fix: rename sw_read/write to arch_read/write

- Update register_file_schema.json: Rename sw_read()/sw_write(value) to
arch_read()/arch_write(value) and clarify their descriptions as
architecturally mandated behavior.
- Drop writable flag from register entries.
- Remove redundant type declarations where const already fixes the
value.
- Adjust the register-file example and README.adoc to reference the new
function names.
- register_file.rb: Expose arch_read/arch_write while retaining sw_*
aliases for backward compatibility with existing YAML.

Author: AnimeshAgarwal28

spec/schemas/register_file_schema.json

@@ -28,13 +28,13 @@
         "when": {
           "$ref": "schema_defs.json#/$defs/requires_entry"
         },
-        "sw_read()": {
+        "arch_read()": {
           "type": "string",
-          "description": "Function that returns the value of the register when read by software. Use this to define special behavior for registers (e.g., x0 always reads as zero)."
+          "description": "Function describing the architecturally defined read behavior. Use this for architecturally mandated effects (e.g., x0 always reads as zero)."
         },
-        "sw_write(value)": {
+        "arch_write(value)": {
           "type": "string",
-          "description": "Function implementing custom write behavior for the register. Given a 'value', return either the value to be written or a modified value. Use this to define special behavior for registers (e.g., x0 ignores writes)."
+          "description": "Function describing the architecturally defined write behavior. Given a 'value', return the architecturally required result (e.g., x0 ignores writes and always yields zero)."
         },
         "caller_saved": {
           "type": "boolean",
@@ -63,11 +63,6 @@
             ]
           },
           "uniqueItems": true
-        },
-        "writable": {
-          "type": "boolean",
-          "default": true,
-          "description": "Whether the register is writable. If false, no sw_write() method is needed."
         }
       }
     },
@@ -85,13 +80,11 @@
       "additionalProperties": false,
       "properties": {
         "$schema": {
-          "type": "string",
           "format": "uri-reference",
           "const": "register_file_schema.json#",
           "description": "Path to schema, relative to <UDB ROOT>/schemas"
         },
         "kind": {
-          "type": "string",
           "const": "register_file"
         },
         "name": {

spec/std/isa/README.adoc

@@ -158,9 +158,9 @@ registers:
   - name: x0    # <2>
     abi_mnemonics: [zero] # <3>
     roles: [zero]
-    sw_read(): | # <4>
+    arch_read(): | # <4>
       return 0;
-    sw_write(value): |
+    arch_write(value): |
       # x0 ignores all writes
   - name: x1
     abi_mnemonics: [ra]
@@ -173,7 +173,7 @@ registers:
 <1> The registers in a register file have identical fixed widths, either an explicit width or from a parameter (e.g. MXLEN or VLEN).
 <2> Each register has a unique name (e.g., x0, x1, x2, etc.). The register's index is inferred from its position in the array (starting from 0).
 <3> Registers can optionally have ABI mnemonics (e.g., ra, sp, fp, etc.).
-<4> Individual registers can define `sw_read()` and `sw_write()` for special behavior (e.g., x0 hardwired to zero).
+<4> Individual registers can define `arch_read()` and `arch_write()` for architecturally mandated behavior (e.g., x0 hardwired to zero).
 <5> A register can have multiple ABI mnemonics (e.g., x8 is known as both s0 and fp).
 
 [source,yaml]

spec/std/isa/register_file/X.yaml

@@ -19,10 +19,11 @@ registers:
   - name: x0
     abi_mnemonics: [zero]
     roles: [zero]
-    writable: false
     description: |
       Register x0 is hardwired with all bits equal to 0.
-    sw_read(): |
+    arch_read(): |
+      return 0;
+    arch_write(value): |
       return 0;
   - name: x1
     abi_mnemonics: [ra]

tools/ruby-gems/udb/lib/udb/obj/register_file.rb

@@ -66,10 +66,20 @@ def caller_saved = @data["caller_saved"]
     def callee_saved = @data["callee_saved"]
 
     sig { returns(T.nilable(String)) }
-    def sw_read = @data["sw_read()"]
+    def arch_read
+      @data["arch_read()"] || @data["sw_read()"]
+    end
+
+    sig { returns(T.nilable(String)) }
+    def arch_write
+      @data["arch_write(value)"] || @data["sw_write(value)"]
+    end
+
+    sig { returns(T.nilable(String)) }
+    def sw_read = arch_read
 
     sig { returns(T.nilable(String)) }
-    def sw_write = @data["sw_write(value)"]
+    def sw_write = arch_write
 
     sig { returns(T.nilable(ExtensionRequirementExpression)) }
     def defined_by_condition