... MARX includes models of the AXAF mirrors, the low- and high-energy transmission grating assemblies (LETG and HETG), and the HRC and ACIS focal plane detectors. We discuss the role of MARX within the ASC Data System and present sample simulated images and spectra for two typical cosmic X-ray sources. ... A continuous emission measure model was constructed based on the published EUVE data (Dupree 1996); the peak emission occurred near 10 7 K. Simulations were run for both the HETG and LETG . ...