View source for Minimal semihosted ARM Cortex-M "Hello World"

The following is a minimal, semihosted "Hello World" D program for ARM Cortex-M processors.

=Goals=  
* Verify the ARM Cortex-M toolchain (compiler-->linker-->debugger)
* Demonstrate that a full port of the D runtime and D standard library are not mandatory
* Demonstrate that linking to C code is not necessary
* Demonstrate that all required assembly code can be done within D
* Provide a starting point, with a low barrier to entry, for developers to begin testing the toolchain, porting the D runtime and libraries to the ARM Cortex-M platform, and programming their ARM Cortex-M software in D

=Tools=
* (host computer) [https://www.archlinux.org/ Arch Linux] 64-bit
* (compiler) [http://wiki.dlang.org/LDC LDC] with ARM backend, or [http://wiki.dlang.org/GDC/Cross_Compiler GDC cross-compiler] for arm-none-eabi 4.9.2
* (GDB client) GNU Tools for [https://launchpad.net/gcc-arm-embedded GNU Tools for ARM Embedded Processors]
* (GDB server) [http://openocd.sourceforge.net/ OpenOCD] 0.9.0
* ([http://en.wikipedia.org/wiki/In-circuit_emulator In Circuit Emulator]) STLink v2 on [http://www.st.com/web/catalog/tools/FM116/SC959/SS1532/PF259090 STM32F429I-DISCO board]

=The Code=
==Program Source Code (start.d)==
<syntaxhighlight lang="D">
module start;
 
version(LDC)
{
  import ldc.llvmasm;
}
 
//Must be stored as second 32-bit word in .text section
alias void function() ISR;
extern(C) immutable ISR ResetHandler = &OnReset;
 
void SendCommand(int command, void* message)
{
  version(LDC)
  {
    __asm
    (
      "mov r0, $0;
      mov r1, $1;
      bkpt #0xAB",
      "r,r,~{r0},~{r1}",
      command, message
    );
  }
  else version(GNU)
  {
    asm
    {
      "mov r0, %[cmd]; 
       mov r1, %[msg]; 
       bkpt #0xAB"
	:                              
	: [cmd] "r" command, [msg] "r" message
	: "r0", "r1", "memory";
    }
  }
}
 
void OnReset()
{
  while(true)
  {
    // Create semihosting message
    uint[3] message =
      [
	2, 			      //stderr
	cast(uint)"hello\r\n".ptr,    //ptr to string
	7                             //size of string
      ];

    //Send semihosting command
    SendCommand(0x05, &message);
  }
}
</syntaxhighlight >

==Minimal Runtime Implementation (object.d) (GDC only)==
GDC requires the following minimal object.d file in compiler's import path, which by default is the compiler's working directory.  If compiling from within folder that contains start.d, simply add object.d to that folder.  It is imported automatically.
<syntaxhighlight lang="D">
module object;
</syntaxhighlight >

=Compiling=
NOTE: You will have to change the -mcpu option for your processor accordingly (e.g. cortex-m0, cortex-m3, etc...)

===LDC===
<source lang="html4strict">
ldc2 -mtriple=thumb-none-linux-eabi -mcpu=cortex-m4 -c -betterC start.d
</source>
For LDC, object.d must NOT exist, or it will try to import it automatically.

===GDC===
<source lang="html4strict">
arm-none-eabi-gdc -mthumb -mcpu=cortex-m4 -fno-emit-moduleinfo -c -fdata-sections start.d 
</source>
For GDC, object.d MUST exist in the compiler's import path, and it is imported automatically.

=Linking=
NOTE: This linker scripts were written specifically for an STM32F4 MCU.  The ''MEMORY'' section and ''_stackStart'' will have to be tailored to your hardware.
==Linker Script (link.ld) ==
<source lang="html4strict">
MEMORY
{
  CCRAM    (rxw) : ORIGIN = 0x10000000, LENGTH =   64k
  SRAM     (rxw) : ORIGIN = 0x20000000, LENGTH =  128k
  FLASH    (rx)  : ORIGIN = 0x08000000, LENGTH = 1024k
}

_stackStart = ORIGIN(CCRAM) + LENGTH(CCRAM);

SECTIONS
{  
  /* We don't need exceptions, and discarding these sections
     prevents linker errors with LDC */
  /DISCARD/ :
  {
    *(.ARM.extab*)
    *(.ARM.exidx*)
  }

  .text :       
  {
    LONG(_stackStart);              /* Initial stack pointer */
    KEEP(start.o(*.ResetHandler))   /* Interrupt vector table (Entry point) */
    
    /* the code */
    *(.text)      
    *(.text*)  
    
    /* for "hello\r\n" string constant */
    . = ALIGN(4);
    *(.rodata)
    *(.rodata*)
    
  }>FLASH
  
  
  /* Need .data, .bss, .ctors and probably more as program becomes
     More complex */
}
</source>

Link with:
<source lang="html4strict">
arm-none-eabi-ld -T link.ld --gc-sections start.o -o start.elf
</source>

=Execution=
Start OpenOCD:
<source lang="html4strict">
openocd -f board/stm32f429discovery.cfg
</source>

Start GDB:
<source lang="html4strict">
arm-none-eabi-gdb start.elf
</source>

In GDB, attach to OpenOCD, reset the hardware, load the executable, and begin execution
<source lang="html4strict">
target remote localhost:3333
monitor arm semihosting enable
monitor reset halt
load
monitor reset init
continue
</source>

Output (in OpenOCD window):
<source lang="html4strict">
hello
hello
...
</source>

[[Category:ARM]]