# NuttX RTOS Porting Guide Last Updated: March 13, 2009

# Table of Contents

1.0 Introduction
2.0 Directory Structure

2.1 Documentation 2.2 arch/

2.2.1 Subdirectory Structure
2.2.2 Summary of Files
2.2.3 Supported Architectures

2.3 configs/

2.3.1 Subdirectory Structure
2.3.2 Summary of Files

2.3.2.1 Board Specific Logic
2.3.2.2 Board Specific Configuration Sub-Directories

2.3.3 Supported Boards

2.4 drivers/
2.5 examples/
2.6 fs/
2.7 graphics/
2.8 include/
2.9 lib/
2.10 mm/
2.11 net
2.12 netutils
2.13 sched/
2.14 tools/
2.15 Makefile

3.0 Configuring and Building

3.1 Configuring NuttX
3.2 Building NuttX

4.0 Architecture APIs

4.1 APIs Exported by Architecture-Specific Logic to NuttX

4.1.1 `up_initialize()`
4.1.2 `up_idle()`
4.1.3 `up_initial_state()`
4.1.4 `up_create_stack()`
4.1.5 `up_use_stack()`
4.1.6 `up_release_stack()`
4.1.7 `up_unblock_task()`
4.1.8 `up_block_task()`
4.1.9 `up_release_pending()`
4.1.10 `up_reprioritize_rtr()`
4.1.11 `_exit()`
4.1.12 `up_assert()`
4.1.13 `up_schedule_sigaction()`
4.1.14 `up_allocate_heap()`
4.1.15 `up_interrupt_context()`
4.1.16 `up_disable_irq()`
4.1.17 `up_enable_irq()`
4.1.18 `up_prioritize_irq()` 4.1.19 `up_putc()`

4.2 APIs Exported by NuttX to Architecture-Specific Logic

4.2.1 `os_start()`
4.2.2 OS List Management APIs
4.2.3 `sched_process_timer()`
4.2.4 `irq_dispatch()`

5.0 NuttX File System
Appendix A: NuttX Configuration Settings
Appendix B: Trademarks

# 1.0 Introduction

Overview This document provides and overview of the NuttX build and configuration logic and provides hints for the incorporation of new processor/board architectures into the build.

See also `arch/README.txt` and `configs/README.txt`.

# 2.0 Directory Structure

Directory Structure. The general directly layout for NuttX is very similar to the directory structure of the Linux kernel -- at least at the most superficial layers. At the top level is the main makefile and a series of sub-directories identified below and discussed in the following paragraphs:

    .
    |-- Makefile
    |-- Documentation
    |   `-- (documentation files)/
    |-- arch/
    |   |-- <arch-name>/
    |   |   |-- include/
    |   |   |   |--<chip-name>/
    |   |   |   |  `-- (chip-specific header files)
    |   |   |   |--<other-chips>/
    |   |   |   `-- (architecture-specific header files)
    |   |   `-- src/
    |   |       |--<chip-name>/
    |   |       |  `-- (chip-specific source files)
    |   |       |--<other-chips>/
    |   |       `-- (architecture-specific source files)
    |   `-- <other-architecture directories>/
    |-- configs/
    |   |-- <board-name>/
    |   |   |-- include/
    |   |   |   `-- (other board-specific header files)
    |   |   |-- src/
    |   |   |   `-- (board-specific source files)
    |   |   |---<config-name>/
    |   |   |   `-- (board configuration-specific source files)
    |   |   `---(other configuration sub-directories for this board)/
    |   `-- <(other board directories)>/
    |-- drivers/
    |   |-- Makefile
    |   |-- (driver-specific sub-directories)/
    |   |   `-- (driver-specific source files)
    |   `-- (common driver source files)
    |-- examples/
    |   `-- (example)/
    |       |-- Makefile
    |       `-- (example source files)
    |-- fs/
    |   |-- Makefile
    |   |-- (file system-specific sub-directories)/
    |   |   `-- (file system-specific source files)
    |   `-- (common file system source files)
    |-- graphics/
    |   |-- Makefile
    |   |-- (feature-specific sub-directories)/
    |   |   `-- (feature-specific source files library source files)
    |   `-- (common graphics-related source files)
    |-- include/
    |   |-- (standard header files)
    |   |-- (standard include sub-directories)
    |   |   `-- (more standard header files)
    |   |-- (non-standard include sub-directories)
    |       `-- (non-standard header files)
    |-- lib/
    |   |-- Makefile
    |   `-- (lib source files)
    |-- mm/
    |   |-- Makefile
    |   `-- (memory management source files)
    |-- net/
    |   |-- Makefile
    |   |-- uip/
    |   |   `-- (uip source files)
    |   `-- (BSD socket source files)
    |-- netutils/
    |   |-- Makefile
    |   |-- (network feature sub-directories)/
    |   |   `-- (network feature source files)
    |   `-- (netutils common files)
    |-- sched/
    |   |-- Makefile
    |   `-- (sched source files)
    `-- tools/
        `-- (miscellaneous scripts and programs)

Configuration Files. The NuttX configuration consists of:

- Processor architecture specific files. These are the files contained in the `arch/`<arch-name>`/` directory and are discussed in a paragraph below.
- Chip/SoC specific files. Each processor 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, clocking logic, general purpose I/O (GIO) 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 configurations. 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 `configs/`<board-name>`/` sub-directories and are discussed in a paragraph below.

## 2.1 Documentation

General documentation for the NuttX OS resides in this directory.

## 2.2 arch

### 2.2.1 Subdirectory Structure

This directory contains several sub-directories, each containing architecture-specific logic. The task of porting NuttX to a new processor consists of add a new subdirectory under `arch/` containing logic specific to the new architecture. The complete board port in is defined by the architecture-specific code in this directory (plus the board-specific configurations in the `config/` subdirectory). Each architecture must provide a subdirectory, <arch-name> under `arch/` with the following characteristics:

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

### 2.2.2 Summary of Files

- `include/`<chip-name>`/` This sub-directory contains chip-specific header files.
- `include/arch.h`: This is a hook for any architecture specific definitions that may be needed by the system. It is included by `include/nuttx/arch.h`.
- `include/types.h`: This provides architecture/toolchain-specific definitions for standard types. This file should `typedef`:
  `sbyte, ubyte, uint8, boolean, sint16, uint16, sint32, uint32`

  and if the architecture supports 64-bit integers

  `sint64, uint64`

  and finally

  `irqstate_t`

  Must be defined to the be 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.

