_mainCRTStartup/ctr0.S and thumb

[Richard Earnshaw]

On 02/08/14 12:17, Torsten@Robitzki.de wrote:
> Hello,
> I try to build software for an arm cortex-m0 (nordic nRF51422) target using binutils (2.24), gcc (4.9) and newlib (2.1.0). I've build newlib using following configure statement:
> 
>     CXX=g++-4.8 CC=gcc-4.8 ${PWD}/../configure --target=arm-none-eabi --prefix=/usr/local/arm-none-eabi --enable-interwork --enable-multilib --disable-nls
> 
> 
> When I execute a very small C++ binary everything works fine until _mainCRTStartup gets called during startup. When I step through the code, step by step using a jtag debugger, I can follow the flow of execution till the first statement in _mainCRTStartup: 
> 
> here the output of arm-none-eabi-objdump -S
> 
>     000000c0 <_mainCRTStartup>:
> 	add	sl, sl, #256
>     #endif
>     #else
>     #ifdef ARM_RDI_MONITOR
> 	/*  Issue Angel SWI to read stack info.  */
> 	movs	r0, #AngelSWI_Reason_HeapInfo
>       c0:	e3b00016 	movs	r0, #22
> 	adr	r1, .LC0	/*  Point at ptr to 4 words to receive data.  */
>       c4:	e28f10e8 	add	r1, pc, #232	; 0xe8
>     ...
> 
> 
> and here the output of the debugger:
> 
>     00000200:  06 48              LDR     R0, [PC, #+0x18]
>     J-Link>s
>     00000202:  00 47              BX      R0
>     J-Link>s
>     000000C0:  16 00              MOV     R6, R2
>     J-Link>s
>     000000C2:  B0 E3              B       #+0x760
>     J-Link>s
>     00000826:  C0 46              MOV     R8, R8
>     ...
> 
> where 00000826 is somewhere in __fp_lock_all(). To me it looks like the cortex-m0 cpu interprets arm32 opcode as thumb code, right?
> 
> According to the linker map, /usr/local/arm-none-eabi/lib/gcc/arm-none-eabi/4.9.0/../../../../arm-none-eabi/lib/thumb/crt0.o is used and mapped to address c0. The disassembly of that file (using arm-none-eabi-objdump -S) yields:
> 
>     /usr/local/arm-none-eabi/lib/gcc/arm-none-eabi/4.9.0/../../../../arm-none-eabi/lib/thumb/crt0.o:     file format elf32-littlearm
> 
> 
>     Disassembly of section .text:
> 
>     00000000 <_mainCRTStartup>:
>             add     sl, sl, #256
>     #endif
>     #else
>     #ifdef ARM_RDI_MONITOR
>         /*  Issue Angel SWI to read stack info.  */
>             movs    r0, #AngelSWI_Reason_HeapInfo
>        0:   e3b00016        movs    r0, #22
>             adr     r1, .LC0        /*  Point at ptr to 4 words to receive data.  */
>        4:   e28f10e8        add     r1, pc, #232    ; 0xe8
> 
> can someone be so kind and shed some light on this? Any pointers on how to investigate this further are highly appreciated.
> 

Cortex-m0 can only execute thumb code.  It doesn't support the A32 (ARM)
instruction set.

If you've not previously built your own toolchain, I'd recommend you
look at a prebuilt embedded set of tools, such as the ones that can be
found here:

https://launchpad.net/gcc-arm-embedded

R.