diff --git a/src/emucore/Thumbulator.cxx b/src/emucore/Thumbulator.cxx
index a9d5f3435..bfb835fd5 100644
--- a/src/emucore/Thumbulator.cxx
+++ b/src/emucore/Thumbulator.cxx
@@ -53,7 +53,7 @@ using Common::Base;
 #endif
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-Thumbulator::Thumbulator(const uInt16* rom_ptr, uInt16* ram_ptr, uInt16 rom_size,
+Thumbulator::Thumbulator(const uInt16* rom_ptr, uInt16* ram_ptr, uInt32 rom_size,
                          const uInt32 c_base, const uInt32 c_start, const uInt32 c_stack,
                          bool traponfatal, Thumbulator::ConfigureFor configurefor,
                          Cartridge* cartridge)
@@ -67,7 +67,7 @@ Thumbulator::Thumbulator(const uInt16* rom_ptr, uInt16* ram_ptr, uInt16 rom_size
     configuration(configurefor),
     myCartridge(cartridge)
 {
-  for(uInt16 i = 0; i < romSize / 2; ++i)
+  for(uInt32 i = 0; i < romSize / 2; ++i)
     decodedRom[i] = decodeInstructionWord(CONV_RAMROM(rom[i]));
 
   setConsoleTiming(ConsoleTiming::ntsc);
diff --git a/src/emucore/Thumbulator.hxx b/src/emucore/Thumbulator.hxx
index 2fd043627..6713ee5c6 100644
--- a/src/emucore/Thumbulator.hxx
+++ b/src/emucore/Thumbulator.hxx
@@ -35,8 +35,8 @@ class Cartridge;
   #define NO_THUMB_STATS
 #endif
 
-#define ROMADDMASK 0x7FFFF      
-#define RAMADDMASK 0x7FFF 
+#define ROMADDMASK 0x7FFFF
+#define RAMADDMASK 0x7FFF
 
 #define ROMSIZE (ROMADDMASK+1)      // 512KB
 #define RAMSIZE (RAMADDMASK+1)      // 32KB
@@ -60,7 +60,7 @@ class Thumbulator
       DPCplus   // cartridges of type DPC+
     };
 
-    Thumbulator(const uInt16* rom_ptr, uInt16* ram_ptr, uInt16 rom_size,
+    Thumbulator(const uInt16* rom_ptr, uInt16* ram_ptr, uInt32 rom_size,
                 const uInt32 c_base, const uInt32 c_start, const uInt32 c_stack,
                 bool traponfatal, Thumbulator::ConfigureFor configurefor,
                 Cartridge* cartridge);
@@ -188,7 +188,7 @@ class Thumbulator
 
   private:
     const uInt16* rom{nullptr};
-    uInt16 romSize{0};
+    uInt32 romSize{0};
     uInt32 cBase{0};
     uInt32 cStart{0};
     uInt32 cStack{0};
diff --git a/src/macos/SerialPortMACOS.cxx b/src/macos/SerialPortMACOS.cxx
index 9006c2c74..745fdc8fc 100644
--- a/src/macos/SerialPortMACOS.cxx
+++ b/src/macos/SerialPortMACOS.cxx
@@ -101,7 +101,7 @@ StringList SerialPortMACOS::portNames()
 
   // Get all possible devices in the '/dev' directory
   FilesystemNode::NameFilter filter = [](const FilesystemNode& node) {
-    return BSPF::startsWithIgnoreCase(node.getPath(), "/dev/tty.usb");
+    return BSPF::startsWithIgnoreCase(node.getPath(), "/dev/cu.usb");
   };
   FSList portList;
   portList.reserve(16);
@@ -111,8 +111,20 @@ StringList SerialPortMACOS::portNames()
 
   // Add only those that can be opened
   for(const auto& port: portList)
-    if(openPort(port.getPath()))
+    if(isValid(port.getPath()))
       ports.emplace_back(port.getPath());
 
   return ports;
 }
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+bool SerialPortMACOS::isValid(const string& port) const
+{
+  // For now, we can only detect whether the port could be opened
+  // Eventually we may extend this to do more intensive checks
+  int handle = open(port.c_str(), O_RDWR | O_NOCTTY | O_NONBLOCK);
+  if(handle > 0)
+    close(handle);
+
+  return handle > 0;
+}
diff --git a/src/macos/SerialPortMACOS.hxx b/src/macos/SerialPortMACOS.hxx
index ea55ddc3d..f16d9ddcd 100644
--- a/src/macos/SerialPortMACOS.hxx
+++ b/src/macos/SerialPortMACOS.hxx
@@ -69,6 +69,15 @@ class SerialPortMACOS : public SerialPort
     */
     StringList portNames() override;
 
+  private:
+    /**
+      Tests whether this port can be opened, and is a valid
+      serial port.
+
+      @return  True if valid, else false
+    */
+    bool isValid(const string& port) const;
+
   private:
     // File descriptor for serial connection
     int myHandle{0};