- `include/irq.h`: This file needs to define some architecture specific functions (usually inline if the compiler supports inlining) and structure. These include:
  - `struct xcptcontext`: This structures represents the saved context of a thread.
  - `irqstate_t irqsave(void)`: Used to disable all interrupts.
  - `void irqrestore(irqstate_t flags)`: Used to restore interrupt enables to the same state as before `irqsave()` was called.``

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

- `src/`<chip-name>`/` This sub-directory contains chip-specific source files.
- `src/Makefile`: 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.
- (architecture-specific source files). The file `include/nuttx/arch.h` identifies all of the APIs that must be provided by the architecture specific logic. (It also includes `arch/`<arch-name>`/arch.h` as described above).

### 2.2.3 Supported Architectures

Archictecture- and Chip-Specific Directories. All processor architecture-specific directories are maintained in sub-directories of the `arch/` directory. Different chips or SoC's may implement the same processor core. Chip-specific logic can be found in sub-directories under the architecture directory. Current architecture/chip directories are summarized below:

- `arch/sim`: A user-mode port of NuttX to the x86 Linux platform is available. The purpose of this port is primarily to support OS feature developement. This port does not support interrupts or a real timer (and hence no round robin scheduler) Otherwise, it is complete.

NOTE: This target will not run on Cygwin probably for many reasons but first off because it uses some of the same symbols as does cygwind.dll.

- `arch/arm`: This directory holds common ARM architectures. At present, this includes the following subdirectories:
  - `arch/arm/include` and `arch/arm/src/common`: Common ARM logic.
  - `arch/arm/include/c5471` and `arch/arm/src/c5471`: TI TMS320C5471 (also called TMS320DM180 or just C5471). NuttX operates on the ARM7 of this dual core processor. This port is complete, verified, and included in the NuttX release 0.1.1.
  - `arch/arm/include/dm320` and `arch/arm/src/dm320`: TI TMS320DM320 (also called just DM320). NuttX operates on the ARM9EJS of this dual core processor. This port complete, verified, and included in the NuttX release 0.2.1.
  - `arch/arm/include/lpc214x` and `arch/arm/src/lpc214x`: These directories provide support for NXP LPC214x family of processors. STATUS: This port is in progress and should be available in the nuttx-0.2.5 release.
- `configs/mcu123-lpc214x`: The mcu123.com lpc214x development board. This is a work in progress.
- `arch/m68322` A work in progress.
- `arch/pjrc-8051`: 8051 Microcontroller. This port is not quite ready for prime time.
- `arch/z16f`: Zilog z16f Microcontroller. This port uses the Zilog z16f2800100zcog Development Kit. This port was released with nuttx-0.3.7.
- `arch/z80`: This directory holds 8-bit ZiLOG architectures. At present, this includes the Zilog z80, ez80Acclaim! and z8Encore! Microcontrollers.
  - `arch/z80/include` and `arch/z80/src/common`: Common logic.
  - `arch/z80/include/z80` and `arch/z80/src/z80`: The Z80 port was released in nuttx-0.3.6 has been verified using only a z80 instruction simulator. The set simulator can be found in the NuttX CVS at http://nuttx.cvs.sourceforge.net/nuttx/misc/sims/z80sim. This port also uses the SDCC toolchain (http://sdcc.sourceforge.net/") (verified with version 2.6.0 and 2.7.0).
  - `arch/z80/include/ez80` and `arch/z80/src/ez80`: The ez80Acclaim! port uses the ZiLOG ez80f0910200kitg development kit, eZ80F091 part, with the Zilog ZDS-II Windows command line tools. The development environment is Cygwin under WinXP. This is a work in progress. Verified ez80 support will be announced in a future NuttX release.
  - `arch/z80/include/z8` and `arch/z80/src/z8`: The Z8Encore! port uses either the ZiLOG z8encore000zco development kit, Z8F6403 part, or the z8f64200100kit development kit, Z8F6423 part with the Zilog ZDS-II Windows command line tools. The development environment is Cygwin under WinXP. The initial release, verified only on the ZDS-II ez8 simulator, was released in nuttx-0.3.9.

Deprecated Architecture Directories. The following architecture directories are deprecated. They have been replaced by the logic in `arm/arm` and will deleted when `arch/arm` is fully verified.

- `arch/c5471`: Replaced with `arch/arm/include/c5471` and `arch/arm/src/c5471`.``
- `arch/dm320`: Replaced with `arch/arm/include/dm320` and `arch/arm/src/dm320`.``

Other ports for the for the TI TMS320DM270 and for MIPS are in various states of progress

## 2.3 configs

The `configs/` subdirectory contains configuration data for each board. These board-specific configurations plus the architecture-specific configurations in the `arch/` subdirectory complete define a customized port of NuttX.

### 2.3.1 Subdirectory Structure

The configs directory contains board specific configuration files. Each board must provide a subdirectory <board-name> under `configs/` with the following characteristics:

    <board-name>
    |-- include/
    |   |-- board.h
    |   `-- (board-specific header files)
    |-- src/
    |   |-- Makefile
    |   `-- (board-specific source files)
    |-- <config1-dir>
    |   |-- Make.defs
    |   |-- defconfig
    |   `-- setenv.sh
    |-- <config2-dir>
    |   |-- Make.defs
    |   |-- defconfig
    |   `-- setenv.sh
    |   ...
    `-- (other board-specific configuration sub-directories)/

### 2.3.2 Summary of Files

#### 2.3.2.1 Board Specific Logic

- `include/`: This directory contains board specific header files. This directory will be linked as `include/arch/board` at configuration time and can be included via `#include <arch/board/header.h>`. These header file can only be included by files in `arch/`<arch-name>`/include/` and `arch/`<arch-name>`/src/`.
- `src/`: This directory contains board specific drivers. This directory will be linked as arch/<arch-name>`/src/board` at configuration time and will be integrated into the build system.
- `src/Makefile`: This makefile will be invoked to build the board specific drivers. It must support the following targets: `libext$(LIBEXT)`, `clean`, and `distclean`.

#### 2.3.2.2 Board Specific Configuration Sub-Directories

The `configs/`<board-name>`/` sub-directory holds all of the files that are necessary to configure Nuttx for the particular board. A board may have various different configurations using the common source files. Each board configuration is described by three files: `Make.defs`, `defconfig`, and `setenv.sh`. Typically, each set of configuration files is retained in a separate configuration sub-directory (<config1-dir>, <config2-dir>, .. in the above diagram). The procedure for configuring NuttX is described below, This paragraph will describe the contents of these configuration files.

- `Make.defs`: This makefile fragment provides architecture and tool-specific build options. It will be included by all other makefiles in the build (once it is installed). This make fragment should define:
  - Tools: CC, LD, AR, NM, OBJCOPY, OBJDUMP
  - Tool options: CFLAGS, LDFLAGS
  - COMPILE, ASSEMBLE, ARCHIVE, CLEAN, and MKDEP macros

  When this makefile fragment runs, it will be passed TOPDIR which is the path to the root directory of the build. This makefile fragment may include ${TOPDIR}/.config to perform configuration specific settings. For example, the CFLAGS will most likely be different if CONFIG_DEBUG=y.

