Single stepping

If the last chapter sounds like cheating, this chapter is going to start with something real. Single stepping is typically provided by processors by setting a flag, when that flag is on, the processor will run a single instruction and then issue an interrupt, the operating system will handle that interrupt to notify the debugger a single stepping happened.

To model this, we will enrich our private interface to include single stepping and also the callback when the single stepping completes.

Interface

class debugger
{
public:
    void step_instruction();
    debugger_virtual_machine_interface* get_debugger_virtual_machine_interface();
};

class virtual_machine_debugging_interface
{
public:
    virtual void set_single_step(bool on) = 0;
};

class debugger_virtual_machine_interface
{
public:
    virtual void on_single_step() = 0;
};

Implementation

Note that a processor will not automatically reset its single stepping flag, so our virtual machine should not. This set of interface can be implemented simply by setting a flag in our virtual machine and break out of the interpretation loop if it is set.

Practical notes

This is exactly how single stepping works in real processors. It is a very important primitive to support debugging.