Architecture-Specific Code

The NuttX configuration consists of:

  • Processor architecture specific files. These are the files contained in the arch/<arch-name>/ directory discussed in this file.

  • Chip/SoC specific files. Each processor architecture is embedded in chip or System-on-a-Chip (SoC) architecture. The full chip architecture includes the processor architecture plus chip-specific interrupt logic, general purpose I/O (GPIO) logic, and specialized, internal peripherals (such as UARTs, USB, etc.).

    These chip-specific files are contained within chip-specific sub-directories in the arch/<arch-name>/ directory and are selected via the CONFIG_ARCH_name selection

  • Board specific files. In order to be usable, the chip must be contained in a board environment. The board configuration defines additional properties of the board including such things as peripheral LEDs, external peripherals (such as network, USB, etc.).

    These board-specific configuration files can be found in the boards/<arch>/<chip>/<board> sub-directories.

This file will address the processor architecture specific files that are contained in the arch/<arch-name>/ directory. The file include/nuttx/arch.h identifies all of the APIs that must be provided by this architecture specific logic. (It also includes arch/<arch-name>/arch.h as described below).

Directory Structure in arch/

The arch/ directory contains architecture-specific logic. The complete board port is defined by the architecture-specific code in this directory plus the board-specific configurations in the boards/ directory. Each architecture must provide a subdirectory <arch-name> under arch/ with the following characteristics:

|-- include/
|   |--<chip-name>/
|   |  `-- (chip-specific header files)
|   |--<other-chips>/
|   |-- arch.h
|   |-- irq.h
|   |-- syscall.h
|   `-- types.h
`-- src/
    |  `-- (chip-specific source files)
    |-- Makefile
    `-- (architecture-specific source files)

Summary of Files


This sub-directory contains chip-specific header files.


This is a hook for any architecture specific definitions that may be needed by the system. It is included by include/nuttx/arch.h


This provides architecture/toolchain-specific definitions for standard types. This file should typedef: _int8_t, _uint8_t, _int16_t, _uint16_t, _int32_t, _uint32_t

and if the architecture supports 64-bit integers: _int24_t, _uint24_t, _int64_t, _uint64_t

NOTE that these type names have a leading underscore character. This file will be included (indirectly) by include/stdint.h and typedef’ed to the final name without the underscore character. This roundabout way of doings things allows the stdint.h to be removed from the include/ directory in the event that the user prefers to use the definitions provided by their toolchain header files.


Must be defined to the size required to hold the interrupt enable/disable state.

This file will be included by include/sys/types.h and be made available to all files.


This file needs to define some architecture-specific functions (usually inline if the compiler supports inlining) and structures. These include:

struct xcptcontext

This structure represents the saved context ofa thread.

irqstate_t up_irq_save(void)

Used to disable all interrupts.

void up_irq_restore(irqstate_t flags)

Used to restore interrupt enables to the same state as before up_irq_save was called.

NOTE: These interfaces are not available to application code but can only be used within the operating system code. And, in general, these functions should never be called directly, not unless you know absolutely well what you are doing. Rather you should typically use the wrapper functions enter_critical_section() and leave_critical_section() as prototyped in include/nuttx/irq.h.

This file must also define NR_IRQS, the total number of IRQs supported by the board.

  • include/syscall.h: This file needs to define some architecture specific functions (usually inline if the compiler supports inlining) to support software interrupts or syscalls that can be used all from user-mode applications into kernel-mode NuttX functions. This file must always be provided to prevent compilation errors. However, it need only contain valid function declarations if the architecture supports the CONFIG_BUILD_PROTECTED or CONFIG_BUILD_KERNELconfigurations.

    See Syscall Layer for details.


This sub-directory contains chip-specific source files.


This makefile will be executed to build the targets src/libup.a and src/up_head.o. The up_head.o file holds the entry point into the system (power-on reset entry point, for example). It will be used in the final link with libup.a and other system archives to generate the final executable.

Supported Architectures

The list of supported architectures can be found in Supported Platforms.