Move exception code into its own file

This is not necessary for the model currently, however internally we wanted to use `internal_error` in a file that was before `riscv_types.sail`. Putting it in its own file makes that possible, and I think it's an organisational improvement even if the model currently does not need it.

Makefile

@@ -96,7 +96,7 @@ SAIL_VM_SRCS += riscv_vmem_tlb.sail
 SAIL_VM_SRCS += riscv_vmem.sail
 
 # Non-instruction sources
-PRELUDE = prelude.sail $(SAIL_XLEN) $(SAIL_FLEN) $(SAIL_VLEN) prelude_mem_metadata.sail prelude_mem.sail
+PRELUDE = prelude.sail riscv_errors.sail $(SAIL_XLEN) $(SAIL_FLEN) $(SAIL_VLEN) prelude_mem_metadata.sail prelude_mem.sail
 
 SAIL_REGS_SRCS = riscv_reg_type.sail riscv_freg_type.sail riscv_regs.sail riscv_pc_access.sail riscv_sys_regs.sail
 SAIL_REGS_SRCS += riscv_pmp_regs.sail riscv_pmp_control.sail

model/riscv_errors.sail

@@ -0,0 +1,23 @@
+/*=======================================================================================*/
+/*  This Sail RISC-V architecture model, comprising all files and                        */
+/*  directories except where otherwise noted is subject the BSD                          */
+/*  two-clause license in the LICENSE file.                                              */
+/*                                                                                       */
+/*  SPDX-License-Identifier: BSD-2-Clause                                                */
+/*=======================================================================================*/
+
+/* model-internal exceptions */
+
+union exception = {
+  Error_not_implemented : string,
+  Error_internal_error  : unit
+}
+
+val not_implemented : forall ('a : Type). string -> 'a
+function not_implemented message = throw(Error_not_implemented(message))
+
+val internal_error : forall ('a : Type). (string, int, string) -> 'a
+function internal_error(file, line, s) = {
+    assert (false, file ^ ":" ^ dec_str(line) ^ ": " ^ s);
+    throw Error_internal_error()
+}

model/riscv_types.sail

@@ -78,23 +78,6 @@ function arch_to_bits(a : Architecture) -> arch_xlen =
     RV128 => 0b11
   }
 
-
-/* model-internal exceptions */
-
-union exception = {
-  Error_not_implemented : string,
-  Error_internal_error  : unit
-}
-
-val not_implemented : forall ('a : Type). string -> 'a
-function not_implemented message = throw(Error_not_implemented(message))
-
-val internal_error : forall ('a : Type). (string, int, string) -> 'a
-function internal_error(file, line, s) = {
-    assert (false, file ^ ":" ^ dec_str(line) ^ ": " ^ s);
-    throw Error_internal_error()
-}
-
 /* privilege levels */
 
 type priv_level = bits(2)