clarify renode port

src/ch03-04-debugging-programs.md

@@ -36,14 +36,18 @@ cargo xtask app-image --gdb-stub mtxchat --feature efuse --feature tls
 
 Then, flash the resulting image to the target device as normal.
 
-## Attaching to the debugger
+## Attaching to the debugger (Renode)
 
-If you're using Renode, then you can connect gdb to `localhost:3333` (may also be `3456` on some systems):
+If you're using Renode, then you can connect gdb to `localhost:3456`:
 
 ```text
-riscv-none-elf-gdb -ex 'tar ext :3333'
+riscv-none-elf-gdb -ex 'tar ext :3456'
 ```
 
+On Renode, port `3333` also exists, but it is useful mainly for debugging machine mode, i.e., when the hardware is in the loader or inside the kernel only.
+
+## Attaching to the debugger (Hardware)
+
 On real hardware, you will first need to re-mux the serial port so that gdb is visible on serial. Then you can connect gdb to the target serial port.
 
 For example, if you have a hardware Precursor device connected to a Raspberry Pi 3B+ with a debug HAT running Raspbian "Buster", you would first run this command in `shellchat` on the hardware device itself: