diff --git a/CMakeLists.txt b/CMakeLists.txt
index a9c9cd0..9bf35de 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.0)
+cmake_minimum_required(VERSION 3.1)
 # 3rd party tools
 find_package(Qt5 COMPONENTS Widgets Qml Quick REQUIRED)
 
diff --git a/src/decn/CMakeLists.txt b/src/decn/CMakeLists.txt
index 3643b00..625c1ec 100644
--- a/src/decn/CMakeLists.txt
+++ b/src/decn/CMakeLists.txt
@@ -1,12 +1,21 @@
 
+#code coverage
+add_library(coverage_config INTERFACE)
+target_compile_options(coverage_config INTERFACE -O0 -g --coverage)
+target_link_libraries(coverage_config INTERFACE --coverage)
+
 # decn library
 add_library(decn decn.c)
 
+# decn library with coverage
+add_library(decn_cover decn.c)
+target_link_libraries(decn_cover PUBLIC coverage_config)
+
 add_executable(decn_test decn_test.c ../utils.c)
-target_link_libraries(decn_test decn)
+target_link_libraries(decn_test decn_cover coverage_config)
 
 add_executable(decn_tests catch_main.cpp decn_tests.cpp ../utils.c)
-target_link_libraries(decn_tests decn mpfr)
+target_link_libraries(decn_tests decn_cover coverage_config mpfr)
 
 # decn prototyping
 add_subdirectory(proto)
diff --git a/src/decn/decn.c b/src/decn/decn.c
index 2225398..ed5a116 100644
--- a/src/decn/decn.c
+++ b/src/decn/decn.c
@@ -30,9 +30,9 @@
 // #define DEBUG_MULT
 // #define DEBUG_MULT_ALL //even more verbose
 // #define DEBUG_DIV
-#define DEBUG_LOG
+// #define DEBUG_LOG
 // #define DEBUG_LOG_ALL //even more verbose
-#define DEBUG_EXP
+// #define DEBUG_EXP
 // #define DEBUG_EXP_ALL //even more verbose
 
 #ifndef DESKTOP
@@ -211,6 +211,7 @@ void build_dec80(__xdata const char* signif_str, exp_t exponent){
 		return;
 	} else if (signif_str[0] == '-'){
 		curr_sign = SIGN_NEG_ZERO;
+		i++;
 	}
 
 	//go through digits
@@ -293,7 +294,7 @@ void build_dec80(__xdata const char* signif_str, exp_t exponent){
 				return;
 			} else {
 				//not zero
-				int8_t new_exponent;
+				exp_t new_exponent = exponent;
 				//write out saved nibble, if it exists
 				// (saved while nibble_i even, nibble_i then incremented to odd)
 				if (nibble_i & 1){ //odd
@@ -307,37 +308,28 @@ void build_dec80(__xdata const char* signif_str, exp_t exponent){
 				if (num_lr_points > 0){ //left count exists
 					assert(DEC80_NUM_LSU*2 > num_lr_points);
 					new_exponent = exponent + (num_lr_points - 1); //1 digit left of implicit point
-					//check for overflow
-#ifdef EXTRA_CHECKS
-					if (new_exponent < exponent || exponent > DEC80_MAX_EXP){
-	#ifdef DEBUG
-						printf("   overflow (new_exp, exp)=(%d,%d)\n",
-						         new_exponent, exponent);
-	#endif
-						set_dec80_NaN(&AccDecn);
-						return;
-					}
-#endif
+					//overflow is checked later, should be impossible to overflow int16_t:
+					assert(new_exponent >= exponent);
 				} else if (num_lr_points < 0) { //right count exists
 					// (num_lr_points represents exponent shift)
 					// (this ends up being a subtraction)
 					new_exponent = exponent + num_lr_points;
-					//check for underflow
-#ifdef EXTRA_CHECKS
-					if (new_exponent > exponent || exponent < DEC80_MIN_EXP){
-	#ifdef DEBUG
-						printf("   underflow (new_exp, exp)=(%d,%d)\n",
-						         new_exponent, exponent);
-	#endif
-						set_dec80_NaN(&AccDecn);
-						return;
-					}
-#endif
-				} else {
-					//no change
-					new_exponent = exponent;
+					//underflow is checked later, should be impossible to overflow int16_t:
+					assert(new_exponent <= exponent);
 				}
+
+				//check for over/underflow of exponent
 				exponent = new_exponent;
+#ifdef EXTRA_CHECKS
+				if (exponent > DEC80_MAX_EXP || exponent < DEC80_MIN_EXP){
+	#ifdef DEBUG
+					printf("   over/underflow (new_exp, exp)=(%d,%d)\n",
+						   new_exponent, exponent);
+	#endif
+					set_dec80_NaN(&AccDecn);
+					return;
+				}
+#endif
 				//set negative bit
 				set_exponent(&AccDecn, exponent, IS_NEG(curr_sign));
 				//normalize
@@ -354,6 +346,12 @@ void build_dec80(__xdata const char* signif_str, exp_t exponent){
 
