diff --git a/arch/crd/MC-1.yaml b/arch/crd/MC-1.yaml
index b6a576993..6ea41b60d 100644
--- a/arch/crd/MC-1.yaml
+++ b/arch/crd/MC-1.yaml
@@ -8,6 +8,9 @@ MC-1:
   # semantic version within the CRD family
   version: "1.0"
 
+  # XLEN used by rakefile
+  base: 32
+
   description: |
     MC-1 can be though of as a minimal 32-bit RISC-V processor with M-mode support:
 
diff --git a/arch/crd/MockCRD-1.yaml b/arch/crd/MockCRD-1.yaml
index 65f803fa0..3d7042712 100644
--- a/arch/crd/MockCRD-1.yaml
+++ b/arch/crd/MockCRD-1.yaml
@@ -6,6 +6,9 @@ MockCRD-1:
 
   family: MockCRDFamily
   
+   # XLEN used by rakefile
+  base: 64
+
   # semantic version within the CRD family
   version: "1.0"
 
@@ -124,7 +127,7 @@ MockCRD-1:
             const: little
         XLEN:
           schema:
-            const: 32
+            const: 64
         CONFIG_PTR_ADDRESS:
           schema:
             const: 0xdeadbeef
diff --git a/backends/crd_doc/tasks.rake b/backends/crd_doc/tasks.rake
index 0d5821872..b97a69bea 100644
--- a/backends/crd_doc/tasks.rake
+++ b/backends/crd_doc/tasks.rake
@@ -13,16 +13,19 @@ Dir.glob("#{$root}/arch/crd/*.yaml") do |f|
   crd_name = File.basename(f, ".yaml")
   crd_obj = YAML.load_file(f, permitted_classes: [Date])
   raise "Ill-formed CRD file #{f}: missing 'family' field" if crd_obj.dig(crd_name, 'family').nil?
+
+  base = crd_obj[crd_name]["base"]
+  raise "Missing CRD base" if base.nil?
   
   file "#{$root}/gen/crd_doc/adoc/#{crd_name}.adoc" => [
     "#{$root}/arch/crd/#{crd_name}.yaml",
     "#{$root}/arch/crd_family/#{crd_obj[crd_name]['family']}.yaml",
     "#{CRD_DOC_DIR}/templates/crd.adoc.erb",
     __FILE__,
-    "#{$root}/.stamps/arch-gen-_64.stamp"
+    "#{$root}/.stamps/arch-gen-_#{base}.stamp"
   ] do |t|
     # TODO: schema validation
-    arch_def = arch_def_for("_64")
+    arch_def = arch_def_for("_#{base}")
     crd = arch_def.crd(crd_name)
     raise "No CRD defined for #{crd_name}" if crd.nil?
 
diff --git a/backends/crd_doc/templates/crd.adoc.erb b/backends/crd_doc/templates/crd.adoc.erb
index 78768ad09..eedea90dc 100644
--- a/backends/crd_doc/templates/crd.adoc.erb
+++ b/backends/crd_doc/templates/crd.adoc.erb
@@ -117,7 +117,7 @@ If the "Allowed Value(s)" is "Any" then any value allowed by the type is accepta
 <% if crd.all_in_scope_ext_params.empty? -%>
 None
 <% else -%>
-[cols="4,2,1,1,3"]
+[cols="4,2,1,1,2"]
 |===
 | Parameter | Type | Allowed Value(s) | Extension(s) | Note
 
@@ -316,7 +316,7 @@ The following instructions are added by this extension:
 
 <% crd.in_scope_ext_params(ext_req).sort.each do |ext_param| -%>
 [[ext-<%= ext_req.name %>-param-<%= ext_param.name %>-def]]
-<%= ext_param.name %>::
+<%= ext_param.name %> &#8658; <%= ext_param.param_db.schema_type %>::
 +
 --
 <%= ext_param.param_db.desc %>
@@ -329,7 +329,7 @@ The following instructions are added by this extension:
 
 <% crd.out_of_scope_params(ext_req.name).sort.each do |param_db| -%>
 [[ext-<%= ext_req.name %>-param-<%= param_db.name %>-def]]
-<%= param_db.name %>::
+<%= param_db.name %> &#8658; <%= param_db.schema_type %>::
 +
 --
 <%= param_db.desc %>
diff --git a/lib/arch_obj_models/crd.rb b/lib/arch_obj_models/crd.rb
index 0be657a2a..2936e8707 100644
--- a/lib/arch_obj_models/crd.rb
+++ b/lib/arch_obj_models/crd.rb
@@ -222,11 +222,6 @@ def value
     end
 
     # @return [String] - # What parameter values are allowed by the CRD.
-    #
-    # Old implementation:
-    #   def schema_pretty_crd_merged_with_param_db
-    #     Schema.new(@param_db.schema).merge!(@schema_crd).to_pretty_s
-    #   end
     def allowed_values
       if (@schema_crd.empty?)
         # CRD doesn't add any constraints on parameter's value.
@@ -236,11 +231,9 @@ def allowed_values
       # Create a Schema object just using information in the parameter database.
       schema_obj = @param_db.schema
 
-      # Merge in constraints imposed by the CRD on the parameter.
-      schema_obj.merge!(@schema_crd)
-
-      # Create string showing allowed values of parameter with CRD constraints added
-      schema_obj.to_pretty_s
+      # Merge in constraints imposed by the CRD on the parameter and then
+      # create string showing allowed values of parameter with CRD constraints added
+      schema_obj.merge(@schema_crd).to_pretty_s
     end
 
     # sorts by name
diff --git a/lib/arch_obj_models/schema.rb b/lib/arch_obj_models/schema.rb
index 48f38ebdf..c5cbbc998 100644
--- a/lib/arch_obj_models/schema.rb
+++ b/lib/arch_obj_models/schema.rb
@@ -122,14 +122,12 @@ def large2hex(value)
       end
     end
 
-    def merge!(other_schema)
+    def merge(other_schema)
       raise ArgumentError, "Expecting Schema" unless (other_schema.is_a?(Schema) || other_schema.is_a?(Hash))
 
-      hash = other_schema.is_a?(Schema) ? other_schema.instance_variable_get(:@schema_hash) : other_schema
+      other_hash = other_schema.is_a?(Schema) ? other_schema.instance_variable_get(:@schema_hash) : other_schema
 
-      @schema_hash.merge!(hash)
-
-      self
+      Schema.new(@schema_hash.merge(other_hash))
     end
 
     def empty?