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HTML

              
                <link href='https://fonts.googleapis.com/css?family=VT323|Source+Sans+Pro:400,400italic' rel='stylesheet' type='text/css'>

<div id="calcBox" class="container">
  <div class="row" id="logoRow">
    <div class="col-xs-3" id="logo">
      <svg id="fcc-logo" class="pull-left">
        <use xlink:href="#free-code-camp-logo"></use>
      </svg><!--</div>
    <div class="col-xs-2" id="logoText">-->
      <div class="pull-left">
        <h6>FREECODECAMP
        <br>CALCULATOR</h6>
      </div>
    </div>
  </div>
  <div id="screenBox" class="container-fluid">
    <div id="screenLogo" class="row">
      <div class="col-xs-2">
        <h5><strong><em>FCC</em></strong></h5>
      </div>
      <div class="col-xs-6 col-xs-offset-4 text-right">
        <h5><em><strong>RPN</strong> SCIENTIFIC</em></h5>
      </div>
    </div>
    <div id="LCDborder">
      <div id="LCD" class="container-fluid">
        <div id="indicatorMem" class="indicator">MEM</div>
        <div id="indicatorOrange" class="indicator"><svg id="indicatorArrowSymbol"><use xlink:href="#unicode-arrow"></use></svg><!-- ⬏ --></div>
        <div class="LCDrow row">
          <div id="row2prefix" class="col-xs-1 rowPrefix">y:</div>
          <div id="row2" class="col-xs-11 rowNumbers">0</div>
        </div>
        <div class="LCDrow row">
          <div id="row1prefix" class="col-xs-1 rowPrefix">x:</div>
          <div id="row1" class="col-xs-11 rowNumbers">0</div>
        </div>
      </div>
    </div>
  </div><!-- #screenBox -->
  <div id="keysBox" class="container-fluid">
    <div id="topRow" class="row">
      <div class="col-xs-2">
        <aside class="disabledShift">∑−</aside>
        <button>∑+</button>
      </div>
      <div class="col-xs-2">
        <aside>y<sup>x<!-- 𝓍 --></sup></aside>
        <button class="btnOpUnary" data-op="inverse"><sup>1</sup>∕<sub>x<!-- 𝓍 --></sub></button>
      </div>
      <div class="col-xs-2">
        <aside>x<!-- 𝓍 --><sup>2</sup></aside>
        <button class="btnOpUnary" data-op="sqrt">√<span id="sqrtLine">x<!-- 𝓍 --></span></button>
      </div>
      <div class="col-xs-2">
        <aside>10<sup>x<!-- 𝓍 --></sup></aside>
        <button class="btnOpUnary" data-op="log">LOG</button>
      </div>
      <div class="col-xs-2">
        <aside>e<sup>x<!-- 𝓍 --></sup></aside>
        <button class="btnOpUnary" data-op="ln">LN</button>
      </div>
      <div class="col-xs-2">
        <aside class="disabledShift">GTO</aside>
        <button>XEQ</button>
      </div>
    </div>
    <div class="row">
      <div class="col-xs-2">
        <aside class="disabledShift"><small>COMPLEX</small></aside>
        <button id="btnStore">STO</button>
      </div>
      <div class="col-xs-2">
        <aside>%</aside>
        <button id="btnRecall">RCL</button>
      </div>
      <div class="col-xs-2">
        <aside>&pi;</aside>
        <button id="btnRollDown">R ↓</button>
      </div>
      <div class="col-xs-2">
        <aside>ASIN</aside>
        <button class="btnOpUnary" data-op="sin">SIN</button>
      </div>
      <div class="col-xs-2">
        <aside>ACOS</aside>
        <button class="btnOpUnary" data-op="cos">COS</button>
      </div>
      <div class="col-xs-2">
        <aside>ATAN</aside>
        <button class="btnOpUnary" data-op="tan">TAN</button>
      </div>
    </div>
    <div class="row">
      <div class="col-xs-4">
        <aside id="asideEnter" class="disabledShift">ALPHA</aside>
        <button id="btnEnter">ENTER</button>
      </div>
      <div class="col-xs-2">
        <aside class="disabledShift"><small>LAST x<!-- 𝓍 --></small></aside>
        <button id="btnSwapXY">x<!-- 𝓍 --><!--≷--><small>⇆</small>y</button>
      </div>
      <div class="col-xs-2">
        <aside class="disabledShift"><small>MODES</small></aside>
        <button id="btnNegation">+/−</button>
