diff --git a/runtimes/web/src/apu.js b/runtimes/web/src/apu.js
index cc318b884..ea2827bd3 100644
--- a/runtimes/web/src/apu.js
+++ b/runtimes/web/src/apu.js
@@ -20,11 +20,17 @@ const NOISE_LENGTH = 0x8000;
 //     440.0 / 44100,
 // ].reverse();
 
+const twelvth_root_of_2 = Math.pow(2, 1/12);
+function midiToFreq(pitch) {
+    return 440 * Math.pow(twelvth_root_of_2, (pitch / 256) - 69);
+}
+
 export class APU {
     constructor () {
         const ctx = new (window.AudioContext || window.webkitAudioContext)();
         this.ctx = ctx;
 
+        this.frequency_mode = 0;
         this.nodes = new Array(4);
         this.gains = new Array(4);
 
@@ -58,8 +64,13 @@ export class APU {
     }
 
     tone (frequency, duration, volume, flags) {
-        const freq1 = frequency & 0xffff;
-        const freq2 = (frequency >> 16) & 0xffff;
+        var freq1 = frequency & 0xffff;
+        var freq2 = (frequency >> 16) & 0xffff;
+
+        if (this.frequency_mode == 1) {
+            freq1 = midiToFreq(freq1);
+            freq2 = midiToFreq(freq2);
+        }
 
         const sustain = (duration & 0xff) / 60;
         const release = ((duration >> 8) & 0xff) / 60;
diff --git a/runtimes/web/src/constants.js b/runtimes/web/src/constants.js
index 4e814c43a..1af898293 100644
--- a/runtimes/web/src/constants.js
+++ b/runtimes/web/src/constants.js
@@ -45,6 +45,7 @@ export const MOUSE_MIDDLE = 4;
 
 export const SYSTEM_PRESERVE_FRAMEBUFFER = 1;
 export const SYSTEM_HIDE_GAMEPAD_OVERLAY = 2;
+export const SYSTEM_MIDI_FREQUENCY_MODE = 4;
 
 // Flags for Runtime.pauseState
 export const PAUSE_UNFOCUSED = 1;
diff --git a/runtimes/web/src/runtime.js b/runtimes/web/src/runtime.js
index 96c445622..d378941db 100644
--- a/runtimes/web/src/runtime.js
+++ b/runtimes/web/src/runtime.js
@@ -324,6 +324,7 @@ export class Runtime {
         if (!this.getSystemFlag(constants.SYSTEM_PRESERVE_FRAMEBUFFER)) {
             this.framebuffer.clear();
         }
+        this.api.frequency_mode = this.getSystemFlag(constants.SYSTEM_MIDI_FREQUENCY_MODE) ? 1 : 0;
 
         this.safeCall(this.wasm.exports.update);