Got the stack display and stack top wrong. Looks like everything else is in the right order but I'll have to test more later.

src/node_modules/buttons.js

@@ -39,7 +39,7 @@ class ButtonMachine {
   }
 
   get stack_top() {
-    return this.stack[0];
+    return this.stack[this.stack.length - 1];
   }
 
   get cur_op() {
@@ -142,14 +142,16 @@ class ButtonMachine {
   }
 
   op_STOR(reg) {
-    if(!this.assert(!reg.includes(reg), `Register ${reg} is not valid. Use ${ButtonMachine.registers_names()}`)) return;
+    const reg_names = ButtonMachine.register_names();
+    if(!this.assert(reg_names.includes(reg), `Register "${reg}" is not valid. Use ${reg_names}`)) return;
 
     this.registers[reg] = this.stack_top;
     this.next();
   }
 
   op_RSTOR(reg) {
-    if(!this.assert(!reg.includes(reg), `Register ${reg} is not valid. Use ${ButtonMachine.registers_names()}`)) return;
+    const reg_names = ButtonMachine.register_names();
+    if(!this.assert(reg_names.includes(reg), `Register "${reg}" is not valid. Use ${reg_names}`)) return;
 
     let val = this.registers[reg];
     this.assert(val !== undefined, `Invalid register ${reg}`);

src/routes/index.svelte

@@ -11,6 +11,7 @@
   let code = [
     ['PUSH', -10], // start at -10
     ['PUSH', 1],  // increment by 1
+    ['STOR', 'AX'],
     ['ADD'],      
     ['JZ', 5],    // if we're at 0 jumps to the end
     ['JUMP', 1]    // the previous test fails so it jumps to loop again
@@ -25,6 +26,7 @@
   const data_ops = new Set(['PUSH', 'JZ', 'JNZ', 'JUMP', 'STOR', 'RSTOR', 'HALT']);
 
   const ops = ButtonMachine.operations();
+  const registers = ButtonMachine.register_names();
 
   const change_op = (i, count) => {
     let cur_op = code[i][0];
@@ -87,6 +89,16 @@
     transition: unset;
     padding: 5px;
   }
+
+  #reg-select {
+    background-color: $gray;
+    width: 100%;
+  }
+
+  #reg-select select {
+    background-color: $gray;
+    width: 100%;
+  }
 </style>
 
 <div class="container grid-lg" id="content" data-testid="buttons-page">
@@ -122,7 +134,15 @@
                   class="btn btn-primary input-group-btn">{op}</button>
 
           {#if op_has_data(op)}
-            {#if op === 'STOR' || op === 'RSTOR' || op == 'HALT'}
+            {#if op === 'STOR' || op === 'RSTOR'}
+              <div class="form-group" id="reg-select">
+                <select class="form-select" bind:value={data}>
+                  {#each registers as reg}
+                    <option>{reg}</option>
+                  {/each}
+                </select>
+              </div>
+            {:else if op == 'HALT'}
               <input class:active-line={ machine.ip == i } 
                      type="text" class="form-input" placeholder="register" bind:value={data}>
             {:else}
@@ -170,7 +190,7 @@
           </tr>
         </thead>
 
-        {#each machine.stack as datum, i} 
+        {#each [...machine.stack].reverse() as datum, i} 
           <tr>
             <td>{datum}</td>
           </tr>