- `defconfig`: This is a configuration file similar to the Linux configuration file. In contains variable/value pairs like:
  - `CONFIG_VARIABLE`=value

  This configuration file will be used at build time:

  1. As a makefile fragment included in other makefiles, and
  2. to generate `include/nuttx/config.h` which is included by most C files in the system.
- `setenv.sh`: This is a script that you can include that will be installed at the toplevel of the directory structure and can be sourced to set any necessary environment variables.

### 2.3.3 Supported Boards

All of the specific boards supported by NuttX are identified below. These are the specific <board-name>'s that may be used to configure NuttX as described below.

- `configs/c5471evm`: This is a port to the Spectrum Digital C5471 evaluation board. The C5471 is a dual core processor from TI with an ARM7TDMI general purpose processor and a c54 SDP. NuttX runs on the ARM core and is built with with a GNU arm-elf toolchain* under Linux or Cygwin. This port is complete, verified, and included in the NuttX release.
- `configs/ez80f0910200kitg` ez80Acclaim! Microcontroller. This port use the Zilog ez80f0910200kitg development kit, eZ80F091 part, and the Zilog ZDS-II Windows command line tools. The development environment is Cygwin under WinXP.
- `configs/m68322evb`: This is a work in progress for the venerable m68322evb board from Motorola.
- `configs/mcu123-lpc214x`: This port is for the NXP LPC2148 as provided on the mcu123.com lpc214x development board. This OS is also built with the arm-elf toolchain* under Linux or Cygwin. The port supports serial, timer0, spi, and usb.
- `configs/ntosd-dm320`: This port uses the Neuros OSD with a GNU arm-elf toolchain* under Linux or Cygwin. See Neuros Wiki for futher information. NuttX operates on the ARM9EJS of this dual core processor. STATUS: This port is code complete, verified, and included in the NuttX 0.2.1 release.
- `configs/olimex-strp711`: This port uses the Olimex STR-P711 board arm-elf toolchain* under Linux or Cygwin. See the Olimex web site for futher information. STATUS: Coding for the basic port -- serial console and system timer -- is complete but untested to problems I am having using OpenOCD with a wiggler clone JTAG.
- `configs/pjrc-8051`: 8051 Microcontroller. This port uses the PJRC 87C52 development system and the SDCC toolchain under Linux or Cygwin. This port is not quite ready for prime time.
- `configs/sim`: A user-mode port of NuttX to the x86 Linux platform is available. The purpose of this port is primarily to support OS feature developement. This port does not support interrupts or a real timer (and hence no round robin scheduler) Otherwise, it is complete.
- `configs/us7032evb1`: This is a port of the Hitachi SH-1 on the Hitachi SH-1/US7032EVB1 board. STATUS: Work has just began on this port.
- `configs/xtrs`: TRS80 Model 3. This port uses a vintage computer based on the Z80. An emulator for this computer is available to run TRS80 programs on a linux platform (http://www.tim-mann.org/xtrs.html).
- `configs/z16f2800100zcog` z16f Microcontroller. This port use the Zilog z16f2800100zcog development kit and the Zilog ZDS-II Windows command line tools. The development environment is Cygwin under WinXP.
- `configs/z80sim`: z80 Microcontroller. This port uses a Z80 instruction set simulator. That simulator can be found in the NuttX CVS here. This port also the SDCC toolchain under Linux or Cygwin(verfied with version 2.6.0).
- `configs/z8encore000zco` z8Encore! Microcontroller. This port use the Zilog z8encore000zco development kit, Z8F6403 part, and the Zilog ZDS-II Windows command line tools. The development environment is Cygwin under WinXP.
- `configs/z8encore000zco` z8Encore! Microcontroller. This port use the Zilog z8f64200100kit development kit, Z8F6423 part, and the Zilog ZDS-II Windows command line tools. The development environment is Cygwin under WinXP.

> * A customized version of the buildroot is available to build these toolchains under Linux or Cygwin.

## 2.4 drivers

This directory holds architecture-independent device drivers.

    drivers/
    |-- Makefile
    |-- bch/
    |   |-- Make.defs
    |   `-- (bch driver source files)
    |-- mmcsd/
    |   |-- Make.defs
    |   `-- (mmcsd driver source files)
    |-- net/
    |   |-- Make.defs
    |   `-- (net driver source files)
    |-- usbdev/
    |   |-- Make.defs
    |   `-- (usbdev driver source files)
    `-- (common driver source files)

## 2.5 examples

Example and test programs to build against.

## 2.6 fs

This directory contains the NuttX file system. This file system is described below.

    fs/
    |-- Makefile
    |-- fat/
    |   |-- Make.defs
    |   `-- (fat file system source files)
    |-- romfs/
    |   |-- Make.defs
    |   `-- (romfs file system source files)
     `-- (common file system source files)

## 2.7 graphics

This directory contains files for graphics/video support under NuttX.

    graphics/
    |-- Makefile
    |-- nxglib/
    |   |-- Make.defs
    |   `-- (NuttX graphics library source files)
    |-- nx/
    |   |-- Make.defs
    |   `-- (Nuttx X-server source files)
    `-- (common file system source files)

## 2.8 include

This directory holds NuttX header files. Standard header files file retained in can be included in the normal fashion:

`include <stdio.h>`
`include <sys/types.h>`
etc.

Directory structure:

    include/
    |-- (standard header files)
    |-- arpa/
    |   `-- (standard header files)
    |-- net/
    |   `-- uip/
    |       `-- (uIP specific header files)
    |-- netinet/
    |   `-- (standard header files)
    |-- nuttx/
    |   `-- (nuttx specific header files)
    `- sys/
        `-- (more standard header files)

## 2.9 lib

This directory holds a collection of standard libc-like functions with custom interfaces into Nuttx.

## 2.10 mm

This is the NuttX memory manager.

## 2.11 net

This directory contains the implementation of the socket APIs. The subdirectory, `uip` contians the uIP port.

## 2.12 netutils

This directory contains most of the network applications. Some of these are original with NuttX (like tftpc) and others were leveraged from the uIP-1.0 apps directory. As the uIP apps/README says, these applications "are not all heavily tested."

    netutils/
    |-- Makefile
    |-- dhcp/
    |   |-- Make.defs
    |   `-- (dhcp source files)
    |-- dhcpd/
    |   |-- Make.defs
    |   `-- (dhcpd source files)
    |-- resolv/
    |   |-- Make.defs
    |   `-- (resolv source files)
    |-- smtp/
    |   |-- Make.defs
    |   `-- (smtp source files)
    |-- telnetd/
    |   |-- Make.defs
    |   `-- (telnetd source files)
    |-- tftpc/
    |   |-- Make.defs
    |   `-- (tftpc source files)
    |-- uiplib/
    |   |-- Make.defs
    |   `-- (uiplib source files)
    |-- weblclient/
    |   |-- Make.defs
    |   `-- (webclient source files)
    |-- webserver/
    |   |-- Make.defs
    |   `-- (webserver source files)
    `-- (netutils common files)

## 2.13 sched

The files forming core of the NuttX RTOS reside here.

