Make sure user has built and installed pdp11.

tools/assemv2

@@ -6,6 +6,11 @@ if [ ! -d tools ]
 then echo 'No tools/ directory, are you running this in the correct dir?'
      exit 1
 fi
+if [ ! -f tools/pdp11 ]
+then echo 'You need to compile Simh with the ke11 patch, and place the'
+     echo 'resulting pdp11 executable into the tools/ directory.'
+     exit 1
+fi
 if [ ! -d build ]
 then mkdir build
 fi