 				return;
 			}
+		} else { //invalid character
+#ifdef DEBUG
+			printf(" invalid character %c at i=%d\n", signif_str[i], i);
+#endif
+			set_dec80_NaN(&AccDecn);
+			return;
 		}
 		i++;
 		assert(i < DECN_BUF_SIZE);
@@ -439,7 +437,7 @@ static int8_t compare_magn(void){ //returns a<b: -1, a==b: 0, a>b: 1
 	//compare signifcands while tracking magnitude
 	for (
 		a_i = 0, b_i = 0;
-		a_i < DEC80_NUM_LSU && b_i < DEC80_NUM_LSU;
+		a_i < DEC80_NUM_LSU;
 		a_i++, b_i++, a_exp+=2, b_exp+=2
 		)
 	{
@@ -662,26 +660,17 @@ void add_decn(void){
 		uint8_t digit100 = AccDecn.lsu[i] + BDecn.lsu[i] + carry;
 		AccDecn.lsu[i] = digit100 % 100;
 		carry = digit100 / 100;
-		assert(carry < 100);
+		assert(carry <= 1);
 	}
 	//may need to rescale number
 	if (carry > 0){
+		assert(carry == 1);
 		exp_t curr_exp = get_exponent(&AccDecn);
 		rel = (AccDecn.exponent < 0); //is_neg?
-#ifdef DEBUG_ADD
-		printf("        carry out: %d", carry);
-#endif
 		//shift right
-		if (carry < 10){
-			shift_right(&AccDecn);
-			AccDecn.lsu[0] += carry*10; //carry gets shifted into most significant digit
-			curr_exp++;
-		} else {
-			shift_right(&AccDecn);
-			shift_right(&AccDecn);
-			AccDecn.lsu[0] = carry;
-			curr_exp+=2;
-		}
+		shift_right(&AccDecn);
+		AccDecn.lsu[0] += 10; //carry gets shifted into most significant digit
+		curr_exp++;
 		//track sign
 		set_exponent(&AccDecn, curr_exp, rel); //rel==is_neg?
 	}
@@ -715,7 +704,8 @@ void mult_decn(void){
 	//calculate new exponent
 	new_exponent = get_exponent(&AccDecn) + get_exponent(&BDecn);
 #ifdef DEBUG_MULT
-		printf("\n new exponent: %d, is_neg: %u", new_exponent, is_neg);
+	printf("\n a_exp: %d, b_exp: %d", get_exponent(&AccDecn), get_exponent(&BDecn));
+	printf("\n new exponent: %d, is_neg: %u", new_exponent, is_neg);
 #endif
 	//do multiply
 	for (i = DEC80_NUM_LSU - 1; i >= 0; i--){
@@ -1079,13 +1069,11 @@ void exp_decn(void){
 	//check if in range
 	copy_decn(&SAVED, &AccDecn); //save = accum
 	set_dec80_zero(&BDecn);
-	BDecn.lsu[0] = 23;
-	BDecn.lsu[1] = 02;
-	BDecn.lsu[2] = 58;
-	BDecn.lsu[2] = 51;
-	BDecn.exponent = 2; //b = 230.25851
+	BDecn.lsu[0] = 29;
+	BDecn.lsu[1] = 47;
+	BDecn.exponent = 2; //b = 294.7
 	negate_decn(&BDecn);
-	add_decn(); //accum = x - 230.25851 (should be negative if in range)
+	add_decn(); //accum = x - 294.7 (should be negative if in range)
 	if (!(AccDecn.exponent < 0)){ //if not negative
 		set_dec80_NaN(&AccDecn);
 		return;
@@ -1249,7 +1237,12 @@ static void set_str_error(void){
 	Buf[5] = '\0';
 }
 
