use virtualbox debug

[splu]

Hi:

I try to use virtualbox debug. Following the document command to start VM.
Then extra "VBoxDbg-Console" and "VirtualBox - Statistics" windows show up.
I try to enter command to execute VM, but reply some message.

g continue execution. Then reply "g: error: The VM is already running"
t execute single step. Then reply "error: VERR_SEM_OUT_OF_TURN: When trying to single step VM"

No matter host is Linux (ubuntu) or Windows (win8). Same result.
Guest OS is DOS.

Do you have any experience in virtualbox debug?

Best Regards
S.P.Lu