Dump memory

The ‘arm-none-eabi-objdump’ command displays information from an object file.

At least one switch must be given.

Useful switches

-d, --disassemble        Display assembler contents of executable sections
-D, --disassemble-all    Display assembler contents of all sections
-f, --file-headers       Display the contents of the overall file header
-g, --debugging          Display debug information in object file
-h, --[section-]headers  Display the contents of the section headers
-S, --source             Intermix source code with disassembly
-t, --syms               Display the contents of the symbol table(s)
-x, --all-headers        Display the contents of all headers

Let’s try the following command, with -f (file header):

$ arm-none-eabi-objdump -f fhc.o

fhc.o:     file format elf32-littlearm
architecture: armv7e-m, flags 0x00000011:
HAS_RELOC, HAS_SYMS
start address 0x00000000

• elf32: the ELF file is designed for 32-bit systems

  ELF means Executable and Linkable Format and is a standard file format used mainly on Unix/Linux systems.

• littlearm:

  short for “little-endian” and means that the LSB (least significant byte or “little end”) is stored first in memory. ARM architecture can operate in both little-endian and big-endian modes.

• architecture:

  armv7e-m: “armv7e-m” refers to the ARMv7-M architecture, where “M” stands for Microcontroller

• flags 0x00000010 (HAS_SYMS):

  the flag “0x00000011” indicates that the object file contains relocation information (bit 0 HAS_RELOC) and symbol information (bit 1 HAS_SYMbolS). Symbols are names associated with addresses in the code and data sections of the executable.

• start address 0x00000000:

  represents the entry point of the executable when it is loaded into memory.

Now, let’s try the command with -h (file header):

$ arm-none-eabi-objdump -h fhc.o

      Sections:
      Idx Name          Size      VMA       LMA       File off  Algn
0 .text         00000108  00000000  00000000  00000034  2**1
                CONTENTS, ALLOC, LOAD, READONLY, CODE
1 .data         00000022  00000000  00000000  0000013c  2**0
                CONTENTS, ALLOC, LOAD, DATA
2 .bss          00000000  00000000  00000000  0000015e  2**0
                ALLOC
3 .debug_line   00000035  00000000  00000000  0000015e  2**0
                CONTENTS, RELOC, READONLY, DEBUGGING, OCTETS
4 .debug_info   00000026  00000000  00000000  00000193  2**0
                CONTENTS, RELOC, READONLY, DEBUGGING, OCTETS
5 .debug_abbrev 00000014  00000000  00000000  000001b9  2**0
                CONTENTS, READONLY, DEBUGGING, OCTETS
6 .debug_aranges 00000020  00000000  00000000  000001d0  2**3
                CONTENTS, RELOC, READONLY, DEBUGGING, OCTETS
7 .debug_str    0000002a  00000000  00000000  000001f0  2**0
                CONTENTS, READONLY, DEBUGGING, OCTETS
8 .ARM.attributes 00000021  00000000  00000000  0000021a  2**0
                CONTENTS, READONLY

• Idx: The index or number of the section.

• Name: The name of the section.

• Size: The size of the section in hexadecimal (in bytes).

• VMA (Virtual Memory Address): The virtual address where the section will be loaded into memory during program execution.

• LMA (Load Memory Address): The load address of the section in memory. This might be different from the VMA in certain situations.

• File off (File Offset): The offset of the section in the file.

• Algn (Alignment): The alignment of the section. The value is in the form of 2 to the power of the specified number.

.data has a size of 34 elements (0X22)

which accounts for the word 0xdeadbeef (32 bits or 4 bytes)

and

the short variable ‘tic’ (2 bytes)

and

hello: .ascii “Aabcdefghijklmnopqrstuvwxyzn” (28 bytes).

.text has a size of 264 bytes (0x108).

These informations can also be retrieved with

      $ arm-none-eabi-size fhc.o

text     data     bss     dec     hex filename
 264       34       0     298     12a fhc.o

• text is the size of all code in your application.

• data is the size of initialized global variables.

• bss is the size of uninitialized global variables or initialized to zero.

• dec and hex are the sum of text, data and bss in decimal and hexadecimal.

SYMBOL TABLE

Let’s look at the symbol table (use the t switch). It displays the static symbols (non-shared):

$ arm-none-eabi-objdump -t fhc.o

SYMBOL TABLE:
00000000 l    d  .text  00000000 .text
00000000 l    d  .data  00000000 .data
00000000 l    d  .bss     00000000 .bss
00000006 l       .data  00000000 hello
00000100 l       .text  00000000 Reset_Handler
00000102 l       .text  00000000 loop
00000000 l    d  .debug_info        00000000 .debug_info
00000000 l    d  .debug_abbrev    00000000 .debug_abbrev
00000000 l    d  .debug_line        00000000 .debug_line
00000000 l    d  .debug_str     0     00000000 .debug_str
00000000 l    d  .debug_aranges   00000000 .debug_aranges
00000000 l    d  .ARM.attributes        00000000 .ARM.attributes
00000004 g       .data            00000000 tic

You can also use ‘readelf’:

       $arm-none-eabi-readelf -s fhc.o

       Symbol table '.symtab' contains 16 entries:
               Num:    Value  Size Type    Bind   Vis      Ndx Name
       0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
       1: 00000000     0 SECTION LOCAL  DEFAULT    1 .text
       2: 00000000     0 SECTION LOCAL  DEFAULT    2 .data
       3: 00000000     0 SECTION LOCAL  DEFAULT    3 .bss
       4: 00000008     0 NOTYPE  LOCAL  DEFAULT    1 $d
       5: 00000006     0 NOTYPE  LOCAL  DEFAULT    2 hello
       6: 00000100     0 NOTYPE  LOCAL  DEFAULT    1 Reset_Handler
 7: 00000100     0 NOTYPE  LOCAL  DEFAULT    1 $t
 8: 00000102     0 NOTYPE  LOCAL  DEFAULT    1 loop
 9: 00000000     0 SECTION LOCAL  DEFAULT    6 .debug_info
10: 00000000     0 SECTION LOCAL  DEFAULT    8 .debug_abbrev
11: 00000000     0 SECTION LOCAL  DEFAULT    4 .debug_line
12: 00000000     0 SECTION LOCAL  DEFAULT   11 .debug_str
13: 00000000     0 SECTION LOCAL  DEFAULT    9 .debug_aranges
14: 00000000     0 SECTION LOCAL  DEFAULT   12 .ARM.attributes
15: 00000004     0 NOTYPE  GLOBAL DEFAULT    2 tic

    with objdump

Column 1 (address): symbol's address
When it is all zeros, it might account for placeholder values to be filled by the linker.

Column 2 (binding): indicates the binding of symbol.
"l" signifies a local symbol, "g" a global symbol.
            "F", "f", "O" symbol is the name of a function, a file, an object.

Column 3 (type): represents the type of the symbol
"d" is a SECTION
" " is NOTYPE

Column 4 (name): name of the symbol.

Column 5 (size): size of the symbol.

Column 6 (section): indicates the section to which the symbol belongs.

Note

relocation at Relocation.

Note

relocation at Disassembling.

$ arm-none-eabi-objdump -T fhc.o

arm-none-eabi-objdump: fhc.o: not a dynamic object
DYNAMIC SYMBOL TABLE:
no symbols