diff --git a/src/devices/cpu/sm510/sm590.h b/src/devices/cpu/sm510/sm590.h
index bf3cf49f7d7..943a41e6297 100644
--- a/src/devices/cpu/sm510/sm590.h
+++ b/src/devices/cpu/sm510/sm590.h
@@ -108,7 +108,7 @@ protected:
 
 	// opcode handlers
 	// 00-3f
-	// adx (same as sm510)
+	virtual void op_adx() override;
 	virtual void op_tax();
 	virtual void op_lblx();
 	// lax (same as sm510)
diff --git a/src/devices/cpu/sm510/sm590op.cpp b/src/devices/cpu/sm510/sm590op.cpp
index 3d116a436fd..99b1c7189b7 100644
--- a/src/devices/cpu/sm510/sm590op.cpp
+++ b/src/devices/cpu/sm510/sm590op.cpp
@@ -16,10 +16,12 @@ void sm590_device::do_branch(u8 pu, u8 pm, u8 pl)
 
 // instruction set
 
-void sm590_device::op_lblx()
+void sm590_device::op_adx()
 {
-	// LBL x: load BL with 4-bit immediate value
-	m_bl = (m_op & 0xf);
+	// ADX x: add immediate value to ACC, skip next on carry
+	m_acc += (m_op & 0xf);
+	m_skip = bool(m_acc & 0x10);
+	m_acc &= 0xf;
 }
 
 void sm590_device::op_tax()
@@ -28,6 +30,12 @@ void sm590_device::op_tax()
 	m_skip = (m_acc == (m_op & 0xf));
 }
 
+void sm590_device::op_lblx()
+{
+	// LBL x: load BL with 4-bit immediate value
+	m_bl = (m_op & 0xf);
+}
+
 void sm590_device::op_lda()
 {
 	// LDA: load ACC with RAM