To achieve what you want you need to call/pinvoke SetWorkingSetSizeEx with 1 GB as minimum (second param) and QUOTA_LIMITS_HARDWS_MIN_ENABLE as fourth param which makes sure that workingset size won't go below minimum you gave even in "high memory pressure" conditions of the system.
The system behaviour also depends on privileges of the caller, depending on OS version etc. you might need SE_INC_WORKING_SET_NAME and/or SE_INC_BASE_PRIORITY_NAME !
Another (nicer) option which uses these APIs "behind the scenes" is a .NET wrapper you can find here.
