How to release the cpu control

[Preston]

Hello all guys:
I try to express the problem clearly.

send and getUnSendMsg are methods DLLImport from aaa DLL (not .net DLL)
program flow:
send---->getUnSendMsg(need to get return value)--->
after run getUnSendMsg, I need to release the cpu control to aaa DLL, then
aaa DLL can use the modem and get the getUnSendMsg return value, I need the
return value to choose what to do next step.
then how to release the CPU control or wait some time, then aaa DLL can do
some thing.