I partially disagree. Qemu, & in general simulators, can really speed up development but for the reason you cited, you need to periodically run your code on real hardware. The second suggestion is to minimize assembly language level code -- this will reduce the temptation to use esoteric h/w specific features that Qemu may not handle! In general micro-optimizations should be left for much later, if ever. Third, as soon as possible set things up to load your kernel over the network or a serial link (or JTAG). Particularly if you use a debugger, you will be running the same kernel over and over again to debug something. Fourth, if you plan to use a debugger like gdb, provide support for remote debugging. That is not a lot of code (basically support for communication, breakpoints, peek and poke).