## 2.14 tools

This directory holds a collection of tools and scripts to simplify configuring, building and maintaining NuttX.

    tools/
    |-- Makefile.mkconfig
    |-- configure.sh
    |-- incdir.sh
    |-- indent.sh
    |-- link.sh
    |-- mkconfig.c
    |-- mkdeps.sh
    |-- mkimage.sh
    |-- mknulldeps.sh
    |-- unlink.sh
    |-- winlink.sh
    `-- zipme

## 2.15 Makefile

The top-level `Makefile` in the `${TOPDIR}` directory contains all of the top-level control logic to build NuttX. Use of this `Makefile` to build NuttX is described below.

# 3.0 Configuring and Building

## 3.1 Configuring NuttX

Manual Configuration. Configuring NuttX requires only copying the board-specific configuration files into the top level directory which appears in the make files as the make variable, `${TOPDIR}`. This could be done manually as follows:

- Copy `configs/`<board-name>`/[`<config-dir>`/]Make.def` to `${TOPDIR}/Make.defs`,
- Copy `configs/`<board-name>`/[`<config-dir>`/]setenv.sh` to `${TOPDIR}/setenv.sh`, and
- Copy `configs/`<board-name>`/[`<config-dir>`/]defconfig` to `${TOPDIR}/.config`

Where <board-name> is the name of one of the sub-directories of the NuttX `configs/` directory. This sub-directory name corresponds to one of the supported boards identified above. And <config-dir> is the optional, specific configuration directory for the board.

Automated Configuration. There is a script that automates these steps. The following steps will accomplish the same configuration:

      cd tools
      ./configure.sh <board-name>[/<config-dir>]

Additional Configuration Steps. The remainder of configuration steps will be performed by `${TOPDIR}/Makefile` the first time the system is built as described below.

## 3.2 Building NuttX

Building NuttX. Once NuttX has been configured as described above, it may be built as follows:

    cd ${TOPDIR}
    source ./setenv.sh
    make

The `${TOPDIR}` directory holds:

- The top level `Makefile` that controls the NuttX build.

That directory also holds:

- The makefile fragment `.config` that describes the current configuration.
- The makefile fragment `Make.defs` that provides customized build targers, and
- The shell script `setenv.sh` that sets up the configuration environment for the build.

The `setenv.sh` contains Linux/Cygwin environmental settings that are needed for the build. The specific environmental definitions are unique for each board but should include, as a minimum, updates to the `PATH` variable to include the full path to the architecture-specific toolchain identified in `Make.defs`. The `setenv.sh` only needs to be source'ed at the beginning of a session. The system can be re-made subsequently by just typing `make`.

First Time Make. Additional configuration actions will be taken the first time that system is built. These additional steps include:

- Auto-generating the file `include/nuttx/config.` using the `${TOPDIR}/.config` file.
- Creating a link to `${TOPDIR}/arch/`<arch-name>`/include` at `${TOPDIR}/include/arch`.
- Creating a link to `${TOPDIR}/configs/`<board-name>`/include` at `${TOPDIR}/include/arch/board`.
- Creating a link to `${TOPDIR}/configs/`<board-name>`/src` at `${TOPDIR}/arch/`<arch-name>`/src/board`
- Creating make dependencies.

# 4.0 Architecture APIs

The file `include/nuttx/arch.h` identifies by prototype all of the APIs that must be provided by the architecture specific logic. The internal OS APIs that architecture-specific logic must interface with also also identified in `include/nuttx/arch.h` or in other header files.

## 4.1 APIs Exported by Architecture-Specific Logic to NuttX

### 4.1.1 `up_initialize()`

Prototype: `void up_initialize(void);`

Description. `up_initialize()` will be called once during OS initialization after the basic OS services have been initialized. The architecture specific details of initializing the OS will be handled here. Such things as setting up interrupt service routines, starting the clock, and registering device drivers are some of the things that are different for each processor and hardware platform.

`up_initialize()` is called after the OS initialized but before the init process has been started and before the libraries have been initialized. OS services and driver services are available.

### 4.1.2 `up_idle()`

Prototype: `void up_idle(void);`

Description. `up_idle()` is the logic that will be executed when their is no other ready-to-run task. This is processor idle time and will continue until some interrupt occurs to cause a context switch from the idle task.

Processing in this state may be processor-specific. e.g., this is where power management operations might be performed.

### 4.1.3 `up_initial_state()`

Prototype: `void up_initial_state(FAR _TCB *tcb);`

Description. A new thread is being started and a new TCB has been created. This function is called to initialize the processor specific portions of the new TCB.

This function must setup the intial architecture registers and/or stack so that execution will begin at tcb->start on the next context switch.

### 4.1.4 `up_create_stack()`

Prototype: `STATUS up_create_stack(FAR _TCB *tcb, size_t stack_size);`

Description. Allocate a stack for a new thread and setup up stack-related information in the TCB.

The following TCB fields must be initialized:

- `adj_stack_size`: Stack size after adjustment for hardware, processor, etc. This value is retained only for debug purposes.
- `stack_alloc_ptr`: Pointer to allocated stack
- `adj_stack_ptr`: Adjusted `stack_alloc_ptr` for HW. The initial value of the stack pointer.

This API is NOT required if `CONFIG_CUSTOM_STACK` is defined.

Inputs:

- `tcb`: The TCB of new task.
- `stack_size`: The requested stack size. At least this much must be allocated.

### 4.1.5 `up_use_stack()`

Prototype: `STATUS up_use_stack(FAR _TCB *tcb, FAR void *stack, size_t stack_size);`

Description. Setup up stack-related information in the TCB using pre-allocated stack memory.

The following TCB fields must be initialized:

- `adj_stack_size`: Stack size after adjustment for hardware, processor, etc. This value is retained only for debug purposes.
- `stack_alloc_ptr`: Pointer to allocated stack
- `adj_stack_ptr`: Adjusted `stack_alloc_ptr` for HW. The initial value of the stack pointer.

This API is NOT required if `CONFIG_CUSTOM_STACK` is defined.

Inputs:

- `tcb`: The TCB of new task.
- `stack_size`: The allocated stack size.

### 4.1.6 `up_release_stack()`

Prototype: `void up_release_stack(FAR _TCB *dtcb);`

Description. A task has been stopped. Free all stack related resources retained int the defunct TCB.

This API is NOT required if `CONFIG_CUSTOM_STACK` is defined.

