arch/x86/include/README.txt ^^^^^^^^^^^^^^^^^^^^^^^^^^^ This directory holds x86-specific header files. The top-level header files in arch/x86/include simply include corresponding header files from lower lower- level chip-specific and architecture-specific directories. Architecture-Specific Directories ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Architecture-specific directories hold common header files for specific x86 architectures. Separating these header file makes it easy to manage such things as differences in sizeof(long) on 32- and 64-bit x86 architectures. i486 This directory holds definitions appropriate for any instantiation of the 32-bit i486 architecture. Chip-Specific directories ^^^^^^^^^^^^^^^^^^^^^^^^^ The same x86 architecture may be realized in different chip implementations. For SoC chips, in particular, on-chip devices and differing interrupt structures may require special, chip-specific definitions in these chip- specific directories. qemu This is the implementation of NuttX on the QEMU x86 simulation.