Eh, it's not that difficult. What do you think uses all the power in your phone? It's the display, WiFi, cellular, etc. Most of the time your CPU is under-clock

Eh, it's not that difficult. What do you think uses all the power in your phone? It's the display, WiFi, cellular, etc. Most of the time your CPU is under-clocked to save power and reduce heat. A lot of the Intel Atom chips will run as passively as an ARM chip will with comparable throughput. The only thing is PCB space, but even then, I wouldn't except modern chip processes not being able to replicate the BIOS setup in a small chip. It should even be possible to get a BIOS into the same die as the processor to save space. By the way, the reason for wanting x86/AMD64 is because OSes that support a BIOS are already built for these architectures, whereas anybody building OSes for ARM doesn't expect such niceties, hence the current state of mobile phone OSes. If ARM gained BIOS support, it would too be suitable.