### 4.1.7 `up_unblock_task()`

Prototype: `void up_unblock_task(FAR _TCB *tcb);`

Description. A task is currently in an inactive task list but has been prepped to execute. Move the TCB to the ready-to-run list, restore its context, and start execution.

This function is called only from the NuttX scheduling logic. Interrupts will always be disabled when this function is called.

Inputs:

- `tcb`: Refers to the tcb to be unblocked. This tcb is in one of the waiting tasks lists. It must be moved to the ready-to-run list and, if it is the highest priority ready to run taks, executed.

### 4.1.8 `up_block_task()`

Prototype: `void up_block_task(FAR _TCB *tcb, tstate_t task_state);`

Description. The currently executing task at the head of the ready to run list must be stopped. Save its context and move it to the inactive list specified by task_state. This function is called only from the NuttX scheduling logic. Interrupts will always be disabled when this function is called.

Inputs:

- `tcb`: Refers to a task in the ready-to-run list (normally the task at the head of the list). It most be stopped, its context saved and moved into one of the waiting task lists. It it was the task at the head of the ready-to-run list, then a context to the new ready to run task must be performed.
- `task_state`: Specifies which waiting task list should be hold the blocked task TCB.

### 4.1.9 `up_release_pending()`

Prototype: `void up_release_pending(void);`

Description. When tasks become ready-to-run but cannot run because pre-emption is disabled, they are placed into a pending task list. This function releases and makes ready-to-run all of the tasks that have collected in the pending task list. This can cause a context switch if a new task is placed at the head of the ready to run list.

This function is called only from the NuttX scheduling logic when pre-emption is re-enabled. Interrupts will always be disabled when this function is called.

### 4.1.10 `up_reprioritize_rtr()`

Prototype: `void up_reprioritize_rtr(FAR _TCB *tcb, ubyte priority);`

Description. Called when the priority of a running or ready-to-run task changes and the reprioritization will cause a context switch. Two cases:

1. The priority of the currently running task drops and the next task in the ready to run list has priority.
2. An idle, ready to run task's priority has been raised above the the priority of the current, running task and it now has the priority.

This function is called only from the NuttX scheduling logic. Interrupts will always be disabled when this function is called.

Inputs:

- `tcb`: The TCB of the task that has been reprioritized
- `priority`: The new task priority

### 4.1.11 `_exit()`

Prototype: `void _exit(int status) noreturn_function;`

Description. This function causes the currently executing task to cease to exist. This is a special case of task_delete().

Unlike other UP APIs, this function may be called directly from user programs in various states. The implementation of this function should diable interrupts before performing scheduling operations.

### 4.1.12 `up_assert()`

Prototype:
`void up_assert(FAR const ubyte *filename, int linenum);` `void up_assert_code(FAR const ubyte *filename, int linenum, int error_code);`

Description. Assertions may be handled in an architecture-specific way.

### 4.1.13 `up_schedule_sigaction()`

Prototype: `void up_schedule_sigaction(FAR _TCB *tcb, sig_deliver_t sigdeliver);`

Description. This function is called by the OS when one or more signal handling actions have been queued for execution. The architecture specific code must configure things so that the 'igdeliver' callback is executed on the thread specified by 'tcb' as soon as possible.

This function may be called from interrupt handling logic.

This operation should not cause the task to be unblocked nor should it cause any immediate execution of sigdeliver. Typically, a few cases need to be considered:

1. This function may be called from an interrupt handler During interrupt processing, all xcptcontext structures should be valid for all tasks. That structure should be modified to invoke sigdeliver() either on return from (this) interrupt or on some subsequent context switch to the recipient task.
2. If not in an interrupt handler and the tcb is NOT the currently executing task, then again just modify the saved xcptcontext structure for the recipient task so it will invoke sigdeliver when that task is later resumed.
3. If not in an interrupt handler and the tcb IS the currently executing task -- just call the signal handler now.

This API is NOT required if `CONFIG_DISABLE_SIGNALS` is defined.

### 4.1.14 `up_allocate_heap()`

Prototype: `void up_allocate_heap(FAR void **heap_start, size_t *heap_size);`

Description. The heap may be statically allocated by defining CONFIG_HEAP_BASE and CONFIG_HEAP_SIZE. If these are not defined, then this function will be called to dynamically set aside the heap region.

This API is NOT required if `CONFIG_HEAP_BASE` is defined.

### 4.1.15 `up_interrupt_context()`

Prototype: `boolean up_interrupt_context(void)`

Description. Return TRUE is we are currently executing in the interrupt handler context.

### 4.1.16 `up_disable_irq()`

Prototype:

    #ifndef CONFIG_ARCH_NOINTC
      void up_disable_irq(int irq);
    #endf

Description. Disable the IRQ specified by 'irq' On many architectures, there are three levels of interrupt enabling: (1) at the global level, (2) at the level of the interrupt controller, and (3) at the device level. In order to receive interrupts, they must be enabled at all three levels.

This function implements enabling of the device specified by 'irq' at the interrupt controller level if supported by the architecture (irqsave() supports the global level, the device level is hardware specific).

If the architecture does not support `up_disable_irq`, `CONFIG_ARCH_NOINTC` should be defined in the NuttX configuration file. Since this API cannot be supported on all architectures, it should be avoided in common implementations where possible.

### 4.1.17 `up_enable_irq()`

Prototype:

    #ifndef CONFIG_ARCH_NOINTC
      void up_enable_irq(int irq);
    #endf

Description. This function implements disabling of the device specified by 'irq' at the interrupt controller level if supported by the architecture (irqrestore() supports the global level, the device level is hardware specific).

If the architecture does not support `up_disable_irq`, `CONFIG_ARCH_NOINTC` should be defined in the NuttX configuration file. Since this API cannot be supported on all architectures, it should be avoided in common implementations where possible.

### 4.1.18 `up_prioritize_irq()`

Prototype:

    #ifdef CONFIG_ARCH_IRQPRIO
      void up_enable_irq(int irq);
    #endf

Description. Set the priority of an IRQ.

If the architecture supports `up_enable_irq`, `CONFIG_ARCH_IRQPRIO` should be defined in the NuttX configuration file. Since this API cannot be supported on all architectures, it should be avoided in common implementations where possible.

### 4.1.19 `up_putc()`

Prototype: `int up_putc(int ch);`

Description. This is a debug interface exported by the architecture-specific logic. Output one character on the console

## 4.2 APIs Exported by NuttX to Architecture-Specific Logic

These are standard interfaces that are exported by the OS for use by the architecture specific logic.

### 4.2.1 `os_start()`

To be provided

### 4.2.2 OS List Management APIs

To be provided

### 4.2.3 `sched_process_timer()`

Prototype: `void sched_process_timer(void);`

Description. This function handles system timer events. The timer interrupt logic itself is implemented in the architecture specific code, but must call the following OS function periodically -- the calling interval must be `MSEC_PER_TICK`.