-int8_t decn_to_str(const dec80* x){
+#ifdef DESKTOP
+int
+#else
+int8_t
+#endif
+decn_to_str(const dec80* x){
 #define INSERT_DOT() Buf[i++]='.'
 	uint8_t i = 0;
 	uint8_t digit100;
@@ -1377,7 +1370,11 @@ int8_t decn_to_str(const dec80* x){
 	//print exponent
 	if (use_sci){
 		//check for overflow
+#ifdef DESKTOP
+		if (exponent > DEC80_MAX_EXP || exponent < DEC80_MIN_EXP){
+#else
 		if (exponent > DECN_MAX_PRINT_EXP || exponent < DECN_MIN_PRINT_EXP){
+#endif
 			set_str_error();
 			return 0;
 		}
@@ -1398,7 +1395,7 @@ int8_t decn_to_str(const dec80* x){
 #ifdef DESKTOP
 //complete string including exponent
 void decn_to_str_complete(const dec80* x){
-	int8_t exponent = decn_to_str(x);
+	int exponent = decn_to_str(x);
 	int i;
 	//find end of string
 	for (i = 0; Buf[i] != '\0'; i++);
diff --git a/src/decn/decn.h b/src/decn/decn.h
index 93da64b..a0f2b70 100644
--- a/src/decn/decn.h
+++ b/src/decn/decn.h
@@ -107,7 +107,13 @@ void exp10_decn(void);
 //Buf should hold at least 18 + 4 + 5 + 1 = 28
 #define DECN_BUF_SIZE 28
 extern __xdata char Buf[DECN_BUF_SIZE];
-int8_t decn_to_str(const dec80* x);
+
+#ifdef DESKTOP
+int
+#else
+int8_t
+#endif
+decn_to_str(const dec80* x);
 
 #ifdef DESKTOP
 //complete string including exponent
diff --git a/src/decn/decn_tests.cpp b/src/decn/decn_tests.cpp
index 8af22c4..970c4ea 100644
--- a/src/decn/decn_tests.cpp
+++ b/src/decn/decn_tests.cpp
@@ -20,10 +20,11 @@
  */
 
 
-#include <string.h>
+#include <string>
 #include <boost/multiprecision/mpfr.hpp>
 #include <catch.hpp>
 #include "decn.h"
+#include "../utils.h"
 
 
 namespace bmp = boost::multiprecision;
@@ -46,6 +47,119 @@ TEST_CASE("build decn"){
 	negate_decn(&AccDecn);
 	decn_to_str_complete(&AccDecn);
 	CHECK_THAT(Buf, Equals("-9234.567890123456"));
+
+	//small positive
+	build_dec80(".1", 3);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("100."));
+
+	build_dec80("0.1", -1);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("0.01"));
+
+	build_dec80("0.01", 3);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("10."));
+
+	//zero
+	build_dec80(".", 3);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("0"));
+
+	build_dec80(".0", 3);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("0"));
+
+	build_dec80("0.", 3);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("0"));
+
+	build_dec80("-0.0", -1);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("0"));
+
+	build_dec80("0", 3);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("0"));
+
+	//small negative
+	build_dec80("-.1", 3);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("-100."));
+
+	build_dec80("-0.1", -1);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("-0.01"));
+
+	build_dec80("-0.001", 3);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("-1."));
+
+	//empty string -> 0
+	build_dec80("", 90);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("0"));
+
+	//too many .
+	build_dec80("..", 3);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("Error"));
+
+	//very long (truncated)
+	build_dec80("12345678901234567890", -2);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("123456789012345678."));
+
+	//overflow
+	build_dec80("100", DEC80_MAX_EXP-1);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("Error"));
+
+	build_dec80("1", DEC80_MAX_EXP+1);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("Error"));
+
+	build_dec80("0.1", DEC80_MAX_EXP+2);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("Error"));
+
+	//underflow
+	build_dec80("10", DEC80_MIN_EXP-2);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("Error"));
+
+	build_dec80("1", DEC80_MIN_EXP-1);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("Error"));
+
+	build_dec80("0.3", DEC80_MIN_EXP);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("Error"));
+	CHECK(decn_is_nan(&AccDecn) == 1);
+
+	//left/right count
+	build_dec80("-100.001", 3);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("-100001."));
+
+	//invalid
+	build_dec80(":", 0);
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("Error"));
+
+	//special number that is not NaN
+	AccDecn.lsu[1] = 3;
+	CHECK(decn_is_nan(&AccDecn) == 0);
+}
+
+TEST_CASE("build_large"){
+	int large_exp = DEC80_MAX_EXP/2 - 50;
+	build_dec80("9.99", large_exp);
+	decn_to_str_complete(&AccDecn);
+	CHECK(AccDecn.exponent == large_exp);
+	std::string expected = "9.99E";
+	expected += std::to_string(large_exp);
+	CHECK_THAT(Buf, Equals(expected));
 }
 
