diff --git a/src/assembler.c b/src/assembler.c
index d3e4e81..290a44b 100644
--- a/src/assembler.c
+++ b/src/assembler.c
@@ -27,13 +27,13 @@ assemble_init(bus_t *b, uint16_t pc)
 }
 
 bool
-assemble_single_implied(assembler_t *a, const char *mnemonic)
+assemble_single_implied(assembler_t *a, char *mnemonic)
 {
 	return assemble_single(a, mnemonic, IMPLIED, 0, 0);
 }
 
 bool
-assemble_single(assembler_t *a, const char *mnemonic, addressing_t mode, uint8_t op1, uint8_t op2)
+assemble_single(assembler_t *a, char *mnemonic, addressing_t mode, uint8_t op1, uint8_t op2)
 {
 	uint8_t *asmbuf;
 	uint8_t bsize;
@@ -50,31 +50,22 @@ assemble_single(assembler_t *a, const char *mnemonic, addressing_t mode, uint8_t
 }
 
 bool
-assemble_single_buf_implied(uint8_t **buf, uint8_t *bsize, const char *mnemonic)
+assemble_single_buf_implied(uint8_t **buf, uint8_t *bsize, char *mnemonic)
 {
 	return assemble_single_buf(buf, bsize, mnemonic, IMPLIED, 0, 0);
 }
 
 
 bool
-assemble_single_buf(uint8_t **buf, uint8_t *bsize, const char *mnemonic, addressing_t mode, uint8_t op1, uint8_t op2)
+assemble_single_buf(uint8_t **buf, uint8_t *bsize, char *mnemonic, addressing_t mode, uint8_t op1, uint8_t op2)
 {
 	instrdef_t id;
 	uint8_t opcode;
 	bool found;
 
-	found = false;
 	opcode = 0;
 
-	/* find the opcode for given mnemonic and addressing mode */
-	while (opcode <= 0xFF)  { /* this is stupid */
-		id = instruction_decode(opcode);
-		if ((strcmp(mnemonic, id.mnemonic) == 0) && (id.mode == mode)) {
-			found = true;
-			break;
-		}
-		opcode++;
-	}
+	found = instruction_opcode_by_mnemonic(mnemonic, mode, &opcode, &id);
 
 	if (!found) {
 		rk65c02_log(LOG_ERROR,
diff --git a/src/assembler.h b/src/assembler.h
index e12d426..e92998f 100644
--- a/src/assembler.h
+++ b/src/assembler.h
@@ -11,12 +11,12 @@ struct assembler {
 
 typedef struct assembler assembler_t;
 
-bool assemble_single_buf_implied(uint8_t **, uint8_t *, const char *);
-bool assemble_single_buf(uint8_t **, uint8_t *, const char *, addressing_t, uint8_t, uint8_t);
+bool assemble_single_buf_implied(uint8_t **, uint8_t *, char *);
+bool assemble_single_buf(uint8_t **, uint8_t *, char *, addressing_t, uint8_t, uint8_t);
 
 assembler_t assemble_init(bus_t *, uint16_t);
-bool assemble_single(assembler_t *, const char *, addressing_t, uint8_t, uint8_t);
-bool assemble_single_implied(assembler_t *, const char *);
+bool assemble_single(assembler_t *, char *, addressing_t, uint8_t, uint8_t);
+bool assemble_single_implied(assembler_t *, char *);
 
 #endif /* _ASSEMBLER_H_ */
 
diff --git a/src/instruction.c b/src/instruction.c
index d52f7d7..9a6ef4f 100644
--- a/src/instruction.c
+++ b/src/instruction.c
@@ -311,3 +311,23 @@ instruction_modify_pc(instrdef_t *id)
 	return id->modify_pc;
 }
 
+/* find instr definition (and opcode) searching by mnemonic and addr mode */
+bool
+instruction_opcode_by_mnemonic(char *mnemonic, addressing_t mode, uint8_t *opcode, instrdef_t *id)
+{
+	bool found;
+
+	found = false;
+
+	while ((*opcode) <= 0xFF)  { /* this is stupid */
+		*id = instruction_decode(*opcode);
+		if ((strcmp(mnemonic, id->mnemonic) == 0) && (id->mode == mode)) {
+			found = true;
+			break;
+		}
+		(*opcode)++;
+	}
+
+	return found;
+}
+
diff --git a/src/instruction.h b/src/instruction.h
index 8b358ba..0c0a0f2 100644
--- a/src/instruction.h
+++ b/src/instruction.h
@@ -56,5 +56,6 @@ uint8_t stack_pop(rk65c02emu_t *);
 void program_counter_increment(rk65c02emu_t *, instrdef_t *);
 bool instruction_modify_pc(instrdef_t *);
 void program_counter_branch(rk65c02emu_t *, int8_t);
+bool instruction_opcode_by_mnemonic(char *, addressing_t, uint8_t *, instrdef_t *);
 
 #endif /* _INSTRUCTION_H_ */