n64: support ROMs bigger than 63.9375 MiB (#1354)

Currently, the n64 core is limited to ROMs whose size is 63.9375 MiB
(also known as 64 MiB - 64 KiB). The reason is twofold: first, only 64
MiB are actually mapped in the PI bus; second, the last 64 KiB are
shadowed by the ISViewer debugging device.

ISViewer addresses sit in the last 64 KiB of the 64th MiB, so when the
ROM is larger than that (for instance, it is exactly 64 MiB), we should
simply disable ISViewer. In the long run, there will be a better
debugging solution which doesn't sit in the PI address space anyway.

Moreover, N64 supports basically ROM of unlimited size, there is no 64
MiB limit. This commit also allows that by simply mapping the whole ROM
into the PI address bus, whatever the actual size is.

ares/n64/cartridge/cartridge.cpp

@@ -47,10 +47,12 @@ auto Cartridge::connect() -> void {
 
   rtc.load();
 
-  isviewer.ram.allocate(64_KiB);
-  isviewer.tracer = node->append<Node::Debugger::Tracer::Notification>("ISViewer", "Cartridge");
-  isviewer.tracer->setAutoLineBreak(false);
-  isviewer.tracer->setTerminal(true);
+  if(rom.size <= 0x03fe'ffff && system.homebrewMode) {
+    isviewer.ram.allocate(64_KiB);
+    isviewer.tracer = node->append<Node::Debugger::Tracer::Notification>("ISViewer", "Cartridge");
+    isviewer.tracer->setAutoLineBreak(false);
+    isviewer.tracer->setTerminal(true);
+  }
 
   debugger.load(node);
 

ares/n64/cartridge/cartridge.hpp

@@ -42,6 +42,8 @@ struct Cartridge {
     Memory::Writable ram;  //unserialized
     Node::Debugger::Tracer::Notification tracer;
 
+    auto enabled() -> bool { return ram.size; }
+
     //isviewer.cpp
     auto messageChar(char c) -> void;
     auto readHalf(u32 address) -> u16;

ares/n64/pi/bus.hpp

@@ -43,13 +43,13 @@ inline auto PI::busRead(u32 address) -> u32 {
     if(cartridge.flash) return cartridge.flash.read<Size>(address);
     return unmapped;
   }
-  if(address <= 0x13fe'ffff) {
-    if(cartridge.rom  ) return cartridge.rom.read<Size>(address);
-    return unmapped;
+  if(cartridge.isviewer.enabled() && address >= 0x13f0'0000 && address <= 0x13ff'ffff) {
+    return cartridge.isviewer.read<Size>(address);
+  }
+  if(address <= 0x1000'0000 + cartridge.rom.size - 1) {
+    return cartridge.rom.read<Size>(address);
   }
-  if(address <= 0x13ff'ffff) return cartridge.isviewer.read<Size>(address);
-  if(address <= 0x7fff'ffff) return unmapped;
-  return unmapped; //accesses here actually lock out the RCP
+  return unmapped;
 }
 
 inline auto PI::writeWord(u32 address, u32 data, Thread& thread) -> void {
@@ -93,18 +93,19 @@ inline auto PI::busWrite(u32 address, u32 data) -> void {
     if(cartridge.flash) return cartridge.flash.write<Size>(address, data);
     return;
   }
-  if(address <= 0x13fe'ffff) {
-    if(cartridge.rom  ) return cartridge.rom.write<Size>(address, data);
-    return;
-  }
-  if(address <= 0x13ff'ffff) {
-    if(system.homebrewMode) {
+  if(address >= 0x13f0'0000 && address <= 0x13ff'ffff) {
+    if(cartridge.isviewer.enabled()) {
       writeForceFinish(); //Debugging channel for homebrew, be gentle
       return cartridge.isviewer.write<Size>(address, data);      
-    } else {
+    } else if (!system.homebrewMode) {
       debug(unhandled, "[PI::busWrite] attempt to write to ISViewer: enable homebrew mode in settings to enable ISViewer emulation");
+    } else {
+      debug(unhandled, "[PI::busWrite] attempt to write to ISViewer: ROM is too big so ISViewer is disabled");
     }
   }
+  if(address <= 0x1000'0000 + cartridge.rom.size - 1) {
+    return cartridge.rom.write<Size>(address, data);
+  }
   if(address <= 0x7fff'ffff) return;
 }