Re: Stack pointer is 0 in a bare metal AArch64 program

[Orlando Arias <orlandoarias@gmail.com>]

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Greetings,

On 5/10/20 10:31 PM, Simon Marchi via Newlib wrote:
> $ ./sim/aarch64/run --trace=on --trace-disasm=on ./sim/aarch64/a.out
> memory:   ERROR: executable is too big: ffffffffffffffff
> insn:      pc = 400168 instr = 58000281
> disasm:   ldr   x1, 0x00000000004001b8
> memory:   read of 0 (8 bytes) from 4001b8
> insn:      pc = 40016c instr = 927cec20
> disasm:   and   x0, x1, #0xfffffffffffffff0
> insn:      pc = 400170 instr = 9100001f
> disasm:   mov   sp, x0
> insn:      pc = 400174 instr = d280001d
> disasm:   mov   x29, #0x0                       // #0
> insn:      pc = 400178 instr = a9bf77fd
> disasm:   stp   x29, x29, [sp, #-16]!

Within libgloss for Aarch64, the stack is initialized using a weak
symbol with the value of 0:

.macro GEN_DWORD name
#if defined(__ILP32__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
    .word \name
    .word 0
#elif defined(__ILP32__) && __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
    .word 0
    .word \name
#else
    .dword \name
#endif
.endm

.Lstack:
    GEN_DWORD __stack
    .weak __stack


If the linker script you are utilizing does not define __stack, then the
weak symbol is used.

As an aside, this behavior in libgloss has always annoyed me. When
working with Cortex-M cores [both ARMv7-M and ARMv8-M], msp is
initialized using the value stored in address 0 of the memory map by the
hardware. Yes, the application is free to change it afterwards, but it
just seems redundant to me.

Cheers,
Orlando.


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