diff --git a/pdpasmhelper.py b/pdpasmhelper.py
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+# MIT License
+#
+# Copyright (c) 2023 Neil Webber
+#
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be included in
+# all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.
+
+# FUNCTIONALITY DISCLAIMER:
+#   This is NOT meant to recreate the entire idea of a PDP-11 assembler.
+#   Rather, it is meant as an ad-hoc assistance for creating and
+#   debugging small test programs, of the sort that are found in pdptest.
+#   As such, the methods here are written on an "as-needed" basis and
+#   are focused around helping to create hand-constructed test code.
+#
+
+class PDP11InstructionAssembler:
+    def __init__(self):
+        # crude but so so easy .. translate string to 6-bit operand mode
+        self.B6MODES = {}
+
+        for i in range(8):
+            rnames = [f"R{i}"]
+            if i == 6:
+                rnames.append("SP")
+            elif i == 7:
+                rnames.append("PC")
+
+            for rn in rnames:
+                self.B6MODES[f"{rn}"] = i              # register direct
+                self.B6MODES[f"({rn})"] = 0o10 | i     # register indirect
+                self.B6MODES[f"({rn})+"] = 0o20 | i    # autoincrement
+                self.B6MODES[f"@({rn})+"] = 0o30 | i   # autoincr deferred
+                self.B6MODES[f"-({rn})"] = 0o40 | i    # autodecrement
+                self.B6MODES[f"@-({rn})"] = 0o50 | i   # autodecr deferred
+            # X(Rn) and @X(Rn) cannot be parsed this simple lookup way
+
+    def operand_parser(self, operand_string, /):
+        """Parse operand_string ('r1', '-(sp)', '4(r5)', etc).
+
+        Returns: sequence: [6 bit code, additional words ...]
+
+        Raises ValueError for syntax errors.
+
+        """
+        # NOTE: Not all forms implemented yet. See FUNCTIONALITY DISCLAIMER.
+
+        # normalize the operand
+        operand = operand_string.upper()
+        s = operand.split()     # this also removes leading/trailing space
+        if len(s) > 1:          # but if there were middle spaces... no good
+            raise ValueError(f"more than one word in '{operand_string}'")
+        operand = s[0]
+
+        # operand should be fully normalized: upper case, no spaces.
+
+        try:
+            return [self.B6MODES[operand]]
+        except KeyError:
+            pass
+        
+        # for convenience
+        def valerr():
+            return ValueError(f"cannot parse '{operand_string}'")
+
+        # see if X(Rn) or @X(Rn)...
+        if operand[0] == '@':
+            mode = 0o70
+            operand = operand[1:]
+        else:
+            mode = 0o60
+
+        # for starters, it must contain one '(' so should split to 2
+        s = operand.split('(')
+        if len(s) != 2:
+            raise valerr()
+        idxstr = s[0]
+
+        # idxstr can only end with a digit or a '.' ... in particular
+        base = 8
+        if idxstr[-1] == '.':
+            base = 10
+            idxstr = idxstr[:-1]
+        elif not idxstr[-1].isdigit():
+            raise valerr()
+
+        idxval = int(idxstr, base)
+
+        # the back end of this, with the '(' put back on,
+        # must end with ')' and must parse
+        if s[1][-1] != ')':
+            raise valerr()
+        try:
+            b6 = self.B6MODES['(' + s[1]]
+        except KeyError:
+            raise valerr() from None
+
+        return [mode | (b6 & 0o07), idxval]
+
+            
+            
+