From e81960a29578bdfff89aae74795a64b4adf9e6f5 Mon Sep 17 00:00:00 2001
From: penfold42 <penfold42@users.noreply.github.com>
Date: Mon, 28 May 2018 14:46:57 +1000
Subject: [PATCH] Added RAMBoard support = RAM at 0x8000

Changed Address decoding code to be clearer
---
 src/main.cpp | 76 ++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 62 insertions(+), 14 deletions(-)

diff --git a/src/main.cpp b/src/main.cpp
index f633446..e6be416 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -203,19 +203,40 @@ DWORD get_fattime() { return 0; }	// If you have hardware RTC return a correct v
 u8 read6502(u16 address)
 {
 	u8 value;
-	if (address & 0x8000)	// address line 15 selects the ROM
+	if (address & 0x8000)
 	{
-		value = roms.Read(address);
+		switch (address & 0xe000) // keep bits 15,14,13
+		{
+			case 0x8000: // 0x8000-0x9fff
+				value = s_u8Memory[address]; // 74LS42 outputs low on pin 1 or pin 2
+				break;
+			case 0xa000: // 0xa000-0xbfff
+			case 0xc000: // 0xc000-0xdfff
+			case 0xe000: // 0xe000-0xffff
+				value = roms.Read(address);
+				break;
+		}
 	}
 	else
 	{
 		// Address lines 15, 12, 11 and 10 are fed into a 74LS42 for decoding
-		u16 addressLines15_12_11_10 = (address & 0x1c00) >> 10;
-		addressLines15_12_11_10 |= (address & 0x8000) >> (15 - 3);
-		if (addressLines15_12_11_10 == 0 || addressLines15_12_11_10 == 1) value = s_u8Memory[address & 0x7ff]; // 74LS42 outputs low on pin 1 or pin 2
-		else if (addressLines15_12_11_10 == 6) value = pi1541.VIA[0].Read(address);	// 74LS42 outputs low on pin 7
-		else if (addressLines15_12_11_10 == 7) value = pi1541.VIA[1].Read(address);	// 74LS42 outputs low on pin 9
-		else value = address >> 8;	// Empty address bus
+		u16 addressLines12_11_10 = (address & 0x1c00) >> 10;
+		switch (addressLines12_11_10)
+		{
+			case 0:
+			case 1:
+				value = s_u8Memory[address & 0x7ff]; // 74LS42 outputs low on pin 1 or pin 2
+				break;
+			case 6:
+				value = pi1541.VIA[0].Read(address);	// 74LS42 outputs low on pin 7
+				break;
+			case 7:
+				value = pi1541.VIA[1].Read(address);	// 74LS42 outputs low on pin 9
+				break;
+			default:
+				value = address >> 8;	// Empty address bus
+				break;
+		}
 	}
 	return value;
 }
@@ -258,12 +279,39 @@ u8 peek6502(u16 address)
 
 void write6502(u16 address, const u8 value)
 {
-	if (address & 0x8000) return; // address line 15 selects the ROM
-	u16 addressLines15_12_11_10 = (address & 0x1c00) >> 10;
-	addressLines15_12_11_10 |= (address & 0x8000) >> (15 - 3);
-	if (addressLines15_12_11_10 == 0 || addressLines15_12_11_10 == 1) s_u8Memory[address & 0x7ff] = value; // 74LS42 outputs low on pin 1 or pin 2
-	else if (addressLines15_12_11_10 == 6) pi1541.VIA[0].Write(address, value);	// 74LS42 outputs low on pin 7
-	else if (addressLines15_12_11_10 == 7) pi1541.VIA[1].Write(address, value);	// 74LS42 outputs low on pin 9
+	if (address & 0x8000)
+	{
+		switch (address & 0xe000) // keep bits 15,14,13
+		{
+			case 0x8000: // 0x8000-0x9fff
+				s_u8Memory[address] = value; // 74LS42 outputs low on pin 1 or pin 2
+				break;
+			case 0xa000: // 0xa000-0xbfff
+			case 0xc000: // 0xc000-0xdfff
+			case 0xe000: // 0xe000-0xffff
+				return;
+		}
+	}
+	else
+	{
+		// Address lines 15, 12, 11 and 10 are fed into a 74LS42 for decoding
+		u16 addressLines12_11_10 = (address & 0x1c00) >> 10;
+		switch (addressLines12_11_10)
+		{
+			case 0:
+			case 1:
+				s_u8Memory[address & 0x7ff] = value; // 74LS42 outputs low on pin 1 or pin 2
+				break;
+			case 6:
+				pi1541.VIA[0].Write(address, value);	// 74LS42 outputs low on pin 7
+				break;
+			case 7:
+				pi1541.VIA[1].Write(address, value);	// 74LS42 outputs low on pin 9
+				break;
+			default:
+				break;
+		}
+	}
 }
 
 void write6502ExtraRAM(u16 address, const u8 value)