The Kernel Debugger is essentially a replacement OS/2 Kernel module that contains a built-in debugger component. The kernel debugger can be used to halt system execution, inspect and alter memory and registers, and display system control blocks. The kernel debugger is described in detail in The OS/2 Debugging Handbook - Volume II, IBM publication number SG24-4641.

Generally the kernel debugger communicates over a serial communications link with a terminal emulator program running on another machine. This allows a user to debug a problem by issuing kernel debugger commands in the emulator program and seeing the results displayed on the debug console.

To automate the debugging process, or to provide a high-level language debugging environment, this interaction with the kernel debugger could instead be handled by a program running on the other machine. This debug engine would interact with the user, convert the user's debugging request to a series of kernel debugger commands, issue them, and then present the response from the kernel debugger back to the user in a user-friendly format.

Communications between the debug engine and the kernel debugger can proceed in one of two modes:

raw (dumb TTY) mode

ASCII characters are sent to the kernel debugger one at a time. The kernel debugger echoes each character. A carriage return (^M or 0x0d) ends a line. The kernel debugger returns data to the debug engine one ASCII character at a time.
packet mode
Packets are sent to the kernel debugger. A packet consists of a fixed sized header followed by zero or more bytes of data. The kernel debugger returns data to the debug engine in packets.

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