diff --git a/extend.c b/extend.c
new file mode 100644
index 0000000..2a323d6
--- /dev/null
+++ b/extend.c
@@ -0,0 +1,77 @@
+/*
+ * Simple program to modify the volume directory (block 2) of a ProDOS
+ * volume to make the directory extensible to more than four blocks (51
+ * entries) when using ProDOS 2.5 or later.  This program can also revert
+ * a volume with an extensible volume directory back to fixed-size directory
+ * provided the directory is still exactly 4 blocks in length.
+ *
+ * Bobbi
+ * March 2020
+ */
+
+#include <prodos.h>
+#include <stdio.h>
+#include <ctype.h>
+#include <string.h>
+
+char buf[512];
+char str[128]; /* Should be enough */
+
+int main(int argc, char *argv[]) {
+	puts("ProDOS 2.5+ Volume Directory Patching Utility");
+	puts("---------------------------------------------");
+	fputs("\nEnter device number> ", stdout);
+	int dev;
+	scanf("%u", &dev);
+	BlockRec br;
+	br.blockDevNum = dev;
+	br.blockDataBuffer = buf;
+	br.blockNum = 2;
+	READ_BLOCK(&br);
+	int rc = toolerror();
+	if (rc) {
+		printf("Block read failed, err=%x\n", rc);
+		exit(255);
+	}
+	if ((buf[0x14] == 0) && (buf[0x15] == 0)) {
+		puts("Fixed-size (4 block, 51 entry) volume directory");
+		printf("Change to ProDOS 2.5+ extensible volume "
+		       "directory ('yes' to confirm)?");
+		scanf("%s", str);
+		if (strcasecmp(str, "yes") == 0) {
+			buf[0x14] = 4;
+			buf[0x15] = 0;
+			WRITE_BLOCK(&br);
+			int rc = toolerror();
+			if (rc) {
+				printf("Block write failed, err=%x\n", rc);
+				exit(255);
+			}
+			puts("** Changed to extensible **");
+		} else
+			puts("No change made.");
+	} else {
+		printf("Extensible volume directory, currently %u blocks\n",
+		       buf[0x14] + 256U * buf[0x15]);
+		if ((buf[0x14] == 4) && (buf[0x15] == 0)) {
+			printf("Revert to classic ProDOS fixed-size volume "
+			       "directory ('yes' to confirm)?");
+			scanf("%s", str);
+			if (strcasecmp(str, "yes") == 0) {
+				buf[0x14] = 0;
+				buf[0x15] = 0;
+				WRITE_BLOCK(&br);
+				int rc = toolerror();
+				if (rc) {
+					printf("Block write failed, err=%x\n",
+					       rc);
+					exit(255);
+				}
+				puts("** Changed to fixed-size **");
+			} else
+				puts("No change made.");
+		}
+	}
+	return 0;
+}
+