diff --git a/j1a/nandland-go/shell.py b/j1a/nandland-go/shell.py
index 343b68b..2c0336a 100644
--- a/j1a/nandland-go/shell.py
+++ b/j1a/nandland-go/shell.py
@@ -58,7 +58,7 @@ def serialize(self):
         for l in lines:
             l = l.split()
             s += [int(b, 16) for b in l[1:17]]
-        s = array.array('B', s).tostring().ljust(8192, chr(0xff))
+        s = array.array('B', s).tobytes().ljust(8192, bytes([0xFF]))
         return array.array('H', s)
 
 if __name__ == '__main__':
diff --git a/j1a/shell.py b/j1a/shell.py
index 97e6576..6ca95a6 100644
--- a/j1a/shell.py
+++ b/j1a/shell.py
@@ -65,7 +65,7 @@ def serialize(self):
         for l in lines:
             l = l.split()
             s += [int(b, 16) for b in l[1:17]]
-        s = array.array('B', s).tostring().ljust(8192, chr(0xff))
+        s = array.array('B', s).tobytes().ljust(8192, bytes([0xFF]))
         return array.array('H', s)
 
 if __name__ == '__main__':
diff --git a/j1a/verilator/shell.py b/j1a/verilator/shell.py
index 40527ff..7cf8487 100644
--- a/j1a/verilator/shell.py
+++ b/j1a/verilator/shell.py
@@ -46,7 +46,7 @@ def serialize(self):
         for l in lines:
             l = l.split()
             s += [int(b, 16) for b in l[1:17]]
-        s = array.array('B', s).tostring().ljust(8192, chr(0xff))
+        s = array.array('B', s).tobytes().ljust(8192, bytes([0xFF]))
         return array.array('H', s)
 
 if __name__ == '__main__':
diff --git a/j1a/verilator/simshell4.py b/j1a/verilator/simshell4.py
index 16d1bab..d6b9141 100644
--- a/j1a/verilator/simshell4.py
+++ b/j1a/verilator/simshell4.py
@@ -46,7 +46,7 @@ def serialize(self):
         for l in lines:
             l = l.split()
             s += [int(b, 16) for b in l[1:17]]
-        s = array.array('B', s).tostring().ljust(8192, chr(0xff))
+        s = array.array('B', s).tobytes().ljust(8192, bytes([0xFF]))
         return array.array('H', s)
 
 if __name__ == '__main__':
diff --git a/j1b/shell.py b/j1b/shell.py
index 4109043..a6b5269 100644
--- a/j1b/shell.py
+++ b/j1b/shell.py
@@ -54,7 +54,7 @@ def serialize(self):
         for l in lines:
             l = l.split()
             s += [int(b, 16) for b in l[1:17]]
-        s = array.array('B', s).tostring().ljust(32768, chr(0xff))
+        s = array.array('B', s).tobytes().ljust(8192, bytes([0xFF]))
         return array.array('i', s)
 
 if __name__ == '__main__':
diff --git a/shell/swapforth.py b/shell/swapforth.py
index 0bb0f6b..4aa03ef 100755
--- a/shell/swapforth.py
+++ b/shell/swapforth.py
@@ -200,12 +200,12 @@ def shellcmd(self, cmd):
                     if dest.endswith('.hex'):
                         open(dest, "w").write("".join(["%08x\n" % (x & 0xffffffff) for x in d]))
                     else:
-                        open(dest, "wb").write(array.array("i", d).tostring())
+                        open(dest, "wb").write(array.array("i", d).tobytes())
                 else:
                     if dest.endswith('.hex'):
                         open(dest, "w").write("".join(["%04x\n" % (x & 0xffff) for x in d]))
                     else:
-                        open(dest, "wb").write(array.array("h", d).tostring())
+                        open(dest, "wb").write(array.array("H", d).tobytes())
         elif cmd.startswith('#setclock'):
             n = datetime.utcnow()
             cmd = "decimal %d %d %d %d %d %d >time&date" % (n.second, n.minute, n.hour, n.day, n.month, n.year)