Add solutions for part 1

projects/12/SysTest/Main.jack

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+// This file is part of www.nand2tetris.org
+// and the book "The Elements of Computing Systems"
+// by Nisan and Schocken, MIT Press.
+// File name: projects/12/SysTest/Main.jack
+
+/** Test program for the OS Sys class. */
+class Main {
+
+    /** Tests the wait method of the Sys class. */
+    function void main() {
+        var char key;
+
+        do Output.printString("Wait test:");
+        do Output.println();
+        do Output.printString("Press any key. After 2 seconds, another message will be printed:");
+
+        while (key = 0) {
+	        let key = Keyboard.keyPressed();
+        }
+        while (~(key = 0)) {
+            let key = Keyboard.keyPressed();
+        }
+        
+        do Sys.wait(2000);
+
+        do Output.println();
+        do Output.printString("Time is up. Make sure that 2 seconds elapsed.");
+        
+        return;
+    }
+}

projects/12/SysTest/Sys.jack

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+// This file is part of www.nand2tetris.org
+// and the book "The Elements of Computing Systems"
+// by Nisan and Schocken, MIT Press.
+// File name: projects/12/Sys.jack
+
+/**
+ * A library that supports various program execution services.
+ */
+class Sys {
+
+    /** Performs all the initializations required by the OS. */
+    function void init() {
+        do Math.init();
+        do Memory.init();
+        do Screen.init();
+        do Keyboard.init();
+        do Output.init();
+        do Main.main();
+        do Sys.halt();
+        return;
+    }
+
+    /** Halts the program execution. */
+    function void halt() {
+        while (true) {}
+        return;
+    }
+
+    /** Waits approximately duration milliseconds and returns.  */
+    function void wait(int duration) {
+        var int i, j;
+        let i = 0;
+        while (i < duration) {
+            let i = i + 1;
+            let j = 0;
+            while (j < 50) {
+                let j = j + 1;
+            }
+        }
+        return;
+    }
+
+    /** Displays the given error code in the form "ERR<errorCode>",
+     *  and halts the program's execution. */
+    function void error(int errorCode) {
+        do Output.printString("ERR<");
+        do Output.printInt(errorCode);
+        do Output.printString(">");
+        do Sys.halt();
+        return;
+    }
+}