Modified memory/MMC to allow 'writePassthrough' (enabled by default) which will pass any write attempt to a writable chunk of memory within the same bank if one is available. Updated tests as needed.

src/memory/mmc.js

@@ -8,6 +8,15 @@ class Switch{
   }
 
   get mem(){return this.__mem[this.__idx];}
+  get wmem(){
+    let sc = this.__mem.length;
+    for (let i=0; i < sc; i++){
+      let idx = (this.__idx + i) % sc;
+      if (this.__mem[idx].writable)
+        return this.__mem[idx];
+    }
+    return null;
+  }
 
   get idx(){return this.__idx;}
   set idx(i){
@@ -15,6 +24,8 @@ class Switch{
       this.__idx = i;
   }
 
+  get length(){return this.__mem.length;}
+
   addMemModule(mem){
     if (!(mem instanceof IMem))
       throw new ValueError("Only IMem instances can be added to MMC Banks.");
@@ -34,6 +45,7 @@ class MMC extends IMem{
     this.__switches = [];
     this.__addr = 0;
     this.__sidx = 0;
+    this.__wpass = true;
   }
 
   get pages(){
@@ -58,6 +70,11 @@ class MMC extends IMem{
     return iw;
   }
 
+  get writePassthrough(){return this.__wpass;}
+  set writePassthrough(p){
+    this.__wpass = (p === true || p === 1);
+  }
+
   get switches(){return this.__switches.length;}
 
   get address(){return this.__addr;}
@@ -81,7 +98,18 @@ class MMC extends IMem{
 
   set byte(b){
     if (this.__switches.length > 0){
-      this.__switches[this.__sidx].mem.byte = b;
+      if (this.__wpass){ // If write passthrough is enabled
+        // Get the *next writable memory
+        // NOTE: If the current memory bank is already writable, it will be
+        // the one used.
+        let wmem = this.__switches[this.__sidx].wmem;
+        if (wmem !== null) // if not null (meaning writable memory found)
+          wmem.byte = b; // write to it.
+      } else {
+        // Attempt a write to the current bank only. If it's not writable,
+        // nothing will happen anyway, so no need to test any harder.
+        this.__switches[this.__sidx].mem.byte = b;
+      }
     }
   }
 

test/unit.src.memory.spec.js

@@ -426,6 +426,8 @@ describe("Testing Memory Module", function(){
     });
 
     it("Write Check 2 (switch between RAM and ROM)", function(){
+      // Disable memory write passthrough for the initial tests...
+      mmc.writePassthrough = false;
       mmc.write(0x0115, 0x16);
       expect(mmc.read(0x0115)).to.equal(0x16);
       mmc.switchBank(0x11);
@@ -434,6 +436,14 @@ describe("Testing Memory Module", function(){
       expect(mmc.read(0x0115)).to.equal(0x00);
       mmc.switchBank(0x10);
       expect(mmc.read(0x0115)).to.equal(0x16);
+
+      // Reenable write passthrough!
+      mmc.writePassthrough = true;
+      mmc.switchBank(0x11);
+      mmc.write(0x0115, 0x17);
+      expect(mmc.read(0x0115)).to.equal(0x00);
+      mmc.switchBank(0x10);
+      expect(mmc.read(0x0115)).to.equal(0x17);
     });
 
     it("Set Address / Get Byte", function(){