Graphics Programs Reference
In-Depth Information
familiar address of EBP minus 4 into the EAX register. The execution of this
instruction is shown below.
(gdb) x/i $eip
0x804839f <main+43>: lea eax,[ebp-4]
(gdb) print $ebp - 4
$2 = (void *) 0xbffff804
(gdb) x/x $2
0xbffff804: 0x00000000
(gdb) i r eax
eax 0xd 13
(gdb) nexti
0x080483a2 6 for(i=0; i < 10; i++)
(gdb) i r eax
eax 0xbffff804 -1073743868
(gdb) x/xw $eax
0xbffff804: 0x00000000
(gdb) x/dw $eax
0xbffff804: 0
(
gdb)
The following
inc
instruction will increment the value found at this address
(now stored in the EAX register) by 1. The execution of this instruction is also
shown below.
(gdb) x/i $eip
0x80483a2 <main+46>: inc DWORD PTR [eax]
(gdb) x/dw $eax
0xbffff804: 0
(gdb) nexti
0x080483a4 6 for(i=0; i < 10; i++)
(gdb) x/dw $eax
0xbffff804: 1
(gdb)
The end result is the value stored at the memory address EBP minus 4
(
0xbffff804
), incremented by 1. This behavior corresponds to a portion of C
code in which the variable
i
is incremented in the for loop.
The next instruction is an unconditional jump instruction.
(gdb) x/i $eip
0x80483a4 <main+48>: jmp 0x804838b <main+23>
(gdb)
When this instruction is executed, it will send the program back to the
instruction at address
0x804838b
. It does this by simply setting EIP to that value.
Looking at the full disassembly again, you should be able to tell which
parts of the C code have been compiled into which machine instructions.
