How to Access the Commodore 128 Machine Language Monitor

The Commodore 128 features a built-in machine language monitor that allows developers and enthusiasts to inspect memory, debug code, and execute assembly instructions directly. This guide explains the specific commands and system modes required to enter and exit this powerful tool efficiently without needing external cartridges.

To access the monitor, you must ensure your Commodore 128 is operating in native C128 mode rather than C64 compatibility mode. The machine language monitor is not available in C64 mode without the use of external hardware modifications. Once the system is booted into C128 mode, you will see the standard BASIC prompt displaying READY..

Enter the command MONITOR and press the Return key. Some versions of the operating system also accept the abbreviated command MONITOR. with a period at the end. Upon successful execution, the screen will clear, and the prompt will change from READY. to a greater-than symbol >. This new prompt indicates that you are now inside the machine language monitor environment.

While in the monitor, you can use various hexadecimal commands to manipulate memory, disassemble code, or assemble new instructions. To exit the monitor and return to the standard BASIC interpreter, simply type the letter X and press Return. The system will return you to the READY. prompt, allowing you to continue running BASIC programs or loading software as usual.