diff --git a/src/devices/cpu/unsp/unsp.cpp b/src/devices/cpu/unsp/unsp.cpp
index 3703735f886..72d8ef35d07 100644
--- a/src/devices/cpu/unsp/unsp.cpp
+++ b/src/devices/cpu/unsp/unsp.cpp
@@ -393,10 +393,10 @@ void unsp_device::state_import(const device_state_entry &entry)
 
 /*****************************************************************************/
 
-void unsp_device::update_nzsc(uint32_t value, int32_t svalue)
+void unsp_device::update_nzsc(uint32_t value, uint16_t r0, uint16_t r1)
 {
 	m_core->m_r[REG_SR] &= ~(UNSP_N | UNSP_Z | UNSP_S | UNSP_C);
-	if (svalue < 0)
+	if (BIT(value, 16) != BIT((r0 ^ r1), 15))
 		m_core->m_r[REG_SR] |= UNSP_S;
 	if (BIT(value, 15))
 		m_core->m_r[REG_SR] |= UNSP_N;
diff --git a/src/devices/cpu/unsp/unsp.h b/src/devices/cpu/unsp/unsp.h
index 38f04ee56df..4515e82bab4 100644
--- a/src/devices/cpu/unsp/unsp.h
+++ b/src/devices/cpu/unsp/unsp.h
@@ -234,7 +234,7 @@ protected:
 	uint16_t pop(uint32_t *reg);
 
 	void update_nz(uint32_t value);
-	void update_nzsc(uint32_t value, int32_t svalue);
+	void update_nzsc(uint32_t value, uint16_t r0, uint16_t r1);
 	bool do_basic_alu_ops(const uint16_t& op0, uint32_t& lres, uint16_t& r0, uint16_t& r1, uint32_t& r2, bool update_flags);
 
 private:
diff --git a/src/devices/cpu/unsp/unsp_other.cpp b/src/devices/cpu/unsp/unsp_other.cpp
index d02e9ee579e..2300f2a5bc0 100644
--- a/src/devices/cpu/unsp/unsp_other.cpp
+++ b/src/devices/cpu/unsp/unsp_other.cpp
@@ -281,46 +281,40 @@ void unsp_device::execute_remaining(const uint16_t op)
 
 bool unsp_device::do_basic_alu_ops(const uint16_t &op0, uint32_t &lres, uint16_t &r0, uint16_t &r1, uint32_t &r2, bool update_flags)
 {
-	int32_t sres = 0;
 	switch (op0)
 	{
 	case 0x00: // Add
 	{
 		lres = r0 + r1;
-		sres = (int16_t)r0 + (int16_t)r1;
 		if (update_flags)
-			update_nzsc(lres, sres);
+			update_nzsc(lres, r0, r1);
 		break;
 	}
 	case 0x01: // Add w/ carry
 	{
 		uint32_t c = (m_core->m_r[REG_SR] & UNSP_C) ? 1 : 0;
 		lres = r0 + r1 + c;
-		sres = (int16_t)r0 + (int16_t)r1 + (int16_t)c;
 		if (update_flags)
-			update_nzsc(lres, sres);
+			update_nzsc(lres, r0, r1);
 		break;
 	}
 	case 0x02: // Subtract
 		lres = r0 + (uint16_t)(~r1) + uint32_t(1);
-		sres = (int16_t)r0 - (int16_t)r1;
 		if (update_flags)
-			update_nzsc(lres, sres);
+			update_nzsc(lres, r0, ~r1);
 		break;
 	case 0x03: // Subtract w/ carry
 	{
 		uint32_t c = (m_core->m_r[REG_SR] & UNSP_C) ? 1 : 0;
 		lres = r0 + (uint16_t)(~r1) + c;
-		sres = (int16_t)r0 - ((int16_t)r1 - (int16_t)c);
 		if (update_flags)
-			update_nzsc(lres, sres);
+			update_nzsc(lres, r0, ~r1);
 		break;
 	}
 	case 0x04: // Compare
 		lres = r0 + (uint16_t)(~r1) + uint32_t(1);
-		sres = (int16_t)r0 - (int16_t)r1;
 		if (update_flags)
-			update_nzsc(lres, sres);
+			update_nzsc(lres, r0, ~r1);
 		return false;
 	case 0x06: // Negate
 		lres = -r1;