 TEST_CASE("small fractions >= 1/10"){
@@ -91,6 +205,24 @@ TEST_CASE("add"){
 	decn_to_str_complete(&AccDecn);
 	//compare result of new acc - b
 	CHECK_THAT(Buf, Equals("-9234.567890123456")); //acc-b
+
+	//add 0
+	build_decn_at(&BDecn, "0", 0);
+	add_decn();
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("-9234.567890123456")); //same
+
+	//carry into MSB
+	build_dec80(          "-82345678901234567.8", -1);
+	build_decn_at(&BDecn, "-87654321098765432.2", -1);
+	add_decn();
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf,Equals("-17000000000000000.")); //acc+b
+
+	//don't negate NaN
+	set_dec80_NaN(&AccDecn);
+	negate_decn(&AccDecn);
+	CHECK(decn_is_nan(&AccDecn));
 }
 
 TEST_CASE("multiply"){
@@ -103,6 +235,14 @@ TEST_CASE("multiply"){
 	mult_decn();
 	decn_to_str_complete(&AccDecn);
 	CHECK_THAT(Buf, Equals("-8527724.41172991849")); //acc*b
+
+	//overflow
+	build_dec80("9.99", DEC80_MAX_EXP/2);
+	build_decn_at(&BDecn, "9.99", DEC80_MAX_EXP/2);
+	mult_decn();
+	decn_to_str_complete(&AccDecn);
+	CHECK_THAT(Buf, Equals("Error")); //acc*b
+
 }
 
 static void div_test(
@@ -126,7 +266,6 @@ static void div_test(
 
 	//calculate actual result
 	bmp::mpfr_float::default_precision(50);
-	bmp::mpfr_float calculated(Buf);
 	std::string a_full_str(a_str);
 	a_full_str += "e" + std::to_string(a_exp);
 	std::string b_full_str(b_str);
@@ -135,12 +274,24 @@ static void div_test(
 // 	CAPTURE(b_full_str);
 	bmp::mpfr_float a_actual(a_full_str);
 	bmp::mpfr_float b_actual(b_full_str);
-	a_actual /= b_actual;
-	bmp::mpfr_float rel_diff = abs((a_actual - calculated) / a_actual);
-	CHECK(rel_diff < 1e-17);
+	a_actual /= b_actual; //calculate actual result
+	if (decn_is_nan(&AccDecn)){
+		//check that NaN result of division by 0
+		CAPTURE(a_actual);
+		CHECK(b_actual == 0);
+	} else {
+		bmp::mpfr_float calculated(Buf);
+		bmp::mpfr_float rel_diff = abs((a_actual - calculated) / a_actual);
+		CHECK(rel_diff < 1e-17);
+	}
 }
 
 TEST_CASE("division"){
+	div_test(
+		"1", 0,
+		"0", 0
+	);
+
 	div_test(
 		"3.14", 60,
 		"-1.5", -2
@@ -198,22 +349,29 @@ static void log_test(
 
