diff --git a/Makefile b/Makefile
index 552b87b..50b3bb2 100644
--- a/Makefile
+++ b/Makefile
@@ -39,6 +39,7 @@ default: fdimage.img
 
 # Host utils
 utils/%: src/utils/%.c
+	mkdir -p utils
 	$(CC) -o $@ $<
 
 # Boot sectors
@@ -46,8 +47,8 @@ boot/%.bs: boot/%.asm
 	$(NASM) $(NASM_ARGS) -DFLOPPY=$(FLOPPY) -o $@ $<
 
 # BIOS option roms
-rom/%.rom: rom/%.asm
-	$(NASM) $(NASM_ARGS) -o $@ $< && utils/fix-rom.sh $@
+rom/%.rom: rom/%.asm utils/fix-rom
+	$(NASM) $(NASM_ARGS) -o $@ $< && utils/fix-rom $@
 
 # Regular COM programs
 com/%.com: com/%.asm
@@ -70,7 +71,8 @@ hdimage.img: boot/mbr.bs fdimage.img
 
 clean:
 	find -name '*.lst' -delete
-	rm -f $$(cat .gitignore) boot/*.bs com/*.com rom/*.rom utils/emul
+	rm -f $$(cat .gitignore) boot/*.bs com/*.com rom/*.rom utils/*
+	rmdir utils
 
 qemu-floppy: fdimage.img $(ROMS)
 	$(QEMU) $(QEMU_ARGS) -boot a -fda fdimage.img
diff --git a/src/utils/fix-rom.c b/src/utils/fix-rom.c
new file mode 100644
index 0000000..05cfc83
--- /dev/null
+++ b/src/utils/fix-rom.c
@@ -0,0 +1,36 @@
+#include <stdio.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <stdlib.h>
+
+struct stat sbuf;
+
+int main(int argc, char** argv) {
+  FILE* fd = fopen(argv[1], "r+");
+  fstat(fileno(fd), &sbuf);
+  if (sbuf.st_size & 0x1F) {
+    fprintf(stderr, "Filesize is not a multiple of 512 bytes\n");
+    exit(1);
+  }
+
+  // Fill out filesize flag
+  fseek(fd, 2, SEEK_SET);
+  fputc(sbuf.st_size >> 9, fd);
+
+  // Calculate checksum
+  fseek(fd, 0, SEEK_SET);
+  off_t i;
+  uint8_t s;
+  for (i=0; i<sbuf.st_size; i++) {
+    s+=fgetc(fd);
+  }
+
+  // Edit last byte so that checksum is 0
+  fseek(fd, -1, SEEK_END);
+  s=fgetc(fd)-s;
+  fseek(fd, -1, SEEK_END);
+  fputc(s, fd);
+
+  fclose(fd);
+}
diff --git a/utils/fix-rom.sh b/utils/fix-rom.sh
deleted file mode 100755
index 9fdeb65..0000000
--- a/utils/fix-rom.sh
+++ /dev/null
@@ -1,13 +0,0 @@
-#!/bin/sh
-
-setbyte() {
-  printf "\x$3"|dd status=none seek="$2" bs=1 count=1 conv=notrunc of="$1"
-}
-
-[ -n "$1" ] || exit 1
-
-setbyte "$1" 2 $(stat -c "%s" "$1"|awk '{printf("%02x",$1/512)}')
-
-setbyte "$1" 41 00
-checksum=$(xxd -p -c 1 "$1"|sed 's/^/0x/'|awk '{s+=$1} END {printf("%02x\n",256-s%256)}')
-setbyte "$1" 41 "$checksum"
\ No newline at end of file