      </div>
      <div class="col-xs-2">
        <aside class="disabledShift"><small>DISP</small></aside>
        <button class="btnNum">E</button>
      </div>
      <div class="col-xs-2">
        <aside class="disabledShift"><small>CLEAR</small></aside>
        <button id="btnBksp"><span id="bkspArrow">⬅</span></button>
      </div>
    </div>
    <div class="row">
      <div class="col-fifth">
        <aside class="disabledShift">BST</aside>
        <button>▲</button>
      </div>
      <div class="col-fifth">
        <aside class="disabledShift"><small>SOLVER</small></aside>
        <button class="btnNum">7</button>
      </div>
      <div class="col-fifth">
        <aside class="disabledShift">∫ f(x<!-- 𝓍 -->)</aside>
        <button class="btnNum">8</button>
      </div>
      <div class="col-fifth">
        <aside class="disabledShift"><small>MATRIX</small></aside>
        <button class="btnNum">9</button>
      </div>
      <div class="col-fifth">
        <aside class="disabledShift"><small>STAT</small></aside>
        <button class="btnOpBinary">÷</button>
      </div>
    </div>
    <div class="row">
      <div class="col-fifth">
        <aside class="disabledShift">SST</aside>
        <button>▼</button>
      </div>
      <div class="col-fifth">
        <aside class="disabledShift"><small>BASE</small></aside>
        <button class="btnNum">4</button>
      </div>
      <div class="col-fifth">
        <aside class="disabledShift"><small>CONVERT</small></aside>
        <button class="btnNum">5</button>
      </div>
      <div class="col-fifth">
        <aside class="disabledShift"><small>FLAGS</small></aside>
        <button class="btnNum">6</button>
      </div>
      <div class="col-fifth">
        <aside class="disabledShift"><small>PROB</small></aside>
        <button class="btnOpBinary">×</button>
      </div>
    </div>
    <div class="row">
      <div class="col-fifth">
        <aside id="asideHidden">_</aside>
        <button id="btnOrange"><div></div></button>
      </div>
      <div class="col-fifth">
        <aside class="disabledShift"><small>ASSIGN</small></aside>
        <button class="btnNum">1</button>
      </div>
      <div class="col-fifth">
        <aside class="disabledShift"><small>CUSTOM</small></aside>
        <button class="btnNum">2</button>
      </div>
      <div class="col-fifth">
        <aside class="disabledShift"><small>PGM.FCM</small/></aside>
        <button class="btnNum">3</button>
      </div>
      <div class="col-fifth">
        <aside class="disabledShift"><small>PRINT</small></aside>
        <button class="btnOpBinary">−</button>
      </div>
    </div>
    <div class="row">
      <div class="col-fifth">
        <aside><!-- OFF --><span id="diagStyle">DIAG</span></aside>
        <button id="btnExit">EXIT</button>
      </div>
      <div class="col-fifth">
        <aside class="disabledShift"><small>TOP.FCN</small></aside>
        <button class="btnNum">0</button>
      </div>
      <div class="col-fifth">
        <aside class="disabledShift"><small>SHOW</small></aside>
        <button class="btnNum">.</button>
      </div>
      <div class="col-fifth">
        <aside class="disabledShift"><small>PRGM</small></aside>
        <button>R/S</button>
      </div>
      <div class="col-fifth">
        <aside class="disabledShift"><small>CATALOG</small></aside>
        <button class="btnOpBinary">+</button>
      </div>
    </div>
  </div><!-- #keysBox -->
</div><!-- #caclBox -->
<footer>
  <p class="text-center"><small>Project created from scratch by <a href="http://andreweast.net" target="_none">Andrew Reid East</a></small></p>
  <p class="text-center"><small><a href="https://en.wikipedia.org/wiki/HP-42S" target="_none">RPN Scientific 42S</a> calculator designed by HP (<a href="http://thomasokken.com/free42/42s.pdf" target="_none">manual</a>)</small></p>
</footer>

