2. Target Dependent Files


This chapter contains outdated and confusing information.

RTEMS has a multi-layered approach to portability. This is done to maximize the amount of software that can be reused. Much of the RTEMS source code can be reused on all RTEMS platforms. Other parts of the executive are specific to hardware in some sense. RTEMS classifies target dependent code based upon its dependencies into one of the following categories.

  • CPU dependent
  • Board dependent
  • Peripheral dependent

2.1. CPU Dependent

This class of code includes the foundation routines for the executive proper such as the context switch and the interrupt subroutine implementations. Sources for the supported processor families can be found in cpukit/score/cpu. A good starting point for a new family of processors is the no_cpu directory, which holds both prototypes and descriptions of each needed CPU dependent function.

CPU dependent code is further subcategorized if the implementation is dependent on a particular CPU model. For example, the MC68000 and MC68020 processors are both members of the m68k CPU family but there are significant differences between these CPU models which RTEMS must take into account.

The source code found in the cpukit/score/cpu is required to only depend upon the CPU model variations that GCC distinguishes for the purposes of multilib’ing. Multilib is the term the GNU community uses to refer to building a single library source multiple times with different compiler options so the binary code generated is compatible. As an example, from GCC’s perspective, many PowerPC CPU models are just a PPC603e. Remember that GCC only cares about the CPU code itself and need not be aware of any peripherals. In the embedded community, we are exposed to thousands of CPU models which are all based upon only a relative small number of CPU cores.

Similarly for the SPARC/ERC32 BSP, the RTEMS_CPU is specified as erc32 which is the name of the CPU model and BSP for this SPARC V7 system on chip. But the multilib variant used is actually v7 which indicates the ERC32 CPU core is a SPARC V7.

2.2. Board Dependent

This class of code provides the most specific glue between RTEMS and a particular board. This code is represented by the Board Support Packages and associated Device Drivers. Sources for the BSPs included in the RTEMS distribution are located in the directory bsps. The BSP source directory is further subdivided based on the CPU family and BSP.

Some BSPs may support multiple board models within a single board family. This is necessary when the board supports multiple variants on a single base board. For example, the Motorola MVME162 board family has a fairly large number of variations based upon the particular CPU model and the peripherals actually placed on the board.

2.3. Peripheral Dependent

This class of code provides a reusable library of peripheral device drivers which can be tailored easily to a particular board. The libchip library is a collection of reusable software objects that correspond to standard controllers. Just as the hardware engineer chooses a standard controller when designing a board, the goal of this library is to let the software engineer do the same thing.

The source code for the reusable peripheral driver library may be found in the directory cpukit/dev or bsps/shared/dev. The source code is further divided based upon the class of hardware. Example classes include serial communications controllers, real-time clocks, non-volatile memory, and network controllers.

2.4. Questions to Ask

When evaluating what is required to support RTEMS applications on a particular target board, the following questions should be asked:

  • Does a BSP for this board exist?
  • Does a BSP for a similar board exists?
  • Is the board’s CPU supported?

If there is already a BSP for the board, then things may already be ready to start developing application software. All that remains is to verify that the existing BSP provides device drivers for all the peripherals on the board that the application will be using. For example, the application in question may require that the board’s Ethernet controller be used and the existing BSP may not support this.

If the BSP does not exist and the board’s CPU model is supported, then examine the reusable chip library and existing BSPs for a close match. Other BSPs and libchip provide starting points for the development of a new BSP. It is often possible to copy existing components in the reusable chip library or device drivers from BSPs from different CPU families as the starting point for a new device driver. This will help reduce the development effort required.

If the board’s CPU family is supported but the particular CPU model on that board is not, then the RTEMS port to that CPU family will have to be augmented. After this is done, development of the new BSP can proceed.

Otherwise both CPU dependent code and the BSP will have to be written.

This type of development often requires specialized skills and there are people in the community who provide those services. If you need help in making these modifications to RTEMS try a search in a search engine with something like “RTEMS support”. The RTEMS Project encourages users to use support services however we do not endorse any providers.

2.5. CPU Dependent Executive Files

The CPU dependent files in the RTEMS executive source code are found in the cpukit/score/cpu/${RTEMS_CPU} directories. The ${RTEMS_CPU} is a particular architecture, e.g. arm, powerpc, riscv, sparc, etc.

Within each CPU dependent directory inside the executive proper is a file named cpu.h which contains information about each of the supported CPU models within that family.

2.6. Board Support Package Structure

The BSPs are all under the bsps directory. The structure in this source subtree is:

  • bsps/shared
  • bsps/${RTEMS_CPU}/shared

The ${RTEMS_CPU} is a particular architecture, e.g. arm, powerpc, riscv, sparc, etc. The shared directories contain code shared by all BSPs or BSPs of a particular architecture. The ${RTEMS_BSP_FAMILY} directories contain BSPs for a particular system on chip (SoC) or processor family.

Use the following structure under the bsps/${RTEMS_CPU}/${RTEMS_BSP_FAMILY}:

  • ata - the legacy ATA/IDE driver
  • btimer - the legacy benchmark timer driver
  • cache - cache controller support
  • clock - the clock driver
  • config - build system configuration files
  • console - the console driver
  • contrib - imports of external sources
    • the layout of external sources should be used as is if possible
  • i2c - the I2C driver
  • include - public header files
  • irq - the interrupt controller support
  • mpci - support for heterogeneous multiprocessing (RTEMS_MULTIPROCESSING)
  • net - legacy network stack drivers
  • rtc - the RTC driver
  • spi - the SPI driver
  • start - everything required to run a minimal application without devices
    • start.S - lowest level startup code
    • bspstart.c - low level startup code
    • bspsmp.c - SMP support
    • linkcmds - a linker command file