Why AMD Athlon X4 845 didn't work on asrock fm2a55m-hd+?
Because the 845K is not on the CPU support list, the only models it supports is the 840, 850, 860, 870, and 880, the 845k is AD845XACKASBX
ASRock > FM2A55M-HD+
You can find detail information on your motherboard from ASRock > FM2A55M-HD+ , and your processor, http://www.cpu-world.com/CPUs/Bulldozer/AMD-Athlon%20X4%20845.html . There is nothing preventing this motherboard support your processor, but you have to install an external graphics card to make the system work. Even though it is not listed in the support list, but I believe this combination would possibly work for you.
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