small update, thanks plgDavid for sending me that doc

src/emu/cpu/ucom4/ucom4.c

@@ -4,8 +4,10 @@
 
   NEC uCOM-4 MCU family cores
   
-  reference: 1981 NEC Microcomputers Catalog (later editions may have errors!)
+  References:
-  also looked at asterick's JavaScript D553 emulator for verification, with permission
+  - 1981 NEC Microcomputers Catalog (later editions may have errors!)
+  - Supplement to uCOM-43 Single Chip Microcomputer Users' Manual
+  I've also looked at asterick's JavaScript D553 emulator for verification, with permission.
 
   TODO:
   - what happens with uCOM-43 opcodes on an uCOM-44/45 MCU?
@@ -52,7 +54,8 @@ ADDRESS_MAP_END
 
 static ADDRESS_MAP_START(data_96x4, AS_DATA, 8, ucom4_cpu_device)
 	AM_RANGE(0x00, 0x3f) AM_RAM
-	AM_RANGE(0x40, 0x5f) AM_RAM AM_MIRROR(0x20)
+	AM_RANGE(0x40, 0x4f) AM_RAM
+	AM_RANGE(0x70, 0x7f) AM_RAM
 ADDRESS_MAP_END
 
 
@@ -199,6 +202,12 @@ void ucom4_cpu_device::device_reset()
 //  execute
 //-------------------------------------------------
 
+void ucom4_cpu_device::increment_pc()
+{
+	// upper bits (field register) don't auto-increment
+	m_pc = (m_pc & ~0xff) | ((m_pc + 1) & 0xff);
+}
+
 void ucom4_cpu_device::fetch_arg()
 {
 	// 2-byte opcodes: STM/LDI/CLI/CI, JMP/CAL, OCD
@@ -206,7 +215,7 @@ void ucom4_cpu_device::fetch_arg()
 	{
 		m_icount--;
 		m_arg = m_program->read_byte(m_pc);
-		m_pc = (m_pc + 1) & m_prgmask;
+		increment_pc();
 	}
 }
 
@@ -222,7 +231,7 @@ void ucom4_cpu_device::execute_run()
 		debugger_instruction_hook(this, m_pc);
 		m_op = m_program->read_byte(m_pc);
 		m_bitmask = 1 << (m_op & 0x03);
-		m_pc = (m_pc + 1) & m_prgmask;
+		increment_pc();
 		fetch_arg();
 		
 		if (m_skip)

src/emu/cpu/ucom4/ucom4.h

@@ -110,8 +110,6 @@ protected:
 	virtual UINT32 execute_input_lines() const { return 1; }
 	virtual void execute_run();
 	
-	void fetch_arg();
-
 	// device_memory_interface overrides
 	virtual const address_space_config *memory_space_config(address_spacenum spacenum = AS_0) const { return(spacenum == AS_PROGRAM) ? &m_program_config :((spacenum == AS_DATA) ? &m_data_config : NULL); }
 
@@ -167,15 +165,23 @@ protected:
 	devcb_write8 m_write_h;
 	devcb_write8 m_write_i;
 	
-	// opcode handlers
+	// misc internal helpers
+	void increment_pc();
+	void fetch_arg();
+
 	UINT8 ram_r();
 	void ram_w(UINT8 data);
 	void pop_stack();
 	void push_stack();
 	UINT8 input_r(int index);
 	void output_w(int index, UINT8 data);
-	void op_illegal();
+
 	bool check_op_43();
+	UINT8 ucom43_reg_r(int index);
+	void ucom43_reg_w(int index, UINT8 data);
+
+	// opcode handlers
+	void op_illegal();
 
 	void op_li();
 	void op_lm();

src/emu/cpu/ucom4/ucom4op.inc

@@ -326,7 +326,7 @@ void ucom4_cpu_device::op_jmpcal()
 	// JMP A: Jump to Address / CAL A: Call Address
 	if (m_op & 0x08)
 		push_stack();
-	m_pc = (m_op & 0x07) << 8 | m_arg;
+	m_pc = ((m_op & 0x07) << 8 | m_arg) & m_prgmask;
 }
 
 void ucom4_cpu_device::op_jcp()
@@ -338,6 +338,7 @@ void ucom4_cpu_device::op_jcp()
 void ucom4_cpu_device::op_jpa()
 {
 	// JPA: Jump to (ACC) in current page
+	m_icount--;
 	m_pc = (m_pc & ~0x3f) | (m_acc << 2);
 }
 
@@ -358,7 +359,7 @@ void ucom4_cpu_device::op_rt()
 void ucom4_cpu_device::op_rts()
 {
 	// RTS: Return from subroutine, skip next
-	pop_stack();
+	op_rt();
 	m_skip = true;
 }
 
@@ -434,6 +435,7 @@ void ucom4_cpu_device::op_tit()
 void ucom4_cpu_device::op_ia()
 {
 	// IA: Input port A to ACC
+	m_icount--;
 	m_acc = input_r(NEC_UCOM4_PORTA);
 }
 
@@ -446,6 +448,7 @@ void ucom4_cpu_device::op_ip()
 void ucom4_cpu_device::op_oe()
 {
 	// OE: Output ACC to port E
+	m_icount--;
 	output_w(NEC_UCOM4_PORTE, m_acc);
 }
 
@@ -483,6 +486,29 @@ inline bool ucom4_cpu_device::check_op_43()
 	return (m_family == NEC_UCOM43);
 }
 
+// extra registers reside in RAM
+enum
+{
+	UCOM43_X = 0,
+	UCOM43_Y,
+	UCOM43_R,
+	UCOM43_S,
+	UCOM43_W,
+	UCOM43_Z,
+	UCOM43_F
+};
+
+inline UINT8 ucom4_cpu_device::ucom43_reg_r(int index)
+{
+	return m_data->read_byte(m_datamask - index) & 0xf;
+}
+
+inline void ucom4_cpu_device::ucom43_reg_w(int index, UINT8 data)
+{
+	m_data->write_byte(m_datamask - index, data & 0xf);
+}
+
+
 // Transfer
 
 void ucom4_cpu_device::op_taw()