<div id="diagBox">
  <header>DIAGNOSTIC DATA</header>
  <p><label>T: <input id="stack3" type="text"></label></p>
  <p><label>Z: <input id="stack2" type="text"></label></p>
  <p><label>Y: <input id="stack1" type="text"></label></p>
  <p><label>X: <input id="stack0" type="text"></label></p>
  <hr>
  <p><label>> <input id="entryReg" type="text"></label></p>
  <div id="showDiagMem">
    <hr>
    <p><label>m: <input type="text" id="diagMem"></label></p>
  </div>
</div>

<!-- Here follows the HIDDEN svg definitions -->
<svg style="display:none;" xmlns="http://www.w3.org/2000/svg" xmlns:svg="http://www.w3.org/2000/svg">
  <symbol id="free-code-camp-logo" viewBox="0 0 400 300">
    <g>
      <path d="m166.8,41.35c0,0 13.4,9.3 13.4,26c0,28.8 -39.1,60.9 -39.1,90.8c0,32.4 24.7,53.5 48.7,59.9c3.3,0.8 4.8,-3.2 0.7,-4.2c-9.7,-2.4 -20.5,-15.1 -20.5,-35c0,-18.3 17.7,-25.4 24.9,-34.4c7.3,-9.1 5.6,-20.8 1.3,-24.1c-4.2,-3.4 -1.4,-7.9 6.3,-2.5c7.8,5.4 14,15.4 11.7,25c-3,12.2 0.9,19.2 7.3,20.1s13.4,-2.3 12.7,-8.2c-0.7,-5.9 -4.2,-12.1 -2.1,-12.1c4.5,0 14.1,11.2 14.1,28.8c0,17.7 -14.6,30.1 -19.1,34.7c-2.7,2.8 0.9,6.4 4.5,4.8c1.9,-0.8 5.8,-3.1 8.2,-4.8c11.6,-8.3 29.6,-24.9 29.6,-55.8c0,-32.3 -12.3,-46.8 -24.6,-58.9c-12.3,-12.1 -15.5,-9 -12,-3.1c10.6,18.1 4.2,24.2 -3.6,24.2c-9,0 -9.5,-20.5 -13,-36.1c-7.8,-34.7 -39.1,-40.6 -45.1,-40.9c-6.1,-0.7 -11.3,2.7 -4.3,5.8z"
      id="svg_3" />
      <path d="m13.7,144.95c0,-39.7 13.8,-75.1 41.4,-106.3c10.1,-11.4 18,-17.2 23.5,-17.2c2.1,0 3.9,0.8 5.6,2.3c1.7,1.5 2.5,3.2 2.5,5.2c0,2.9 -3.5,7.7 -10.6,14.5c-28.8,27.7 -43.2,61.9 -43.2,102.6c0,45.2 14.9,82 44.8,110.5c6,5.7 9,10.3 9,13.7c0,2.2 -0.8,4.1 -2.5,5.7s-3.5,2.5 -5.7,2.5c-6.9,0 -16.7,-8.1 -29.4,-24.2c-23.6,-30.1 -35.4,-66.5 -35.4,-109.3z"
      id="svg_4" />
      <path d="m386.3,154.95c0,39.8 -13.8,75.2 -41.4,106.4c-10.1,11.4 -18,17.2 -23.5,17.2c-2.1,0 -3.9,-0.8 -5.6,-2.3c-1.7,-1.5 -2.5,-3.3 -2.5,-5.3c0,-2.8 3.5,-7.6 10.6,-14.4c28.8,-27.6 43.2,-61.8 43.2,-102.7c0,-45.1 -14.9,-82 -44.8,-110.5c-6,-5.7 -9,-10.2 -9,-13.5c0,-2.2 0.8,-4.1 2.5,-5.8c1.6,-1.7 3.5,-2.5 5.7,-2.5c6.8,0 16.5,8.1 29.3,24.3c23.6,30.2 35.5,66.6 35.5,109.1z"
      id="svg_5" />
      <path d="m294,259.05l-174.7,0c-5.7,0 -10.4,-4.6 -10.4,-10.4s4.6,-10.4 10.4,-10.4l174.7,0c5.7,0 10.4,4.6 10.4,10.4s-4.7,10.4 -10.4,10.4z" id="svg_6" />
    </g>
  </symbol>
</svg>
<svg style="display:none;" xmlns="http://www.w3.org/2000/svg" xmlns:svg="http://www.w3.org/2000/svg">
  <!-- Created with SVG-edit - https://svg-edit.googlecode.com/ -->
  <symbol id="unicode-arrow" viewBox="0 0 260 120">
    <g>
      <title>Layer 1</title>
      <rect x="0" y="100" width="220" height="20"/>
      <rect x="200" y="0"  width="20" height="120"/>
      <rect x="180" y="20" width="20" height="20"/>
      <rect x="160" y="40" width="20" height="20"/>
      <rect x="240" y="40" width="20" height="20"/>
      <rect x="220" y="20" width="20" height="20"/>
    </g>
  </symbol>
</svg>
              
