[model] Fence instruction: Don't do anything

Author: wallento

README.rst

@@ -17,13 +17,13 @@ First install it:
 
 Create random assembler sequences
 `````````````````````````````````
-    
+
 Create a random assembler sequence:
 
 ::
 
     riscv-random-asm
-    
+
 You can tweak the assembler output:
 
 ::

riscvmodel/isa.py

@@ -257,12 +257,13 @@ def execute(self, model: State):
 
 @isa("fence", opcode=0b0001111, funct3=0b000, funct7=0b0000000)
 class InstructionFENCE(Instruction):
-    pass
-
+    def execute(self, model: State):
+        pass
 
 @isa("fence.i", opcode=0b0001111, funct3=0b001, funct7=0b0000000)
 class InstructionFENCEI(Instruction):
-    pass
+    def execute(self, model: State):
+        pass
 
 
 @isa("ecall", opcode=0b1110011, funct3=0b000, funct12=0b000000000000)