diff --git a/src/A_extension.cpp b/src/A_extension.cpp
index d1ebc95..595da0b 100644
--- a/src/A_extension.cpp
+++ b/src/A_extension.cpp
@@ -44,7 +44,7 @@ op_A_Codes A_extension::decode() {
 	case A_AMOMAXU:
 		return OP_A_AMOMAXU;
 		break;
-	default:
+	[[unlikely]] default:
 		return OP_A_ERROR;
 		break;
 
@@ -405,7 +405,7 @@ bool A_extension::process_instruction(Instruction &inst) {
 	case OP_A_AMOMAXU:
 		Exec_A_AMOMAXU();
 		break;
-	default:
+	[[unlikely]] default:
 		std::cout << "A instruction not implemented yet" << std::endl;
 		inst.dump();
 		NOP();
diff --git a/src/BASE_ISA.cpp b/src/BASE_ISA.cpp
index 650b457..7678899 100644
--- a/src/BASE_ISA.cpp
+++ b/src/BASE_ISA.cpp
@@ -1316,7 +1316,7 @@ bool BASE_ISA::process_instruction(Instruction &inst) {
 	case OP_SFENCE:
 		Exec_SFENCE();
 		break;
-	default:
+	[[unlikely]] default:
 		std::cout << "Wrong instruction" << std::endl;
 		inst.dump();
 		NOP();
diff --git a/src/BusCtrl.cpp b/src/BusCtrl.cpp
index cc5c6ef..52fa20a 100644
--- a/src/BusCtrl.cpp
+++ b/src/BusCtrl.cpp
@@ -37,7 +37,7 @@ void BusCtrl::b_transport(tlm::tlm_generic_payload &trans,
 	case TRACE_MEMORY_ADDRESS / 4:
 		trace_socket->b_transport(trans, delay);
 		break;
-	default:
+	[[likely]] default:
 		memory_socket->b_transport(trans, delay);
 		break;
 	}
diff --git a/src/CPU.cpp b/src/CPU.cpp
index 7587ca4..90f6ace 100644
--- a/src/CPU.cpp
+++ b/src/CPU.cpp
@@ -162,7 +162,7 @@ void CPU::CPU_thread(void) {
 
 		/* check what type of instruction is and execute it */
 		switch (inst->check_extension()) {
-		case BASE_EXTENSION:
+		[[likely]] case BASE_EXTENSION:
 			PC_not_affected = exec->process_instruction(*inst);
 			incPCby2 = false;
 			break;
@@ -178,7 +178,7 @@ void CPU::CPU_thread(void) {
 			PC_not_affected = a_inst->process_instruction(*inst);
 			incPCby2 = false;
 			break;
-		default:
+		[[unlikely]] default:
 			std::cout << "Extension not implemented yet" << std::endl;
 			inst->dump();
 			exec->NOP();
@@ -193,7 +193,7 @@ void CPU::CPU_thread(void) {
 		/* Process IRQ (if any) */
 		cpu_process_IRQ();
 
-		/* Fixed instruction time to 10 ns (i.e. 100 MHz)*/
+		/* Fixed instruction time to 10 ns (i.e. 100 MHz) */
 //#define USE_QK
 #ifdef USE_QK
 		// Model time used for additional processing
diff --git a/src/C_extension.cpp b/src/C_extension.cpp
index e550177..1c8f32b 100644
--- a/src/C_extension.cpp
+++ b/src/C_extension.cpp
@@ -33,7 +33,7 @@ op_C_Codes C_extension::decode() {
 		case C_FSW:
 			return OP_C_FSW;
 			break;
-		default:
+		[[unlikely]] default:
 			return OP_C_ERROR;
 			break;
 		}
@@ -90,7 +90,7 @@ op_C_Codes C_extension::decode() {
 		case C_BNEZ:
 			return OP_C_BNEZ;
 			break;
-		default:
+		[[unlikely]] default:
 			return OP_C_ERROR;
 			break;
 		}
@@ -131,13 +131,13 @@ op_C_Codes C_extension::decode() {
 			return OP_C_SWSP;
 			break;
 		case C_FWWSP:
-		default:
+		[[unlikely]] default:
 			return OP_C_ERROR;
 			break;
 		}
 		break;
 
-	default:
+	[[unlikely]] default:
 		return OP_C_ERROR;
 		break;
 
@@ -745,7 +745,7 @@ bool C_extension::process_instruction(Instruction &inst) {
 	case OP_C_AND:
 		Exec_C_AND();
 		break;
-	default:
+	[[unlikely]] default:
 		std::cout << "C instruction not implemented yet" << std::endl;
 		inst.dump();
 		NOP();
diff --git a/src/M_extension.cpp b/src/M_extension.cpp
index ef59fd6..5cda739 100644
--- a/src/M_extension.cpp
+++ b/src/M_extension.cpp
@@ -35,7 +35,7 @@ op_M_Codes M_extension::decode() {
 	case M_REMU:
 		return OP_M_REMU;
 		break;
-	default:
+	[[unlikely]] default:
 		return OP_M_ERROR;
 		break;
 	}
@@ -275,7 +275,7 @@ bool M_extension::process_instruction(Instruction &inst) {
 	case OP_M_REMU:
 		Exec_M_REMU();
 		break;
-	default:
+	[[unlikely]] default:
 		std::cout << "M instruction not implemented yet" << std::endl;
 		inst.dump();
 		//NOP(inst);