Rewrote .load() method.

src/memory/mmc.js

   set address(a){
     if (a >= 0 && a < this.size){
       this.__addr = a;
-      offset = 0;
+      let offset = 0;
       for (let s=0; s < this.__switches.length; s++){
         if (a >= offset && a < offset + this.__switches[s].mem.size){
-          this.__sidx = b;
+          this.__sidx = s;
           this.__switches[s].mem.address = a - offset;
           break;
         } else {
     }
   }
 
-  get byte(){return (this.__switches.length > 0) ? this.__switches[this.__sidx].mem.byte : -1;}
+  get byte(){
+    return (this.__switches.length > 0) ? this.__switches[this.__sidx].mem.byte : -1;
+  }
+
   set byte(b){
     if (this.__switches.length > 0){
       this.__switches[this.__sidx].mem.byte = b;
     if (addr < 0 || addr > this.size)
       throw new RangeError("Memory address out of range.");
     let offset = 0;
-    for (let i = 0; i < this.__switches.length; i++){
-      let mem = this.__switches[i].mem;
-      let od = data;
-      if (addr >= offset && addr < offset + mem.size){
-        if ((addr - offset) + d.length >= mem.size)
-          od = data.slice(0, mem.size);
-        mem.load(addr - offset, od);
+    let doff = 0;
+    let addre = addr + data.length;
+    for (let s = 0; s < this.__switches.length; s++){
+      let mem = this.__switches[s].mem;
+      if (addr < offset + mem.size && addre >= offset){
+        let a = (addr > offset) ? addr - offset : 0;
+        let dlen = (mem.size - a < data.length - doff) ? 
+          mem.size - a : data.length - doff; 
+        mem.load(a, data.slice(doff, doff + dlen));
+        doff += dlen;
       }
-      data = data.slice(mem.size);
       offset += mem.size;
-      if (d.length > 0)
-        addr = offset;
     }
     return this;
   }