Why does .bss section load to UFM flash memory in MAX10 QSYS project?

OGavr · ‎09-18-2018

I use Quartus 16.1 and MAX10 device 10M16DAF256I7G. I create project with NIOS QSYS. I place NIOS code in UFM memory, and RAM in onchip RAM. I create BSP automatic and not change it. I know, that .rwdata section is relocable and loaded in flash. But I see, that .bss section is placed in flash too. It write in UFM 4 kB zeroes. Why does it do it?

This is objdump output:

proj.elf: file format elf32-littlenios2

proj.elf

architecture: nios2:r1, flags 0x00000112:

EXEC_P, HAS_SYMS, D_PAGED

start address 0x00008180

Program Header:

LOAD off 0x00001000 vaddr 0x00008000 paddr 0x00008000 align 2**12

filesz 0x00000020 memsz 0x00000020 flags r-x

LOAD off 0x00001020 vaddr 0x00008020 paddr 0x00008020 align 2**12

filesz 0x000029fc memsz 0x000029fc flags r-x

LOAD off 0x00004000 vaddr 0x00010000 paddr 0x0000aa1c align 2**12

filesz 0x00000124 memsz 0x0000131c flags rw-

Sections:

Idx Name Size VMA LMA File off Algn

0 .entry 00000020 00008000 00008000 00001000 2**5

CONTENTS, ALLOC, LOAD, READONLY, CODE

1 .exceptions 00000160 00008020 00008020 00001020 2**2

CONTENTS, ALLOC, LOAD, READONLY, CODE

2 .text 00002808 00008180 00008180 00001180 2**2

CONTENTS, ALLOC, LOAD, READONLY, CODE

3 .rodata 00000094 0000a988 0000a988 00003988 2**2

CONTENTS, ALLOC, LOAD, READONLY, DATA

4 .rwdata 00000124 00010000 0000aa1c 00004000 2**2

CONTENTS, ALLOC, LOAD, DATA, SMALL_DATA

5 .bss 000011f8 00010124 0000ab40 00004124 2**2

ALLOC, SMALL_DATA

6 .onchip_flash_0_data 00000000 0000ab40 0000ab40 00004124 2**0

CONTENTS

7 .onchip_memory2_0 00000000 0001131c 0001131c 00004124 2**0

CONTENTS

8 .comment 00000023 00000000 00000000 00004124 2**0

CONTENTS, READONLY

.bss is UFM address.

I not change linker.x script. At this scrip I find:

    .bss :  
    {
        __bss_start = ABSOLUTE(.);
        PROVIDE (__sbss_start = ABSOLUTE(.));
        PROVIDE (___sbss_start = ABSOLUTE(.));
 
        *(.dynsbss)
        *(.sbss .sbss.* .gnu.linkonce.sb.*)
        *(.sbss2 .sbss2.* .gnu.linkonce.sb2.*)
        *(.scommon)
 
        PROVIDE (__sbss_end = ABSOLUTE(.));
        PROVIDE (___sbss_end = ABSOLUTE(.));
 
        *(.dynbss)
        *(.bss .bss.* .gnu.linkonce.b.*)
        *(COMMON)
 
        . = ALIGN(4);
        __bss_end = ABSOLUTE(.);
    } > onchip_memory2_0

I try add NOLOAD option to .bss with no effect.

GuaBin_N_Intel · ‎09-20-2018

Each of section can be put to different memory regions by user through linker's memory map. Refer to https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/hb/nios2/n2sw_nii5v2gen2.pdf, 7.15.2.1 and figure 12

.text—All remaining code

.rodata—The read-only data

.rwdata—Read-write data

.bss—Zero-initialized data