diff --git a/ares/n64/cartridge/cartridge.cpp b/ares/n64/cartridge/cartridge.cpp
index 4be91ef257..36d430b197 100644
--- a/ares/n64/cartridge/cartridge.cpp
+++ b/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);
 
diff --git a/ares/n64/cartridge/cartridge.hpp b/ares/n64/cartridge/cartridge.hpp
index 23a2c6d878..9efbf30a4a 100644
--- a/ares/n64/cartridge/cartridge.hpp
+++ b/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;
diff --git a/ares/n64/pi/bus.hpp b/ares/n64/pi/bus.hpp
index ad9b3348f8..893e855ff6 100644
--- a/ares/n64/pi/bus.hpp
+++ b/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;
 }