diff --git a/inc/BASE_ISA.h b/inc/BASE_ISA.h
index b697cbf..e595628 100644
--- a/inc/BASE_ISA.h
+++ b/inc/BASE_ISA.h
@@ -1787,7 +1787,7 @@ namespace riscv_tlm {
          * @brief Decodes opcode of instruction
          * @return opcode of instruction
          */
-        opCodes decode() {
+        opCodes decode() const {
             switch (opcode()) {
                 case LUI:
                     return OP_LUI;
diff --git a/inc/C_extension.h b/inc/C_extension.h
index 331e7d0..90e6ada 100644
--- a/inc/C_extension.h
+++ b/inc/C_extension.h
@@ -182,7 +182,7 @@ namespace riscv_tlm {
             return aux;
         }
 
-        inline void set_imm_I(std::uint32_t value) {
+        inline void set_imm_I(std::uint32_t value) const {
             this->m_instr.range(31, 20) = value;
         }