Firts code: Simply want to execute ADD instruction in C code.
By default, AT&T format is used for assembly representation. Therefore, add %ecx, %eax means eax <- eax+ecx. In order to use Intel representation, -masm=intel should be used in gcc command as below:
$ gcc -masm=intel -g machine1.c -o machine1
OPCODE of the ADD instruction is 01 c1 which means 00000001 11000001 where
OP=000000 --> add
D=0 --> REG field is the source operand
W=1 --> 16,32,64-bit registers.
MOD=11 --> register to register instruction
REG=000 --> eax code
R/M=001 --> ecx code
To test that, launch GDB and follow these steps.
1- Tell it to use intel representation with set disassembly-flavor intel.
2- Use disass /r main to see the assembly code of the source file.
3- Put a breakpoint at the beginning of the main function with break main or b main.
4- Run the program inside GDB with run or r.
5- Print registers with info registers or i r.
6- Continue to the next line with next or n
7- Verify the content of ecx.
$ gdb ./machine1
(gdb) set disassembly-flavor intel
(gdb) disass /r main
Dump of assembler code for function main:
0x00000000004004ed <+0>: 55 push rbp
0x00000000004004ee <+1>: 48 89 e5 mov rbp,rsp
0x00000000004004f1 <+4>: 01 c1 add ecx,eax
0x00000000004004f3 <+6>: 5d pop rbp
0x00000000004004f4 <+7>: c3 ret
End of assembler dump.
(gdb) break main
Breakpoint 1 at 0x4004f1: file machine.c, line 3.
(gdb) run
Starting program: /home/mahmood/Documents/./machine
Breakpoint 1, main () at machine.c:3
3 __asm__("add %ecx, %eax");
Missing separate debuginfos, use: debuginfo-install glibc-2.17-196.el7.x86_64
(gdb) info registers eax ecx
eax 0x4004ed 4195565
ecx 0x400500 4195584
(gdb) next
4 }
(gdb) info registers eax ecx
eax 0x4004ed 4195565
ecx 0x8009ed 8391149
(gdb) q
A debugging session is active.
Inferior 1 [process 35165] will be killed.
Quit anyway? (y or n) y