 	//calculate actual result
 	bmp::mpfr_float::default_precision(50);
-	bmp::mpfr_float calculated(Buf);
 	std::string x_full_str(x_str);
-	   x_full_str += "e" + std::to_string(x_exp);
-	// 	CAPTURE(x_full_str);
+	x_full_str += "e" + std::to_string(x_exp);
+	CAPTURE(x_full_str);
 	bmp::mpfr_float x_actual(x_full_str);
-	if (base10){
-		x_actual = log10(x_actual);
-	} else {
-		x_actual = log(x_actual);
-	}
 	CAPTURE(x_actual);
-	bmp::mpfr_float rel_diff = abs((x_actual - calculated) / x_actual);
-	CHECK(rel_diff < 3e-16); //TODO
+	if (decn_is_nan(&AccDecn)){
+		//check that NaN is from result of log(-)
+		CHECK(x_actual <= 0);
+	} else {
+		if (base10){
+			x_actual = log10(x_actual);
+		} else {
+			x_actual = log(x_actual);
+		}
+		bmp::mpfr_float calculated(Buf);
+		bmp::mpfr_float rel_diff = abs((x_actual - calculated) / x_actual);
+		CHECK(rel_diff < 3e-16); //TODO
+	}
 }
 
 TEST_CASE("log"){
+	log_test("0", 0);
+	log_test("-1", 0);
 	log_test("0.155", 0);
 	log_test("10", 0);
 	log_test("1.1", 10);
@@ -221,19 +379,27 @@ TEST_CASE("log"){
 	log_test("2.02", 0);
 	log_test("1.5", 0, true);
 	log_test("9", 99);
+	log_test("123", 12345);
 }
 
 static void exp_test(
 	//input
 	const char* x_str, int x_exp,
-	double epsilon=6e-16
+	double epsilon=6e-16,
+	bool base10=false
 )
 {
 	CAPTURE(x_str); CAPTURE(x_exp);
+	CAPTURE(base10);
 	build_dec80(x_str, x_exp);
-	exp_decn();
+	if (base10){
+		exp10_decn();
+	} else {
+		exp_decn();
+	}
 	decn_to_str_complete(&AccDecn);
 	CAPTURE(Buf);  // exp(x)
+	CAPTURE(AccDecn.exponent);
 
 	//calculate actual result
 	bmp::mpfr_float::default_precision(50);
@@ -241,12 +407,24 @@ static void exp_test(
 	std::string x_full_str(x_str);
 	x_full_str += "e" + std::to_string(x_exp);
 	bmp::mpfr_float x_actual(x_full_str);
+	if (base10){
+		x_actual *= log(10);
+	}
 	x_actual = exp(x_actual);
 	CAPTURE(x_actual);
 	bmp::mpfr_float rel_diff = abs((x_actual - calculated) / x_actual);
 	CHECK(rel_diff < epsilon);
 }
 
+static void exp10_test(
+	//input
+	const char* x_str, int x_exp,
+	double epsilon=3e-15
+)
+{
+	exp_test(x_str, x_exp, epsilon, true);
+}
+
 TEST_CASE("exp"){
 	exp_test("4.4", 0);
 	exp_test("0.155", 0);
@@ -257,6 +435,25 @@ TEST_CASE("exp"){
 	exp_test("2.02", -10);
 	exp_test("2.02", 0);
 	exp_test("1.5", 0);
+	exp_test("294.69999999", 0, 8e-15);
+
+	//do not operate on NaN
+	set_dec80_NaN(&AccDecn);
+	exp_decn();
+	CHECK(decn_is_nan(&AccDecn)); //still NaN
+}
+
+
+TEST_CASE("exp10"){
+	exp10_test("4.4", 0);
+	exp10_test("0.155", 0);
+	exp10_test("9.999", 0);
+	exp10_test("10", 0);
+	exp10_test("10.001", 0);
+	exp10_test("2.02", -10);
+	exp10_test("2.02", 0);
+	exp10_test("1.5", 0);
+	exp10_test("127", 0, 3e-14);
 }
 
 static void pow_test(
@@ -306,3 +503,8 @@ TEST_CASE("power"){
 		"201", 0
 	);
 }
+
+TEST_CASE("u32str corner"){
+	u32str(0, &Buf[0], 10);
+	CHECK_THAT(Buf, Equals("0"));
+}