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Real-Time System Design. Developing a Cross Compiler and libraries for a target system. Why port a cross compiler?. In real-time and embedded systems you will frequently come across new hardware for which there either isn’t a compiler or the current compiler doesn’t meet your specifications

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real time system design

Real-Time System Design

Developing a Cross Compiler and libraries for a target system

why port a cross compiler
Why port a cross compiler?
  • In real-time and embedded systems you will frequently come across new hardware for which there either isn’t a compiler or the current compiler doesn’t meet your specifications
  • In this situation one way forward is to port a compiler for this new board
what compiler to select
What compiler to select?
  • Short of writing a new compiler yourself you will probably want to port an existing compiler
  • Selection of the base compiler is important
  • The target hardware might force a choice
    • If it is only supported by one compiler
    • If it runs an OS that is close to a particular compiler
selection criteria
Selection Criteria
  • There are a number of criteria that can be used
    • Mature compiler
    • Portable
    • Widely used/known
    • Large tool set
    • Runs on same/compatible hardware
    • Variety of OS/RTOS interfaces
    • Good library support
    • Supports a number of languages
    • Good documentation
    • Open source/open standard
hardware considerations
Hardware considerations
  • If your selected compiler does not have support for the processor you will have to
    • Write a back end for the compiler to generate the correct assembler code
    • Port many low level startup and compiler routines
    • Write linker scripts and library files
operating system considerations
Operating System considerations
  • If your compiler is not supported by the target OS then you will have to
    • Write initialisation routines – crt0.o for example
    • Port library routines
    • Port some compiler startup routines
  • Frequently boards have very minimal OS/monitors
using gcc as an example
Using GCC as an example
  • GCC scores very highly on many of the points in slide four
  • I will use it as an example
  • Although the name implies the C compiler and this is what I will concentrate on, it does support a number of other languages – Ada, C++, Java, etc
building a cross compiler
Building a cross compiler
  • Firstly select the host machine, although it can help if it is the same processor and OS, it is frequently the case that neither are supported on the host machine
  • However it is very useful that the cross compiler runs in native mode on your host machine
    • i.e. you can compiler at least some of your cross compiler with the host’s native compiler
  • The link between GNU and Linux is obvious therefore I will use a GNU i686 Linux platform
cross compiler tools
Cross compiler tools
  • Aside from the compiler itself you will require a number of tools. Key ones are
    • A cross linker – gas in GNU
    • A cross assembler – gld in GNU
    • A downloader – will depend upon the target
  • Others that will be very useful are
    • A librarian – ranlib in GNU
    • An object copier – objcopy in GNU
    • A debugger – GDB in GNU
    • A symbol table dumper – nm in GNU
building the cross utilities
Building the cross utilities
  • The GNU suite comes with a set of compiler utilities that are bundled together called binutils.
  • You must build binutils before building the cross compiler.
  • It is possible to use other tools with the compiler – it may be required by an on-board OS for example, however GNU does work well with its own tools.
  • You must check compatibility issues between the GCC release and binutils
building the cross compiler 1
Building the cross compiler #1
  • The first pass of the cross compiler must be built with the native compiler
  • This will be a i686 program that generates target output
  • There are a variety of target outputs to configure GNU with
  • The standard is
    • CPU-VENDOR-OS – i.e.
    • m68k-sun-bsd4.2
    • arm-unknown-elf
    • m88k-unknown-a.out
using a gnu template
Using a GNU template
  • Selecting the correct template is important
  • Firstly
    • Select the correct processor and check that the exact family is supported – i.e. m68k and 68040. This means that GCC can generate the correct assembler code
  • Then
    • If you have the correct manufacturer and OS life is simple, Otherwise,
    • Go for the nearest (or unknown) and at least get the code format correct – coff, elf and so on
  • In the later option you will have to do some porting and interfacing.
creating a gnu template
Creating a GNU template
  • GCC is built with portability in mind.
  • It is possible to create new assembler and OS interfaces for GNU
  • This requires learning BDF to create binary definition files for the new target
  • You may have to write a version of gas, the GNU assembler.
  • This is non-trivial, and first off try to use what already exists.
building the cross compiler 2
Building the cross compiler #2
  • As stated earlier, most of the cross compiler will be generated in native mode
  • However GNU will need to generate target code and some of this may be difficult or impossible for the native compiler
  • The code generation may require extra (target) library code that it can’t generate
    • For example, the m68k -68000 option only has 16 bit multiple and divide instructions (mulu/s and divs/u) and the compiler requires 32 bit library functions. These must be created by either a 68000 compiler or assembler.
building the cross compiler 21
Building the cross compiler #2
  • The compiler itself requires some special startup functions written in target code
    • The compiler has some functions _exit for example
    • These can be suppressed using the –nostdfunc option
  • All of the above cannot be generate by the native compiler. They may have to be hand coded or stubbed out if not required.
includes and libraries
Includes and libraries
  • The standard include files - /usr/include – will probably not work with the target and a minimal set must generated
  • Much is standard and can be copied, so can be taken from the target OS
  • GCC, like most C compilers, requires a minimal, standard library, libc.a. Again this will have to be created.
startup files
Startup files
  • The cross compiler will generate all the target code from your program code and the linker will load in any library code, in the correct format.
  • However there is an initialisation file that is required to initialise and execute the target code
  • This is crt0.o on GNU – cstart on Crossware and OS9