            
!

CSS

              
                /* Background pattern from subtlepatterns.com */

$DEBUG: false;

$calcBG: #3C3C36;
$silverLogo: #909289;
$logoBG: #32302B;
$LCDborder: #51514C;
$LCDborderLogo: white;
$LCDsubBorder: #58595A;
$LCDbg: #6B836B;
$LCDtext: #0B2327;
$buttonBezel: #433C38;
$buttonBezelTopRow: #51514C;
$buttonBG: #212121;
$buttonText: #D5D3CF;
$buttonOrange: #FC8C4A;

body {
  font-family: "Source Sans Pro", sans-serif;
}
svg {
  fill: currentColor; /* Lets SVG take on parent element's text color */
  vertical-align: bottom;
}
small {
  font-size: 80%;
}
button.disabled {
  border: 2px inset $buttonBG;
  color: #444;
}

#diagStyle {
  font-family: Consolas, monospace;
}
#diagBox {
  display: none;
  @if $DEBUG {
    display: initial;
  }
  header {
    background-color: $LCDborder;
    text-align: center;
    color: $LCDborderLogo;
    padding: 5px 0;
  }
  p {
    margin: 5px;
    padding-left: 3px;
    background-color: $LCDbg;
  }
  input {
    background-color: $LCDbg;
    border: none;
    font-family: VT323, Consolas, monospace;
    font-size: 120%;
    color: $LCDtext;
  }
  position: absolute;
  top: 10px;
  left: 10px;
  border: 2px solid $calcBG;
  border-radius: 8px;
  background-color: $silverLogo;
  font-size: 120%;
  color: $LCDtext;
}

#calcBox {
  background-color: $calcBG;
  background-image: url(http://dev.andreweast.net/FreeCodeCamp/pattern_colored_ticks_@2X.png);
  //background-image: url(pattern_colored_ticks_@2X.png);
  width: 318px;
  border: 2px outset $calcBG;
  border-radius: 12px;
  margin-top: 10px;
  margin-bottom: 10px;
}

