diff --git a/03_uart1/src/main.rs b/03_uart1/src/main.rs
index 660261e8..012e54a2 100644
--- a/03_uart1/src/main.rs
+++ b/03_uart1/src/main.rs
@@ -42,6 +42,7 @@ fn main() {
     uart.getc(); // Press a key first before being greeted
     uart.puts("Hello Rustacean!\n");
 
+    // echo everything back
     loop {
         uart.send(uart.getc());
     }
diff --git a/05_uart0/kernel8.img b/05_uart0/kernel8.img
index fe9766c1..0f7e74d7 100755
Binary files a/05_uart0/kernel8.img and b/05_uart0/kernel8.img differ
diff --git a/05_uart0/src/main.rs b/05_uart0/src/main.rs
index 2d4da612..2d90506f 100644
--- a/05_uart0/src/main.rs
+++ b/05_uart0/src/main.rs
@@ -72,8 +72,11 @@ fn main() {
         uart.hex(mbox.buffer[6]);
         uart.hex(mbox.buffer[5]);
         uart.puts("\n");
+    } else {
+        uart.puts("Unable to query serial!\n");
     }
 
+    // echo everything back
     loop {
         uart.send(uart.getc());
     }
diff --git a/05_uart0/src/uart.rs b/05_uart0/src/uart.rs
index bb526696..c410c1cc 100644
--- a/05_uart0/src/uart.rs
+++ b/05_uart0/src/uart.rs
@@ -173,8 +173,11 @@ impl Uart {
         let mut n;
 
         for i in 0..8 {
+            // get highest tetrad
             n = d.wrapping_shr(28 - i * 4) & 0xF;
 
+            // 0-9 => '0'-'9', 10-15 => 'A'-'F'
+            // Add proper offset for ASCII table
             if n > 9 {
                 n += 0x37;
             } else {