fix: add writable property to register entry and update file name pattern

Changes:
- Add 'writable' boolean property to register schema to indicate if a register is writable.
- Update gpr.yaml to X.yaml, register directory to register_file.
- Restrict regex for register_file_name to a single character.
- Update README.adoc.

Signed-off-by: Animesh Agarwal <[email protected]>

Author: AnimeshAgarwal28

spec/schemas/register_file_schema.json

@@ -63,6 +63,11 @@
             ]
           },
           "uniqueItems": true
+        },
+        "writable": {
+          "type": "boolean",
+          "default": true,
+          "description": "Whether the register is writable. If false, no sw_write() method is needed."
         }
       }
     },

spec/schemas/schema_defs.json

@@ -64,7 +64,7 @@
     },
     "register_file_name": {
       "type": "string",
-      "pattern": "^[A-Za-z][A-Za-z0-9_.-]*$",
+      "pattern": "^[A-Za-z]$",
       "description": "Register file name"
     },
     "register_alias": {

spec/std/isa/README.adoc

@@ -170,7 +170,7 @@ registers:
     roles: [callee_saved, frame_pointer]
 ----
 
-<1> Registers can have either a fixed architecture width or take their width from a parameter (e.g., MXLEN, VLEN).
+<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).

spec/std/isa/register/gpr.yaml renamed to spec/std/isa/register_file/X.yaml

@@ -1,30 +1,29 @@
 # Copyright (c) Animesh Agarwal
 # SPDX-License-Identifier: BSD-3-Clause-Clear
 
-# yaml-language-server: $schema=../../../schemas/register_file_schema.json
+# yaml-language-server: $schema=../../../../schemas/register_file_schema.json
 
 $schema: register_file_schema.json#
 kind: register_file
 name: X
 long_name: General Purpose Registers
 description: |
   The 'X' register file contains the general-purpose integer registers. Each register
-  is architecturally XLEN bits wide, unless XLEN<MXLEN. Register 'x0' is hardwired to
-  zero, while the remaining registers hold architecturally visible integer state used
-  by the calling convention and execution environment.
+  is MXLEN bits wide, unless XLEN<MXLEN. Register 'x0' is hardwired to zero, while
+  the remaining registers hold architecturally visible integer state used by the
+  calling convention and execution environment.
 definedBy: I
 register_class: general_purpose
 register_length: MXLEN
 registers:
   - name: x0
     abi_mnemonics: [zero]
     roles: [zero]
+    writable: false
     description: |
       Register x0 is hardwired with all bits equal to 0.
     sw_read(): |
       return 0;
-    sw_write(value): |
-      # x0 ignores all writes
   - name: x1
     abi_mnemonics: [ra]
     caller_saved: true