Electronic – maximum supported RAM for Snapdragon 410

To amplify stevenvh's answer, any type of logic, when given an input signal, will take some time to produce an output signal; memory is often very slow compared with other logic. Often, there will be a guarantee that the output signal will become valid within a certain amount of time, but that's it. In particular, it's possible that the signal might change several times within that interval, and there will be no indication, prior to the end of that interval, that the signal has achieved its final "correct" value.

When a typical microcontroller or microprocessor reads a byte (or word, or whatever unit) of memory, it generates an address and, some time later, looks at the value output by the memory and acts upon it. Between the time the controller generates the address and the time it looks at the value from memory, it doesn't care when or whether the output signals from the memory change. On the other hand, if the signal from memory hasn't stabilized to its final value by the time the controller looks at it, the controller will misread the memory as having held whatever value was being output at the moment it looked. Normally the controller would look at the value from memory as soon as it was ready to do something with it, but if the memory's value wouldn't be ready then, that might not work. Consequently, many controllers have an option to wait a bit longer after they're ready to process data from memory, to ensure that the output from memory is actually valid. Note that adding such delay will slow things down (the controller would have been happy to act on the data from memory sooner), but will not affect correctness of operation (unless things are slowed down so much that other timing obligations cannot be met).

Given your baseline of 1GHz ARM you are looking at a multilayer board (8 layers say) with a high pin count BGA package. An AM3539 is a 324-Pin PBGA package. Assuming you don't go package on package for the memory, you are looking at an awful lot of high speed interconnects, all of which have to be balanced and impedence matched. You'd start with a 5 grand licence for Altium Designer or something similar.

You can have a look at the hardware platforms like the Beaglebone and the Pandaboard which will give you an excellent idea of the scale involved. There are some terrifying videos on YouTube (example) showing you the routing involved with DDR3.

What I'm doing currently is designing a baseboard around a SoM - a system on module (A Gumstix). All the horror is taken care of, yet you have a lot of low level access to pins.