Implement V5+ call variants and fix double-byte operand decoding

New opcodes: op_call_vn, op_call_vn2, op_call_vs2, op_catch,
op_check_arg_count. All call variants delegate to existing _call().
ZRoutine now tracks arg_count for check_arg_count.

Fixed zopdecoder double-byte operand parsing for call_vs2/call_vn2:
the old code called _parse_operands_byte() twice, but this method
reads both type byte AND operands together. The second call would
read operand data as a type byte. Refactored into _read_type_byte()
+ _parse_operand_list() so both type bytes are read before any
operand data.

Also fixed the double-byte detection: was checking opcode[0:7] (7
bits = 0x7A for call_vn2) instead of opcode_num (5 bits = 0x1A).
The check never matched, so double-byte opcodes were always
mis-parsed.

src/mudlib/zmachine/zcpu.py

@@ -644,8 +644,9 @@ class ZCpu:
         self._stackmanager.pop_stack()
 
     def op_catch(self, *args):
-        """TODO: Write docstring here."""
-        raise ZCpuNotImplemented
+        """Store the current stack frame index (for throw opcode)."""
+        frame_index = self._stackmanager.get_stack_frame_index()
+        self._write_result(frame_index)
 
     def op_quit(self, *args):
         """Quit the game."""
@@ -793,9 +794,9 @@ class ZCpu:
         """Set the given window as the active window."""
         self._ui.screen.select_window(window_num)
 
-    def op_call_vs2(self, *args):
-        """TODO: Write docstring here."""
-        raise ZCpuNotImplemented
+    def op_call_vs2(self, routine_addr, *args):
+        """Call routine with up to 7 arguments and store the result."""
+        self._call(routine_addr, args, True)
 
     def op_erase_window(self, window_number):
         """Clear the window with the given number. If # is -1, unsplit
@@ -884,13 +885,13 @@ class ZCpu:
         """TODO: Write docstring here."""
         raise ZCpuNotImplemented
 
-    def op_call_vn(self, *args):
-        """TODO: Write docstring here."""
-        raise ZCpuNotImplemented
+    def op_call_vn(self, routine_addr, *args):
+        """Call routine with up to 3 arguments and discard the result."""
+        self._call(routine_addr, args, False)
 
-    def op_call_vn2(self, *args):
-        """TODO: Write docstring here."""
-        raise ZCpuNotImplemented
+    def op_call_vn2(self, routine_addr, *args):
+        """Call routine with up to 7 arguments and discard the result."""
+        self._call(routine_addr, args, False)
 
     def op_tokenize(self, *args):
         """TODO: Write docstring here."""
@@ -908,9 +909,10 @@ class ZCpu:
         """TODO: Write docstring here."""
         raise ZCpuNotImplemented
 
-    def op_check_arg_count(self, *args):
-        """TODO: Write docstring here."""
-        raise ZCpuNotImplemented
+    def op_check_arg_count(self, arg_number):
+        """Branch if the Nth argument was passed to the current routine."""
+        current_frame = self._stackmanager._call_stack[-1]
+        self._branch(arg_number <= current_frame.arg_count)
 
     ## EXT opcodes (opcodes 256-284)
 

src/mudlib/zmachine/zopdecoder.py

@@ -122,14 +122,16 @@ class ZOpDecoder:
 
         opcode_num = opcode[0:5]
 
-        # Parse the types byte to retrieve the operands.
-        operands = self._parse_operands_byte()
-
-        # Special case: opcodes 12 and 26 have a second operands byte.
-        if opcode[0:7] == 0xC or opcode[0:7] == 0x1A:
+        # Read all type bytes FIRST, before parsing any operands.
+        # call_vs2 (VAR:12) and call_vn2 (VAR:26) have two type bytes;
+        # all others have one. Both type bytes must be read before
+        # operand data starts in the stream.
+        operand_types = self._read_type_byte()
+        if opcode_type == OPCODE_VAR and opcode_num in (0xC, 0x1A):
             log("Opcode has second operand byte")
-            operands += self._parse_operands_byte()
+            operand_types += self._read_type_byte()
 
+        operands = self._parse_operand_list(operand_types)
         return (opcode_type, opcode_num, operands)
 
     def _parse_opcode_extended(self):
@@ -179,25 +181,31 @@ class ZOpDecoder:
 
         return operand
 
-    def _parse_operands_byte(self):
-        """Parse operands given by the operand byte and return a list of
-        values.
-        """
+    def _read_type_byte(self):
+        """Read one operand type byte and return a list of type codes."""
         operand_byte = BitField(self._get_pc())
-        operands = []
-        for operand_type in [
+        return [
             operand_byte[6:8],
             operand_byte[4:6],
             operand_byte[2:4],
             operand_byte[0:2],
-        ]:
+        ]
+
+    def _parse_operand_list(self, operand_types):
+        """Parse operands from a list of type codes, stopping at ABSENT."""
+        operands = []
+        for operand_type in operand_types:
             operand = self._parse_operand(operand_type)
             if operand is None:
                 break
             operands.append(operand)
-
         return operands
 
+    def _parse_operands_byte(self):
+        """Read one type byte and parse its operands. Used by extended
+        opcodes and other callers that always have a single type byte."""
+        return self._parse_operand_list(self._read_type_byte())
+
     # Public funcs that the ZPU may also need to call, depending on the
     # opcode being executed:
 

src/mudlib/zmachine/zstackmanager.py

@@ -54,6 +54,7 @@ class ZRoutine:
         self.start_addr = start_addr
         self.return_addr = return_addr
         self.program_counter = 0  # used when execution interrupted
+        self.arg_count = len(args)  # track number of args passed
 
         if stack is None:
             self.stack = []