HelenOS sources
kernel/arch/amd64/include/arch/ddi/ddi.h
kernel/arch/ia32/include/arch/ddi/ddi.h
kernel/generic/include/ddi/ddi.h
uspace/lib/c/arch/abs32le/include/libarch/ddi.h
uspace/lib/c/arch/amd64/include/libarch/ddi.h
uspace/lib/c/arch/arm32/include/libarch/ddi.h
uspace/lib/c/arch/arm64/include/libarch/ddi.h
uspace/lib/c/arch/ia32/include/libarch/ddi.h
uspace/lib/c/arch/ia64/include/libarch/ddi.h
uspace/lib/c/arch/mips32/include/libarch/ddi.h
uspace/lib/c/arch/ppc32/include/libarch/ddi.h
uspace/lib/c/arch/riscv64/include/libarch/ddi.h
uspace/lib/c/arch/sparc64/include/libarch/ddi.h
uspace/lib/c/include/ddi.h
uspace/lib/posix/include/libc/ddi.h
HelenOS homepage, sources at GitHub