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Debug the program by using Qemu

Qemu supports many cpu models, since we are developing programs for ARM, we will use - qemu-system-arm. We can Execute/Debug in Qemu by attaching to GDB. Open a terminal window and execute the following command.

$ qemu-system-arm -cpu cortex-m3 -S -gdb tcp::1234 -nographic -kernel add.elf
  • -cpu cortex-m3 is the option for simulating cortex-m3 processor
  • -S Do not start CPU at startup
  • -gdb, tcp::1234 Attach to GDB through tcp port 1234
  • -nographic disable features we don’t need
  • -kernel add.elf tells qemu to execute directly from our object file.

Run/Debug the program (in another terminal window)

$ arm-none-eabi-gdb add.elf
$ (gdb) target remote localhost:1234
$ (gdb) load
$ (gdb) break _start
$ (gdb) continue

The program should break at _start label. Now, we can exmine the memory, registers, or dis-assemble the code.

  • Dumping memory contents
(gdb) x/18wx 0x0
0x0 <pfnVectors>:	0x20004000	0x00000049	0x00000059	0x00000059
0x10 <pfnVectors+16>:	0x00000059	0x00000059	0x00000059	0x00000000
0x20 <pfnVectors+32>:	0x00000000	0x00000000	0x00000000	0x00000059
0x30 <pfnVectors+48>:	0x00000059	0x00000000	0x00000059	0x00000059
0x40 <pfnVectors+64>:	0x00000059	0x00000059
(gdb)

The above memory dump is vector table. The 1st entry is 0x20004000 which is our Top_Of_Stack (_estack) and the 2nd entry is our program _start address - 0x00000048 (instead of 0x00000048 it is 0x00000049 is stored why?). We can verify these addresses from our above symbol table dump.

  • Dis-assembling
(gdb) x/4iw _start
=> 0x48 <_start>:	mov.w	r0, #5
   0x4c <_start+4>:	mov.w	r1, #4
   0x50 <_start+8>:	add.w	r2, r1, r0
   0x54 <stop>:	bl	0x54 <stop>
(gdb)
  • Single step through the code
(gdb)step
  • Verify Registers
(gdb)info registers
  • Quit gdb
(gdb) quit
A debugging session is active.
Inferior 1 [Remote target] will be killed.
Quit anyway? (y or n) y

Typing y followed by return takes you back to the terminal prompt.

Please note that when you quit gdb, qemu is also terminated.

QEMU: Terminated via GDBstub

This start-up guide clearly demonstrated that without the hardware, we can write programs for ARM and simulate it in Qemu.

  • Ref:

    • [http://shukra.cedt.iisc.ernet.in/edwiki/EmSys:Starting_Cortex-M3Development_Using_the_GNU_Tool_Chain-Part_1](http://shukra.cedt.iisc.ernet.in/edwiki/EmSys:Starting_Cortex-M3_Development_Using_the_GNU_Tool_Chain-_Part_1)
    • https://github.com/arunkuttiyara/cortex_m3_development

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