#logoRow {
  color: $silverLogo;
  padding-top: 20px;
  padding-bottom: 10px;
}
#logo {
  background-color: $logoBG;
  border: 2px groove $calcBG;
  border-radius: 2px;
  height: 37px;
  width: 135px;
  padding-left: 3px;
  padding-right: 3px;
  margin-left: 25px;
  padding-top: 3px;
  padding-bottom: 3px;
}
#logo svg{
  height: 26px;
  width: 35px;
}
#logo h6 {
  margin: 0 0 0 3px;
}

#screenBox {
  border-top-left-radius: 8px;
  border-top-right-radius: 8px;
  background-color: $LCDborder;
  color: $LCDborderLogo;
}
#screenLogo h5 {
  padding-left: 10px;
  padding-right: 10px;
  margin-top: 15px;
  margin-bottom: 5px;
}

#LCDborder {
  background-color: $LCDsubBorder;
  padding: 10px 3px;
  border: 1px inset $LCDsubBorder; 
}

#LCD {
  font-family: VT323, Consolas, monospace;
  font-variant: small-caps;
  background-color: $LCDbg;
  color: $LCDtext;
  padding-top: 15px;
  padding-left: 3px;
  padding-right: 3px;
}

.indicator {
  display: none;
  color: $LCDbg;
  background-color: $LCDtext;
  border-radius: 2px;
  font-size: 0.9em;
  line-height: 0.9;
  padding: 2px 4px;
  margin-top: -11px;
  position: absolute;
}
#indicatorMem {
  margin-left: 176px
}
#indicatorOrange {
  margin-left: 39px;
  color: $buttonOrange;
  padding: 1px 5px;
}
#indicatorArrowSymbol {
  width: 12px;
  height: 13px;
}

.LCDrow {
  line-height: 1.2;
  font-size: 1.8em;
  //background-color: rgba(50,50,50,0.5);
  margin-left: -3px;
  margin-right: -3px;
}
.LCDrow > div {
  padding-left: 3px;
  padding-right: 3px;
}
#row1, #row2 {
  padding-left: 8px;
}

#keysBox {
  border-bottom-left-radius: 12px;
  border-bottom-right-radius: 12px;
  background-color: $buttonBezel;
  color: $buttonText;
  border-right: 2px inset $buttonBG;
  border-bottom: 2px inset $buttonBG;
  border-left: 2px inset #999;
  margin-bottom: 15px;
  padding-top: 5px;
  padding-bottom: 10px;
  padding-left: 5px;
  padding-right: 5px;
}
#keysBox > div {
  height: 46px;
  margin-right: -5px;
  margin-left: -5px;
}
#keysBox > div > div {
  padding-left: 5px;
  padding-right: 5px;
}
#keysBox button {
  background-color: $buttonBG;
  border: 2px outset $buttonBG;
  border-radius: 3px;
  font-size: 1em;
  padding: 0;
  width: 30px;
  margin-left: 3px;
  margin-right: 3px;
  height: 100%;
}
#keysBox button:active {
  border: 2px inset $buttonBG;
}
#keysBox button:focus {
  outline: gray auto 5px;
  //outline: none;
}
#keysBox aside {
  font-size: 0.8em;
  color: $buttonOrange;
  text-align: center;
  width: 34px;
}
#keysBox aside.disabledShift {
  color: #964412; //#933903;
  // filter: saturate(50%);
}
.col-fifth {
  width: 20%; // 1/5th, similair to Bootstrap's 1/12th grid columns
  float: left;
  position: relative;
  min-height: 1px;
}
#keysBox .col-fifth button {
  width: 39.25px;
}
#keysBox .col-fifth aside {
  width: 43.25px;
}

button#btnEnter { //note: "eleent#id" is necessary here to override specificty of "#parent element"
  width: 76px;
}
aside#asideEnter {
  width: 82px; //btnEnter's width of 76px + 3*2 margins
}
aside#asideHidden {
  color: $buttonBezel;
}
#btnOrange > div {
  background-color: $buttonOrange;
  width: 90%;
  height: 16px;
  margin: 2px auto;
}
// #topRow {
//   background-color: $buttonBezelTopRow;
// }
#sqrtLine {
  border-top: 1px solid $buttonText;
}
// #bkspArrow {
//   font-size: 150%;
//   line-height: 20px;
// }
.btnNum {
  font-weight: bold;
}
              
            
!

