使用汇编程序的gdb :打印进位标志的状态

2024-04-21 13:26| 来源: 网络整理| 查看: 265

使用gdb的另一个很好的选择是在eflag上添加一个观察点。

GDB Examining the Symbol Table

这是一个12345和23456的程序，可以在tutorialspoint中找到一个很好的教程：TutorialPoint assembly tutorial

unroot@unroot-VirtualBox:~/NASM$ gdb -q add5dig Reading symbols from add5dig...done. (gdb) set listsize 100 (gdb) list 0 1 section .text 2 global _start 3 4 _start: 5 mov ecx,5 ;number of digits in each number to be added 6 mov esi,4 ;used to move the esi pointer to point at the rightmost digits to be summed 7 clc ;clear carry flag 8 9 add_loop: ;iterates through add_loop, decrementing ecx after each iteration until ecx equals 0 10 mov al,[num1+esi] ;mov 11 adc al,[num2+esi] ;add with carry, nicely sets carry to 0 if there is no carry and to 1 if there is a carry 12 aaa ;ascii adjust after addition 13 pushf ;push flags onto stack 14 or al,30h ;OR value with 0b0001 1110. (essentially adds 0x30 to the ascii adjusted value, ascii 0x30 is '0', which converts the number to its ascii representation for proper display) 15 popf ;pop flags from stack 16 17 mov [sum+esi],al ;moves sum of the two digits into the correct space in memory 18 dec esi ;point esi at the next digit to the left 19 loop add_loop ;checks if exc==0, loops if not, continues if yes 20 ;printing message 21 mov edx,len 22 mov ecx,msg 23 mov ebx,1 24 mov eax,4 25 int 0x80 26 ;printing sum 27 mov edx,5 28 mov ecx,sum 29 mov ebx,1 30 mov eax,4 31 int 0x80 32 ;exiting 33 mov eax,1 34 int 0x80 35 36 section .data 37 msg db 'The sum is:',0xa 38 len equ $ - msg 39 num1 db '12345' 40 num2 db '23456' 41 sum db ' ' ;reserves 5 spaces in memory for sum (gdb) break _start Breakpoint 1 at 0x8048080 (gdb) break add_loop Breakpoint 2 at 0x804808b (gdb) run Starting program: /home/unroot/NASM/add5dig Breakpoint 1, 0x08048080 in _start () (gdb) watch $eflags Watchpoint 3: $eflags (gdb) info registers eax 0x0 0 ecx 0x0 0 edx 0x0 0 ebx 0x0 0 esp 0xbffff0b0 0xbffff0b0 ebp 0x0 0x0 esi 0x0 0 edi 0x0 0 eip 0x8048080 0x8048080 eflags 0x202 [ IF ] cs 0x73 115 ss 0x7b 123 ds 0x7b 123 es 0x7b 123 fs 0x0 0 gs 0x0 0 (gdb) cont Continuing. Breakpoint 2, 0x0804808b in add_loop () (gdb) info registers eax 0x0 0 ecx 0x5 5 edx 0x0 0 ebx 0x0 0 esp 0xbffff0b0 0xbffff0b0 ebp 0x0 0x0 esi 0x4 4 edi 0x0 0 eip 0x804808b 0x804808b eflags 0x202 [ IF ] cs 0x73 115 ss 0x7b 123 ds 0x7b 123 es 0x7b 123 fs 0x0 0 gs 0x0 0 (gdb) cont Continuing. Watchpoint 3: $eflags Old value = [ IF ] New value = [ CF AF IF ] 0x08048098 in add_loop () (gdb) info registers eax 0x101 257 ecx 0x5 5 edx 0x0 0 ebx 0x0 0 esp 0xbffff0b0 0xbffff0b0 ebp 0x0 0x0 esi 0x4 4 edi 0x0 0 eip 0x8048098 0x8048098 eflags 0x213 [ CF AF IF ] cs 0x73 115 ss 0x7b 123 ds 0x7b 123 es 0x7b 123 fs 0x0 0 gs 0x0 0 (gdb)

当程序添加'6‘和'5’时，观察点捕捉到eflags寄存器中的进位标志和调整标志已被更改

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