The ever increasing pressure for both high spectral and high angular resolution spectrograph imposes an increasing complexity on astronomical instrument control software, now a critical component in the instrument design. To achieve the accuracy required to maintain the image of the target within its 0.2 arcsec entrance slit, the Observation Control software(OS) for the ESO-VLT CRIRES instrument must take into account a number of optical phenomena (differential atmospheric refraction, distortion, etc.), some of them time dependent, even when observing an object moving at a rate different from the object used for auto-guiding. Five internal software control loops adjust the position of mechanical devices and/or the telescope on top of the OS standard functionalities (e.g. monitoring, exposure handling). Besides internal activities, the OS must promptly reply to sequential commands as well as simultaneous interruptions/adjustments from operator via GUI interface. The required advanced synchronization mechanisms are implemented as an extension to the OS framework (a tool collecting the general features of all instrument OS) while allowing for maintainability and future generalization.