The HP-41 was fascinating for an obscure reason called &quot;synthetic programming&quot;. It was a series of key presses and operations that unlocked a mode in the calculators that allowed you to do all kinds of things that were never allowed in the first place, such as controlling individual pixels on the screen.<p>It was absolutely magical<p><a href="http:&#x2F;&#x2F;www.hpmuseum.org&#x2F;prog&#x2F;synth41.htm" rel="nofollow">http:&#x2F;&#x2F;www.hpmuseum.org&#x2F;prog&#x2F;synth41.htm</a>