JS

              
                var DEBUG = true;

$(document).ready(function() {
  $("button").addClass("disabled");
  
  var stack = new RPNStack();
  var trigModeDegrees = true;
  
  var shiftMode = false;
  var shiftToggle = function() {
    shiftMode = (shiftMode) ? false : true;
    $("#indicatorOrange").toggle(shiftMode);
  };
  var shiftOff = function() {
    shiftMode = false;
    $("#indicatorOrange").toggle(shiftMode);
  }
  $("#btnOrange").on("click", function() {
    shiftToggle();
  }).removeClass("disabled");
  
  $("#btnExit").on("click", function() { 
    if (shiftMode)
      $("#diagBox").toggle(); 
    else
      ; // exit functionality?
    shiftOff();
  }).removeClass("disabled");
  
  $(".btnNum").on("click", function() { 
    stack.addToEntryRegister(this.innerHTML); 
    shiftOff();
  }).removeClass("disabled");
  
  $("#btnEnter").on("click", function() {
    if (!shiftMode) {
      stack.commitEntryRegister();
      stack.keyEnter();
    } else {
      //key "ALPHA" not implemented
    }
    shiftOff();
  }).removeClass("disabled");
  
  $(".btnOpBinary").on("click", function() {
    stack.commitEntryRegister();
    var result = 0; 
    switch (this.innerHTML) {
      case "+":
        // var op1 = stack.pop();
        // var op2 = stack.pop();
        // result = op1 + op2;
        // console.log(op1 + "+" + op2 + "=" + result);
        result = stack.pop() + stack.pop();
        break;
      case "−":
        var op2 = stack.pop();
        result = stack.pop() - op2;
        break;
      case "×":
        result = stack.pop() * stack.pop();
        break;
      case "÷":
        var op2 = stack.pop();
        result = stack.pop() / op2;
        break;
      default:
        console.log("btnOpBinary passed bad operator: " + this.innerHTML);
    }
    stack.push(result);
    shiftOff();
  }).removeClass("disabled");
  
  $(".btnOpUnary").on("click", function() {
    stack.commitEntryRegister();
    switch (this.dataset.op) {
      case "sqrt":
        if (shiftMode) {
          var base = stack.pop();
          result = base * base; 
        } else {
          result = Math.sqrt(stack.pop());
        }
        break;
      case "inverse":
        if (shiftMode) {
          var exponent = stack.pop();
          result = Math.pow(stack.pop(), exponent);
        } else {
          result = 1 / stack.pop();
        }
        break;
      case "ln":
        if (shiftMode) {
          result = Math.pow(Math.E, stack.pop());
        } else {
          //TODO: check for NaN results of the log functions, ie. log(negative)
          result = Math.log(stack.pop()); //in JS, Math.log(x) means ln(x)
        }
        break;
      case "log":
        if (shiftMode) {
          result = Math.pow(10, stack.pop());
        } else {
          result = Math.log10(stack.pop());
        }
        break;
      case "sin":
        if (!shiftMode) {
          result = Math.sin((trigModeDegrees) ? (stack.pop() * (Math.PI / 180)) : stack.pop());
        } else {
          result = Math.asin((trigModeDegrees) ? (stack.pop() * (Math.PI / 180)) : stack.pop());
        }
        break;
      case "cos":
        if (!shiftMode) {
          result = Math.cos((trigModeDegrees) ? (stack.pop() * (Math.PI / 180)) : stack.pop());
        } else {
          result = Math.acos((trigModeDegrees) ? (stack.pop() * (Math.PI / 180)) : stack.pop());
        }
        break;
      case "tan":
        if (!shiftMode) {
          result = Math.tan((trigModeDegrees) ? (stack.pop() * (Math.PI / 180)) : stack.pop());
        } else {
          result = Math.atan((trigModeDegrees) ? (stack.pop() * (Math.PI / 180)) : stack.pop());
        }
      break;
      default:
    }
    stack.push(result);
    shiftOff();
  }).removeClass("disabled");
  
