From 758d336563ab64a8667f58cd39f933da894f3c05 Mon Sep 17 00:00:00 2001
From: Preston Skupinski <skupinsk@cse.msu.edu>
Date: Wed, 11 May 2011 20:31:12 -0400
Subject: [PATCH] added emulation mode page one swap wrapping

---
 cpu.js        | 15 +++++++++++++--
 test/tests.js | 15 +++++++++++++++
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/cpu.js b/cpu.js
index 0aab334..73c7794 100644
--- a/cpu.js
+++ b/cpu.js
@@ -175,11 +175,22 @@ var MMU = {
   memory: { 0: {} },
 
   pull_byte: function() {
-    return this.memory[this.cpu.r.dbr][++this.cpu.r.s];
+    if(this.cpu.p.e&&(this.cpu.r.s===0x1ff)) {
+      this.cpu.r.s = 0x100;
+      return this.memory[this.cpu.r.dbr][this.cpu.r.s]; 
+    } else {
+      return this.memory[this.cpu.r.dbr][++this.cpu.r.s];
+    }
   },
 
   push_byte: function(b) {
-    this.memory[this.cpu.r.dbr][this.cpu.r.s--] = b;
+    if(this.cpu.p.e&&(this.cpu.r.s===0x100)) {
+      var result = this.memory[this.cpu.r.dbr][this.cpu.r.s];
+      this.cpu.r.s = 0x1ff;
+      return result;
+    } else {
+      this.memory[this.cpu.r.dbr][this.cpu.r.s--] = b;
+    }
   },
 
   read_byte: function(location) {
diff --git a/test/tests.js b/test/tests.js
index b997061..97fbe39 100644
--- a/test/tests.js
+++ b/test/tests.js
@@ -23,9 +23,24 @@ function run_tests() {
   test_cmp();
   test_subroutines();
   test_mvn_and_mvp();
+  test_emulation_mode();
+}
+
+function test_emulation_mode() {
+  module("Emulation Mode");
+  test("Make sure pulling from the stack when the stack register is at 0x1ff"+
+       "causes the stack register to pull from 0x100.", function() {
+    var cpu = new CPU_65816();
+    cpu.execute("a9fe8d0001a90068");
+    equals(cpu.r.s, 0x100, "The stack register should be 0x100 after the "+
+                           "pull operation.");
+    equals(cpu.r.a, 0xfe,  "The accumulator should be 0xfe after the pull "+
+                           "operation."); 
+  });
 }
 
 function test_mvn_and_mvp() {
+  module("MVN and MVP");
   test("Test a short example program for MVP", function() {
     var cpu = new CPU_65816();
     cpu.execute("18fbe230a9ab8dff0fa9cd8d0010c230a90100a20010a00020440000");