Add opcode 0x0XXX and some comments

emulator.js

@@ -3,12 +3,13 @@ let default_font = [0b11110000,0b10010000,0b10010000,0b10010000,0b11110000,0b001
 class CHIP8_Emulator {
   constructor() {
     this.RAM = new Uint8Array(0x1000).fill(0x00);
-    this.PC = 0x0200;
-    this.SP = 0x00;
+    this.PC = 0x0200; // Program counter
+    this.SP = 0x00; // Stack pointer
     this.I = 0x0000;
-    this.DT = 0x0;
-    this.ST = 0x0;
+    this.DT = 0x0; // Delay timer
+    this.ST = 0x0; // Sound timer
     this.vRegisters = new Uint8Array(16).fill(0x00);
+
     this.DISPLAY = new Array(32)
     for (let i = 0; i < 32; i++) {
       this.DISPLAY[i] = new Array(64).fill(0);
@@ -57,7 +58,7 @@ class CHIP8_Emulator {
       }
     } else if (opcode == 0x00EE) { 
       // Return from a subroutine
-    }else if (firstByte == 0x1) {
+    } else if (firstByte == 0x1) {
       // Jump to location nnn
       this.PC = getNNN(opcode);
     } else if (firstByte == 0x2) {
@@ -167,14 +168,17 @@ class CHIP8_Emulator {
       let kk = getKK(opcode);
       if (kk == 0x9E) {
         // Skip next instruction if key with the value of Vx is pressed
-        
+
       } else if (kk == 0xA1) {
         // Skip next instruction if key with the value of Vx is not pressed.
       } else {
         return 0;
       }
     } else if (firstByte == 0xF) {
 
+    } else if (firstByte == 0x0) {
+      // Jump to a machine code routine at nnn
+      // Ignored by modern interpreters
     } else {
       console.log("Invalid opcode", opcode)
       return 0;