  $("#btnRollDown").on("click", function() {
    if (!shiftMode) {
      //TODO: this also needs to (conditionally?) commit eRegister!
      stack.rollStackDown();
    } else {
      stack.commitEntryRegister();
      stack.push(Math.PI);
    }
    shiftOff();
  }).removeClass("disabled");
  
  $("#btnNegation").on("click", function() {
    stack.negation();
    shiftOff();
  }).removeClass("disabled");
  
  $("#btnSwapXY").on("click", function() {
    stack.swap();
    shiftOff();
  }).removeClass("disabled");
  
  $("#btnBksp").on("click", function() {
    stack.bksp();
    shiftOff();
  }).removeClass("disabled");
  
  $("#btnStore").on("click", function() {
    if (!shiftMode) {
      stack.sto();
    } else {
      //button "COMPLEX" not implemeted
    }
    shiftOff();
  }).removeClass("disabled");
  $("#btnRecall").on("click", function() {
    if (!shiftMode) {
      stack.rcl();
    } else {
      stack.commitEntryRegister();
      var percent = stack.pop();
      var total = stack.pop();
      var part = percent * total / 100;
      stack.push(total); //HP 42s leaves the total on the stack
      stack.push(part);
    }
    shiftOff();
  }).removeClass("disabled");
  
  //this function must be after all other event binding to allow jQuery to call them in that order!
  // $("button").on("click", function() { 
  //   shiftOff();
  //  });
});

//An object that simulate's the HP-42S internal memory.
//Has three registers:
//  1. An "Entry Register", which stores a string (to allow LTR entry), and is validated after entry is committed
//  2. The 4-item stack. X & Y, which are show on the LCD, and Z & T above.
//     The stack copies the top item "T" down if the stack is popped()
//  3. A single memory location
//Use the "DIAG" function on the "Exit" key to view all the memory locations.
//To separate functionality and maintain privacy within this object, all GUI functions are outside this class, as if the hardware was sending button commands to a processor & memory IC
var RPNStack = function() {
  //using C#-style underscore to designate Private variables
  var _stack = [0,0,0,0]; // X, Y, Z, T
  var _entryRegister = "";
  var _hasDecimal = false;
  var _hasExponent = false;
  var _wasLastEntryHardCommit = false; //controls if new input onto the entryRegister should NOT push the stack up. ie. after [ENTER]
  var _mem = 0;
  
  
  function debugLog(func) {
    console.log(func + ":" + _stack[0] + "," + _stack[1] + "," + _stack[2] + "," + _stack[3] + "," + _entryRegister);
  }
  
  //Pushes the registers "onto the GUI busses", to use the hardware term
  var refresh = function() {
    //LCD locations
    if (_entryRegister === "") {
      $("#row1").text(_stack[0]);
    } else {
      $("#row1").text(_entryRegister + "_");
    }
    $("#row2").text(_stack[1]);
    
    //Memory idicator on the LCD
    if (_mem === 0) {
      $("#indicatorMem, #showDiagMem").hide();
    } else {
      $("#indicatorMem, #showDiagMem").show();
    }
    
    //DIAG text boxes
    $("#entryReg").val(_entryRegister);
    $("#diagMem").val(_mem);
    for (var i = 0; i < _stack.length; ++i)
      $("#stack" + i).val(_stack[i]);
    
    if (DEBUG) debugLog("refresh");
  }
  
  this.pop = function() {
    var result = _stack[0];
    //return x. roll down & copy T down onto Z
    _stack[0] = _stack[1];
    _stack[1] = _stack[2];
    _stack[2] = _stack[3];
    if (DEBUG) debugLog("pop");
    if (DEBUG) refresh();
    return result;
  };
  this.push = function(num) {
    //add a new value at the bottom, push the rest upward
    _stack[3] = _stack[2]; //lose value in T
    _stack[2] = _stack[1];
    _stack[1] = _stack[0];
    _stack[0] = num;
    if (DEBUG) debugLog("push");
    refresh();
  };
  
