diff --git a/build/simh/boot b/build/simh/boot
index 270172d9..6709fb3c 100644
--- a/build/simh/boot
+++ b/build/simh/boot
@@ -2,5 +2,6 @@ set cpu its
 set cpu idle
 set tim y2k
 set rp0 rp06
+set lhdh enable
 at rp0 out/rp0.dsk
 b rp0
diff --git a/build/simh/init b/build/simh/init
index 3b37f0b7..948d0660 100644
--- a/build/simh/init
+++ b/build/simh/init
@@ -1,5 +1,6 @@
 set cpu its
 set tim y2k
+set lhdh enable
 at tu0 out/minsys.tape
 at tu1 out/salv.tape
 set rp0 rp06