To make this software "work" (install and boot) on your router, follow these standard Cisco upgrade steps: Preparation Verify Space:
The M9 designation indicates the 9th maintenance rebuild of the 15.8(3) train. By this stage in the software development lifecycle, Cisco has ironed out the major bugs present in earlier versions (like the initial 15.8(3)M1 or M2). c800universalk9mzspa1583m9bin work
to previous releases once a newer version (like 15.9.x) has been installed, as it may impair router functionality or cause boot errors. Security Changes To make this software "work" (install and boot)