  //Add a single digit to the Entry Register(or decimal or "E" for scientific notation)
  this.addToEntryRegister = function(digit) {
    if (DEBUG) console.log("addToEntryReg("+digit+")");
    //Preserve the value in x
    //  unless the last keystroke was "ENTER", then don't push the stack up.
    if (_entryRegister === "") {
      if (!_wasLastEntryHardCommit)
        this.push(0);
      else
        _wasLastEntryHardCommit = false;
    }
    
    if (digit[0] >= "0" && digit[0] <= "9") {
      _entryRegister += digit[0];
    } else if (digit[0] === "." && !_hasDecimal) {
      if (_entryRegister === "")
        _entryRegister = "0.";
      else
        _entryRegister += ".";
      _hasDecimal = true;
    } else if (digit[0] === "E" && !_hasExponent) {
      if (_entryRegister === "")
        _entryRegister = "1e";
      else
        _entryRegister += "e";
      _hasExponent = true;
    }
    
    if (DEBUG) debugLog("addEReg");
    refresh();
  }
  this.commitEntryRegister = function() {
    if (_entryRegister !== "") {
      _stack[0] = parseFloat(_entryRegister);
      _entryRegister = "";
      _hasDecimal = false;
      _hasExponent = false;
    }
    if (DEBUG) debugLog("commitEReg");
  }
  
  this.rollStackDown = function() {
    //rotate the whole stack, erase no values;
    var temp = _stack[0];
    _stack[0] = _stack[1];
    _stack[1] = _stack[2];
    _stack[2] = _stack[3];
    _stack[3] = temp;
    if (DEBUG) debugLog("rollDown");
    refresh();
  };
  
  this.keyEnter = function() {
    this.push(_stack[0]);
    _wasLastEntryHardCommit = true;
    if (DEBUG) debugLog("enter");
  };
  
  this.negation = function() {
    if (_entryRegister !== "") {
      if (_entryRegister[0] === "-")
        _entryRegister = _entryRegister.slice(1);
      else
        _entryRegister = "-" + _entryRegister;
    } else {
      _stack[0] = _stack[0] * -1;
    }
    if (DEBUG) debugLog("negation");
    refresh();
  };
  
  this.swap = function() {
    this.commitEntryRegister();
    var temp = _stack[0];
    _stack[0] = _stack[1];
    _stack[1] = temp;
    if (DEBUG) debugLog("swapX&Y");
    refresh();
  };
  
  this.bksp = function () {
    if (_entryRegister !== "") {
      //TODO: check how clearing the entryRegister interacts with _wasLastEntryHardCommit ie. should it flip _wasLastEntryHardCommit? (try on real calc)
      if (_entryRegister.length === 1) {
        _entryRegister = "";
      } else {
        _entryRegister = _entryRegister.slice(0, _entryRegister.length - 1);
        if (_entryRegister === "-")
          _entryRegister = "";
      }
    } else {
      _stack[0] = 0;
    }
    if (DEBUG) debugLog("backspace");
    refresh();
  };
  
  this.sto = function() {
    this.commitEntryRegister();
    _mem = _stack[0];
    if (DEBUG) debugLog("sto mem=" + _mem);
    refresh(); //also shows/hides indicatorMem
  }
  this.rcl = function() {
    this.commitEntryRegister();
    this.push(_mem);
    if (DEBUG) debugLog("rcl mem=" + _mem);
    refresh();
  }
  
  //Part of constructor: call refresh() after all private variables are set.
  refresh();
};

              
            
!
999px

Console