Minimal semihosted ARM Cortex-M "Hello World"

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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

The Code

Program Source Code (start.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);
  }
}

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.

module object;

Compiling

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

LDC

ldc2 -march=thumb -mcpu=cortex-m4 -c start.d

For LDC, object.d must NOT exist, or it will try to import it automatically.

GDC

arm-none-eabi-gdc -mthumb -mcpu=cortex-m4 -fno-emit-moduleinfo -c -fdata-sections start.d

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)

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 */
}

Link with:

arm-none-eabi-ld -T link.ld --gc-sections start.o -o start.elf

Execution

Start OpenOCD:

openocd -f board/stm32f429discovery.cfg

Start GDB:

arm-none-eabi-gdb start.elf

In GDB, attach to OpenOCD, reset the hardware, load the executable, and begin execution

target remote localhost:3333
monitor arm semihosting enable
monitor reset halt
load
monitor reset init
continue

Output (in OpenOCD window):

hello
hello
...