diff --git a/tools/blkup.c b/tools/blkup.c
index fb09ad2..39cbcf5 100644
--- a/tools/blkup.c
+++ b/tools/blkup.c
@@ -3,6 +3,7 @@
 #include <fcntl.h>
 #include <unistd.h>
 #include <string.h>
+#include <stdbool.h>
 
 #include "common.h"
 
@@ -30,29 +31,43 @@ int main(int argc, char **argv)
     char s[0x40];
     char buf[1024] = {0};
     sendcmdp(fd, ": _ 1024 0 DO KEY DUP .x I BLK( + C! LOOP ;");
+    sendcmdp(fd, ": Z BLK( 1024 0 FILL ;");
 
     int returncode = 0;
     while (fread(buf, 1, 1024, fp)) {
-        sendcmd(fd, "_");
+        bool allzero = true;
         for (int i=0; i<1024; i++) {
-            putchar('.');
-            fflush(stdout);
-            write(fd, &buf[i], 1);
-            usleep(1000); // let it breathe
-            mread(fd, s, 2); // read hex pair
-            s[2] = 0; // null terminate
-            unsigned char c = strtol(s, NULL, 16);
-            if (c != buf[i]) {
-                // mismatch!
-                fprintf(stderr, "Mismatch at bno %d (%d) %d != %d.\n", blkno, i, buf[i], c);
-                // we don't exit now because we need to "consume" our whole program.
-                returncode = 1;
+            if (buf[i] != 0) {
+                allzero = false;
+                break;
             }
-            usleep(1000); // let it breathe
         }
-        readprompt(fd);
-        if (returncode) break;
-        memset(buf, 0, 1024);
+        if (allzero) {
+            sendcmdp(fd, "Z");
+            putchar('Z');
+            fflush(stdout);
+        } else {
+            sendcmd(fd, "_");
+            for (int i=0; i<1024; i++) {
+                putchar('.');
+                fflush(stdout);
+                write(fd, &buf[i], 1);
+                usleep(1000); // let it breathe
+                mread(fd, s, 2); // read hex pair
+                s[2] = 0; // null terminate
+                unsigned char c = strtol(s, NULL, 16);
+                if (c != buf[i]) {
+                    // mismatch!
+                    fprintf(stderr, "Mismatch at bno %d (%d) %d != %d.\n", blkno, i, buf[i], c);
+                    // we don't exit now because we need to "consume" our whole program.
+                    returncode = 1;
+                }
+                usleep(1000); // let it breathe
+            }
+            readprompt(fd);
+            if (returncode) break;
+            memset(buf, 0, 1024);
+        }
         sprintf(s, "%d BLK> ! BLK!", blkno);
         sendcmdp(fd, s);
         blkno++;