited to 440 bytes, padding is not required. It is only used by legacy BIOS boot.")
 	flag.StringVar(&nodeParams, "node_parameters", "", "Path to Node Parameters to be written to the ESP (default: don't write Node Parameters)")
 	flag.StringVar(&outputPath, "out", "", "Path to the resulting disk image or block device")
 	flag.Int64Var(&cfg.PartitionSize.Data, "data_partition_size", 2048, "Override the data partition size (default 2048 MiB). Used only when generating image files.")
@@ -95,6 +97,14 @@
 		cfg.NodeParameters = np
 	}
 
+	if biosBootCodePayload != "" {
+		bp, err := os.ReadFile(biosBootCodePayload)
+		if err != nil {
+			log.Fatalf("while opening BIOS bootcode at %q: %v", biosBootCodePayload, err)
+		}
+		cfg.BIOSBootCode = bp
+	}
+
 	// TODO(#254): Build and use dynamically-grown block devices
 	cfg.Output, err = blockdev.CreateFile(outputPath, 512, 10*1024*1024)
 	if err != nil {