diff --git a/04_mailboxes/kernel8.img b/04_mailboxes/kernel8.img
index ec6de0e7..19db1d41 100755
Binary files a/04_mailboxes/kernel8.img and b/04_mailboxes/kernel8.img differ
diff --git a/04_mailboxes/main.c b/04_mailboxes/main.c
index 4af4d252..2f4f00e2 100644
--- a/04_mailboxes/main.c
+++ b/04_mailboxes/main.c
@@ -36,8 +36,8 @@ void main()
     mbox[1] = MBOX_REQUEST;         // this is a request message
     
     mbox[2] = MBOX_TAG_GETSERIAL;   // get serial number command
-    mbox[3] = 0;                    // no input arguments
-    mbox[4] = 0;
+    mbox[3] = 8;                    // buffer size
+    mbox[4] = 8;
     mbox[5] = 0;                    // clear output buffer
     mbox[6] = 0;
 
diff --git a/05_uart0/kernel8.img b/05_uart0/kernel8.img
index 50bd4543..c4b9750f 100755
Binary files a/05_uart0/kernel8.img and b/05_uart0/kernel8.img differ
diff --git a/05_uart0/main.c b/05_uart0/main.c
index 4af4d252..2f4f00e2 100644
--- a/05_uart0/main.c
+++ b/05_uart0/main.c
@@ -36,8 +36,8 @@ void main()
     mbox[1] = MBOX_REQUEST;         // this is a request message
     
     mbox[2] = MBOX_TAG_GETSERIAL;   // get serial number command
-    mbox[3] = 0;                    // no input arguments
-    mbox[4] = 0;
+    mbox[3] = 8;                    // buffer size
+    mbox[4] = 8;
     mbox[5] = 0;                    // clear output buffer
     mbox[6] = 0;