Merge pull request #5238 from DavidHaywood/140619

xavix - store the extra codebank byte in a private stack when making long calls etc.

src/devices/cpu/m6502/oxavix.lst

@@ -4,8 +4,8 @@
 
 callf_xa3
 	read(SP);
-	write(SP, get_codebank());
+	write_special_stack(get_codebank());
-	dec_SP();
+	dec_special_stack();
 	TMP2 = read_pc();
 	TMP = read_pc();
 	//read(SP);
@@ -33,8 +33,8 @@ retf_imp
 	PC = read(SP);
 	inc_SP();
 	PC = set_h(PC, read(SP));
-	inc_SP();
+	inc_special_stack();
-	TMP2 = read(SP);
+	TMP2 = read_special_stack();
 	set_codebank(TMP2);
 	read_pc();
 	prefetch();
@@ -47,9 +47,9 @@ brk_xav_imp
 	} else {
 		read_pc();
 	}
-	write(SP, get_codebank());
+	write_special_stack(get_codebank());
 	set_codebank(0x00); // epo_efdx, rad_ping and rad_mtrk strongly suggest that interrupts must force bank 0 as code jumps to a ROM pointer stored earlier / a fixed pointer to a rom address in bank 0
-	dec_SP();
+	dec_special_stack();
 	write(SP, PC >> 8);
 	dec_SP();
 	write(SP, PC);
@@ -115,8 +115,8 @@ rti_xav_imp
 	PC = read(SP);
 	inc_SP();
 	PC = set_h(PC, read(SP));
-	inc_SP();
+	inc_special_stack();
-	TMP2 = read(SP);
+	TMP2 = read_special_stack();
 	set_codebank(TMP2);
 	prefetch();
 

src/devices/cpu/m6502/xavix.cpp

@@ -93,12 +93,19 @@ void xavix_device::device_start()
 	m_lowbus_space = &space(5);
 	m_extbus_space = &space(6);
 
-	state_add(XAVIX_DATABANK, "DATBNK", m_databank).callimport().formatstr("%2s");;
+	state_add(XAVIX_DATABANK, "DATBNK", m_databank).callimport().formatstr("%2s");
 	state_add(XAVIX_CODEBANK, "CODBNK", m_codebank).callimport().formatstr("%2s");
+
+	save_item(NAME(m_special_stack));
+	save_item(NAME(m_special_stackpos));
 }
 
 void xavix_device::device_reset()
 {
+	m_special_stackpos = 0xff;
+	for (int i = 0; i < 0x100; i++)
+		m_special_stack[i] = 0x00;
+
 	set_codebank(0);
 	set_databank(0);
 	m6502_device::device_reset();
@@ -126,6 +133,28 @@ xavix_device::mi_xavix_normal::mi_xavix_normal(xavix_device *_base)
 	base = _base;
 }
 
+void xavix_device::write_special_stack(uint8_t data)
+{
+	m_special_stack[m_special_stackpos&0xff] = data;
+}
+
+void xavix_device::dec_special_stack()
+{
+	m_special_stackpos--;
+}
+
+void xavix_device::inc_special_stack()
+{
+	m_special_stackpos++;
+}
+
+uint8_t xavix_device::read_special_stack()
+{
+	return m_special_stack[m_special_stackpos&0xff];
+}
+
+
+
 inline uint8_t xavix_device::read_full_data(uint32_t addr)
 {
 	return read_full_data((addr & 0xff0000)>>16, addr & 0xffff);
@@ -194,7 +223,7 @@ inline uint8_t xavix_device::read_full_data_sp(uint8_t databank, uint16_t adr)
 			if (adr == 0xfe)
 				return m_codebank;
 			else if (adr == 0xff)
-				return databank;
+				return m_databank;
 
 			return m_lowbus_space->read_byte(adr);
 		}
@@ -275,6 +304,9 @@ inline void xavix_device::write_full_data(uint8_t databank, uint16_t adr, uint8_
 				return;
 			}
 
+			// actually it is more likely that all zero page and stack operations should go through their own handlers, and never reach here
+			// there is code that explicitly uses pull/push opcodes when in the high banks indicating that the stack area likely isn't
+			// mapped normally
 			if ((adr & 0x7fff) >= 0x200)
 			{
 				m_extbus_space->write_byte((databank << 16) | adr, val);

src/devices/cpu/m6502/xavix.h

@@ -131,6 +131,18 @@ protected:
 	void set_databank(uint8_t bank);
 	uint8_t get_databank();
 
+	void write_special_stack(uint8_t data);
+	void dec_special_stack();
+	void inc_special_stack();
+	uint8_t read_special_stack();
+
+	/* we store the additional 'codebank' used for far calls in a different, private stack
+	   this seems to be neccessary for 'rad_hnt2' not to crash when bringing up the calibration / score table screens
+	   and also for ekara 'a1' cart not to crash shortly after going ingame
+	   it's possible however the stack format is just incorrect, so the exact reason for this being needed does
+	   need further research */
+	uint8_t m_special_stack[0x100];
+	uint8_t m_special_stackpos;
 };
 
 enum {