### 4.2.4 `irq_dispatch()`

Prototype: `void irq_dispatch(int irq, FAR void *context);`

Description. This function must be called from the achitecture- specific logic in order to dispaly an interrupt to the appropriate, registered handling logic.

# 5.0 NuttX File System

Overview. NuttX includes an optional, scalable file system. This file-system may be omitted altogther; NuttX does not depend on the presence of any file system.

Pseudo Root File System. Or, a simple in-memory, psuedo file system can be enabled. This simple file system can be enabled setting the CONFIG_NFILE_DESCRIPTORS option to a non-zero value (see Appendix A). This is an in-memory file system because it does not require any storage medium or block driver support. Rather, file system contents are generated on-the-fly as referenced via standard file system operations (open, close, read, write, etc.). In this sense, the file system is psuedo file system (in the same sense that the Linux `/proc` file system is also referred to as a psuedo file system).

Any user supplied data or logic can be accessed via the psuedo-file system. Built in support is provided for character and block drivers in the `/dev` psuedo file system directory.

Mounted File Systems The simple in-memory file system can be extended my mounting block devices that provide access to true file systems backed up via some mass storage device. NuttX supports the standard `mount()` command that allows a block driver to be bound to a mountpoint within the psuedo file system and to a a file system. At present, NuttX supports only the VFAT file system.

Comparison to Linux From a programming perspective, the NuttX file system appears very similar to a Linux file system. However, there is a fundamental difference: The NuttX root file system is a psuedo file system and true file systems may be mounted in the psuedo file system. In the typical Linux installation by comparison, the Linux root file system is a true file system and psuedo file systems may be mounted in the true, root file system. The approach selected by NuttX is intended to support greater scalability from the very tiny platform to the moderate platform.

# Appendix A: NuttX Configuration Settings

The following variables are recognized by the build (you may also include architecture-specific settings).

## Architecture selection

The following configuration itemes select the architecture, chip, and board configuration for the build.

- `CONFIG_ARCH`: Identifies the arch subdirectory
- `CONFIG_ARCH_name`: For use in C code
- `CONFIG_ARCH_CHIP`: Identifies the arch/*/chip subdirectory
- `CONFIG_ARCH_CHIP_name`: For use in C code
- `CONFIG_ARCH_BOARD`: Identifies the configs subdirectory and hence, the board that supports the particular chip or SoC.
- `CONFIG_ARCH_BOARD_name`: For use in C code
- `CONFIG_ENDIAN_BIG`: Define if big endian (default is little endian).
- `CONFIG_ARCH_NOINTC`: Define if the architecture does not support an interrupt controller or otherwise cannot support APIs like up_enable_irq() and up_disable_irq().
- `CONFIG_ARCH_IRQPRIO`: Define if the architecture suports prioritizaton of interrupts and the up_prioritize_irq() API.

Some architectures require a description of the RAM configuration:

- `CONFIG_DRAM_SIZE`: Describes the installed DRAM.
- `CONFIG_DRAM_START`: The start address of DRAM (physical)
- `CONFIG_DRAM_VSTART`: The start address of DRAM (virtual)

General build options:

- `CONFIG_RRLOAD_BINARY`: Make the rrload binary format used with BSPs from ridgerun.com using the `tools/mkimage.sh` script.
- `CONFIG_INTELHEX_BINARY`: Make the Intel HEX binary format used with many different loaders using the GNU objcopy program This option hould not be selected if you are not using the GNU toolchain.
- `CONFIG_MOTOROLA_SREC`: Make the Motorola S-Record binary format used with many different loaders using the GNU objcopy program Should not be selected if you are not using the GNU toolchain.
- `CONFIG_RAW_BINARY`: mmke a raw binary format file used with many different loaders using the GNU objcopy program. This option should not be selected if you are not using the GNU toolchain.
- `CONFIG_HAVE_LIBM`: Toolchain supports libm.a

## General OS setup

- `CONFIG_EXAMPLE`: identifies the subdirectory in examples that will be used in the build.
- `CONFIG_DEBUG`: enables built-in debug options
- `CONFIG_DEBUG_VERBOSE`: enables verbose debug output
- `CONFIG_DEBUG_SCHED`: enable OS debug output (disabled by default)
- `CONFIG_DEBUG_MM`: enable memory management debug output (disabld by default)
- `CONFIG_DEBUG_NET`: enable network debug output (disabled by default)
- `CONFIG_DEBUG_FS`: enable file system debug output (disabled by default)
- `CONFIG_DEBUG_LIB`: enable C library debug output (disabled by default)
- `CONFIG_ARCH_LOWPUTC`: architecture supports low-level, boot time console output
- `CONFIG_MM_REGIONS`: If the architecture includes multiple regions of memory to allocate from, this specifies the number of memory regions that the memory manager must handle and enables the API mm_addregion(start, end);
- `CONFIG_TICKS_PER_MSEC`: The default system timer is 100Hz or `TICKS_PER_MSEC`=10. This setting may be defined to inform NuttX that the processor hardware is providing system timer interrupts at some interrupt interval other than 10 msec.
- `CONFIG_RR_INTERVAL`: The round robin timeslice will be set this number of milliseconds; Round robin scheduling can be disabled by setting this value to zero.
- `CONFIG_SCHED_INSTRUMENTATION`: enables instrumentation in scheduler to monitor system performance
- `CONFIG_TASK_NAME_SIZE`: Spcifies that maximum size of a task name to save in the TCB. Useful if scheduler instrumentation is selected. Set to zero to disable.
- `CONFIG_START_YEAR, CONFIG_START_MONTH, CONFIG_START_DAY - Used to initialize the internal time logic.`
- `CONFIG_JULIAN_TIME`: Enables Julian time conversions
- `CONFIG_DEV_CONSOLE`: Set if architecture-specific logic provides /dev/console. Enables stdout, stderr, stdin.
- `CONFIG_MUTEX_TYPES`: Set to enable support for recursive and errorcheck mutexes. Enables `pthread_mutexattr_settype()`.
- `CONFIG_PRIORITY_INHERITANCE`: Set to enable support for priority inheritance on mutexes and semaphores. Priority inheritance is a strategy of addessing priority inversion. Details of the NuttX implementation of priority inheritance is discussed elsewhere.
- `CONFIG_SEM_PREALLOCHOLDERS`: This setting is only used if priority inheritance is enabled. It defines the maximum number of different threads (minus one) that can take counts on a semaphore with priority inheritance support. This may be set to zero if priority inheritance is disabled OR if you are only using semaphores as mutexes (only one holder) OR if no more than two threads participate using a counting semaphore.
- `CONFIG_SEM_NNESTPRIO`: If priority inheritance is enabled, then this setting is the maximum number of higher priority threads (minus 1) than can be waiting for another thread to release a count on a semaphore. This value may be set to zero if no more than one thread is expected to wait for a semaphore.

