Mostly embedded firmware, low level communications related stuff, as in shepherding frames on the wire. Industrial controllers and networks. Making microprocessors on the same chip, same board, or adjacent boards talk to each other. C, C++, Verilog, Lisp, ARM and other asm. Various ARM9's, Cortex-M1, Cortex-M3, multi-core Cortex-A9, TI DSP, and roll-my-own CPU, plus historic CPUs like Z80 and 68K which I have not touched in years. x86 only if I have to. Actel, Altera, or Xilinx FPGA. CPU and chip architecture. FreeRTOS, roll-my-own RTOS, or bare metal. Linux ports and drivers, Debian on SBC. Normally I do not admit to knowing Windows or vxWorks. I do admit to owning several PDP-11s.