The following can be used to disable categories of APIs supported by the OS. If the compiler supports weak functions, then it should not be necessary to disable functions unless you want to restrict usage of those APIs.

There are certain dependency relationships in these features.

- `mq_notify()` logic depends on signals to awaken tasks waiting for queues to become full or empty.
- `pthread_condtimedwait()` depends on signals to wake up waiting tasks.

`CONFIG_DISABLE_CLOCK`, `CONFI_DISABLE_POSIX_TIMERS`, `CONFIG_DISABLE_PTHREAD`, `CONFIG_DISABLE_SIGNALS`, `CONFIG_DISABLE_MQUEUE`, `CONFIG_DISABLE_MOUNTPOUNT`

## Miscellaneous libc settings

- `CONFIG_NOPRINTF_FIELDWIDTH`: sprintf-related logic is a little smaller if we do not support fieldwidthes

## Allow for architecture optimized implementations

The architecture can provide optimized versions of the following to improve sysem performance.

`CONFIG_ARCH_MEMCPY`, `CONFIG_ARCH_MEMCMP`, `CONFIG_ARCH_MEMMOVE`, `CONFIG_ARCH_MEMSET`, `CONFIG_ARCH_STRCMP`, `CONFIG_ARCH_STRCPY`, `CONFIG_ARCH_STRNCPY`, `CONFIG_ARCH_STRLEN`, `CONFIG_ARCH_BZERO`, `CONFIG_ARCH_KMALLOC`, `CONFIG_ARCH_KZMALLOC`, `ONFIG_ARCH_KFREE`,

## Sizes of configurable things (0 disables)

- `CONFIG_MAX_TASKS`: The maximum number of simultaneously active tasks. This value must be a power of two.
- `CONFIG_NPTHREAD_KEYS`: The number of items of thread- specific data that can be retained
- `CONFIG_NFILE_DESCRIPTORS`: The maximum number of file descriptors (one for each open)
- `CONFIG_NFILE_STREAMS`: The maximum number of streams that can be fopen'ed
- `CONFIG_NAME_MAX`: The maximum size of a file name.
- `CONFIG_STDIO_BUFFER_SIZE`: Size of the buffer to allocate on fopen. (Only if CONFIG_NFILE_STREAMS > 0)
- `CONFIG_NUNGET_CHARS`: Number of characters that can be buffered by ungetc() (Only if CONFIG_NFILE_STREAMS > 0)
- `CONFIG_PREALLOC_MQ_MSGS`: The number of pre-allocated message structures. The system manages a pool of preallocated message structures to minimize dynamic allocations
- `CONFIG_MQ_MAXMSGSIZE`: Message structures are allocated with a fixed payload size given by this settin (does not include other message structure overhead.
- `CONFIG_PREALLOC_WDOGS`: The number of pre-allocated watchdog structures. The system manages a pool of preallocated watchdog structures to minimize dynamic allocations
- `CONFIG_DEV_PIPE_SIZE`: Size, in bytes, of the buffer to allocated for pipe and FIFO support (default is 1024).

## File Systems

- `CONFIG_FS_FAT`: Enable FAT filesystem support.
- `CONFIG_FAT_SECTORSIZE`: Max supported sector size.
- `CONFIG_FS_ROMFS`: Enable ROMFS filesystem support

## SPI-based MMC/SD driver

- `CONFIG_MMCSD_NSLOTS`: Number of MMC/SD slots supported by the driver. Default is one.
- `CONFIG_MMCSD_READONLY`: Provide read-only access. Default is Read/Write

## Network Support

### TCP/IP and UDP support via uIP

- `CONFIG_NET`: Enable or disable all network features
- `CONFIG_NET_IPv6`: Build in support for IPv6
- `CONFIG_NSOCKET_DESCRIPTORS`: Maximum number of socket descriptors per task/thread.
- `CONFIG_NET_NACTIVESOCKETS`: Maximum number of concurrent socket operations (recv, send, etc.). Default: `CONFIG_NET_TCP_CONNS`+`CONFIG_NET_UDP_CONNS`.
- `CONFIG_NET_SOCKOPTS`: Enable or disable support for socket options.
- `CONFIG_NET_BUFSIZE`: uIP buffer size
- `CONFIG_NET_TCP`: TCP support on or off
- `CONFIG_NET_TCP_CONNS`: Maximum number of TCP connections (all tasks).
- `CONFIG_NET_TCPBACKLOG`: Incoming connections pend in a backlog until `accept()` is called. The size of the backlog is selected when `listen()` is called.
- `CONFIG_NET_TCP_READAHEAD_BUFSIZE`: Size of TCP read-ahead buffers
- `CONFIG_NET_NTCP_READAHEAD_BUFFERS`: Number of TCP read-ahead buffers (may be zero)
- `CONFIG_NET_MAX_LISTENPORTS`: Maximum number of listening TCP ports (all tasks).
- `CONFIG_NET_TCPURGDATA`: Determines if support for TCP urgent data notification should be compiled in. Urgent data (out-of-band data) is a rarely used TCP feature that is very seldom would be required.
- `CONFIG_NET_UDP`: UDP support on or off
- `CONFIG_NET_UDP_CHECKSUMS`: UDP checksums on or off
- `CONFIG_NET_UDP_CONNS`: The maximum amount of concurrent UDP connections
- `CONFIG_NET_ICMP`: Enable minimal ICMP support. Includes built-in support for sending replies to received ECHO (ping) requests.
- `CONFIG_NET_ICMP_PING`: Provide interfaces to support application level support for sending ECHO (ping) requests and associating ECHO replies.
- `CONFIG_NET_PINGADDRCONF`: Use "ping" packet for setting IP address
- `CONFIG_NET_STATISTICS`: uIP statistics on or off
- `CONFIG_NET_RECEIVE_WINDOW`: The size of the advertised receiver's window
- `CONFIG_NET_ARPTAB_SIZE`: The size of the ARP table
- `CONFIG_NET_BROADCAST`: Incoming UDP broadcast support
- `CONFIG_NET_MULTICAST`: Outgoing multi-cast address support
- `CONFIG_NET_LLH_LEN`: The link level header length
- `CONFIG_NET_FWCACHE_SIZE`: number of packets to remember when looking for duplicates

### UIP Network Utilities

- `CONFIG_NET_DHCP_LIGHT`: Reduces size of DHCP
- `CONFIG_NET_RESOLV_ENTRIES`: Number of resolver entries

## USB Device-Side Support

### USB Device Controller Driver

- `CONFIG_USBDEV`: Enables USB device support
- `CONFIG_USBDEV_ISOCHRONOUS`: Build in extra support for isochronous endpoints
- `CONFIG_USBDEV_DUALSPEED`: Hardware handles high and full speed operation (USB 2.0)
- `CONFIG_USBDEV_SELFPOWERED`: Will cause USB features to indicate that the device is self-powered
- `CONFIG_USBDEV_MAXPOWER`: Maximum power consumption in mA
- `CONFIG_USBDEV_TRACE`: Enables USB tracing for debug
- `CONFIG_USBDEV_TRACE_NRECORDS`: Number of trace entries to remember

### USB Serial Device Class Driver

- `CONFIG_USBSER`: Enable compilation of the USB serial driver
- `CONFIG_USBSER_EPINTIN`: The logical 7-bit address of a hardware endpoint that supports interrupt IN operation
- `CONFIG_USBSER_EPBULKOUT`: The logical 7-bit address of a hardware endpoint that supports bulk OUT operation
- `CONFIG_USBSER_EPBULKIN`: The logical 7-bit address of a hardware endpoint that supports bulk IN operation
- `CONFIG_USBSER_NWRREQS` and `CONFIG_USBSER_NRDREQS`: The number of write/read requests that can be in flight
- `CONFIG_USBSER_VENDORID` and `CONFIG_USBSER_VENDORSTR`: The vendor ID code/string
- `CONFIG_USBSER_PRODUCTID` and `CONFIG_USBSER_PRODUCTSTR`: The product ID code/string
- `CONFIG_USBSER_RXBUFSIZE` and `CONFIG_USBSER_TXBUFSIZE`: Size of the serial receive/transmit buffers

### USB Storage Device Configuration

- `CONFIG_USBSTRG`: Enable compilation of the USB storage driver
- `CONFIG_USBSTRG_EP0MAXPACKET`: Max packet size for endpoint 0
- `CONFIG_USBSTRGEPBULKOUT` and `CONFIG_USBSTRG_EPBULKIN`: The logical 7-bit address of a hardware endpoints that support bulk OUT and IN operations
- `CONFIG_USBSTRG_NWRREQS` and `CONFIG_USBSTRG_NRDREQS`: The number of write/read requests that can be in flight
- `CONFIG_USBSTRG_BULKINREQLEN` and `CONFIG_USBSTRG_BULKOUTREQLEN`: The size of the buffer in each write/read request. This value needs to be at least as large as the endpoint maxpacket and ideally as large as a block device sector.
- `CONFIG_USBSTRG_VENDORID` and `CONFIG_USBSTRG_VENDORSTR`: The vendor ID code/string
- `CONFIG_USBSTRG_PRODUCTID` and `CONFIG_USBSTRG_PRODUCTSTR`: The product ID code/string
- `CONFIG_USBSTRG_REMOVABLE`: Select if the media is removable

## Graphics related configuration settings

- `CONFIG_NX` Enables overall support for graphics library and NX

### NX configuration setting

- `CONFIG_NX_MULTIUSER`: Configures NX in multi-user mode.
- `CONFIG_NX_NPLANES`: Some YUV color formats requires support for multiple planes, one for each color component. Unless you have such special hardware, this value should be undefined or set to 1.
- `CONFIG_NX_DISABLE_1BPP`, `CONFIG_NX_DISABLE_2BPP`, `CONFIG_NX_DISABLE_4BPP`, `CONFIG_NX_DISABLE_8BPP` `CONFIG_NX_DISABLE_16BPP`, `CONFIG_NX_DISABLE_24BPP`, and `CONFIG_NX_DISABLE_32BPP`: NX supports a variety of pixel depths. You can save some memory by disabling support for unused color depths.
- `CONFIG_NX_PACKEDMSFIRST`: If a pixel depth of less than 8-bits is used, then NX needs to know if the pixels pack from the MS to LS or from LS to MS
- `CONFIG_NX_MOUSE`: Build in support for mouse input.
- `CONFIG_NX_KBD`: Build in support of keypad/keyboard input.
- `CONFIG_NXTK_BORDERWIDTH`: Specifies with with of the border (in pixels) used with framed windows. The default is 4.
- `CONFIG_NXTK_BORDERCOLOR1` and `CONFIG_NXTK_BORDERCOLOR2`: Specify the colors of the border used with framed windows. `CONFIG_NXTK_BORDERCOLOR2` is the shadow side color and so is normally darker. The default is medium and dark grey, respectively
- `CONFIG_NXTK_AUTORAISE`: If set, a window will be raised to the top if the mouse position is over a visible portion of the window. Default: A mouse button must be clicked over a visible portion of the window.
- `CONFIG_NXFONTS_CHARBITS`: The number of bits in the character set. Current options are only 7 and 8. The default is 7.
- `CONFIG_NXFONT_SANS`: At present, there is only one font. But if there were were more, then this option would select the sans serif font.

### NX Multi-user only options

- `CONFIG_NX_BLOCKING` Open the client message queues in blocking mode. In this case, `nx_eventhandler()` will not return until a message is received and processed.
- `CONFIG_NX_MXSERVERMSGS` and `CONFIG_NX_MXCLIENTMSGS` Specifies the maximum number of messages that can fit in the message queues. No additional resources are allocated, but this can be set to prevent flooding of the client or server with too many messages (`CONFIG_PREALLOC_MQ_MSGS` controls how many messages are pre-allocated).

## Stack and heap information

- `CONFIG_BOOT_FROM_FLASH`: Some configurations support XIP operation from FLASH.
- `CONFIG_STACK_POINTER`: The initial stack pointer
- `CONFIG_IDLETHREAD_STACKSIZE`: The size of the initial stack. This is the thread that (1) performs the inital boot of the system up to the point where user_start() is spawned, and (2) there after is the IDLE thread that executes only when there is no other thread ready to run.
- `CONFIG_USERMAIN_STACKSIZE`: The size of the stack to allocate for the main user thread that begins at the user_start() entry point.
- `CONFIG_PTHREAD_STACK_MIN`: Minimum pthread stack size
- `CONFIG_PTHREAD_STACK_DEFAULT`: Default pthread stack size
- `CONFIG_HEAP_BASE`: The beginning of the heap
- `CONFIG_HEAP_SIZE`: The size of the heap

# Appendix B: Trademarks

- ARM, ARM7 ARM7TDMI, ARM9, ARM926EJS are trademarks of Advanced RISC Machines, Limited.
- Cygwin is a trademark of Red Hat, Incorporated.
- Linux is a registered trademark of Linus Torvalds.
- LPC2148 is a trademark of NXP Semiconductors.
- TI is a tradename of Texas Instruments Incorporated.
- UNIX is a registered trademark of The Open Group.
- VxWorks is a registered trademark of Wind River Systems, Incorporated.
- ZDS, ZNEO, Z16F, Z80, and Zilog are a registered trademark of Zilog, Inc.

NOTE: NuttX is not licensed to use the POSIX trademark. NuttX uses the POSIX standard as a development guideline only.