Conditional Branching & Loop

Conditional Branching
 * JXX - JA, JAE, JE, JG, JZ, JNZ etc
 * These Jump depends on the state of eflags
    - Zero Flag (ZF)
    - Parity Flag (PF)
    - OverFlowFlag (OF)
    - Sign Flag (SF)
    - Carry Flag (CF)
 * Only Short & Near jumps are supported, Far Jumps not allowed

Program Start here

.data

    HelloWorld:

        .asciz "Hello Earth!\n"

    ZeroFlagSet:

        .asciz "Zero Flag was Set!\n"

    ZeroFlagNotSet:

        .asciz "Zero Flag Not Set!\n"

.text

    .globl _start

    _start:

        nop

        movl $10, %eax

        xorl %eax, %eax  #To set Zero Flag

        jz FlagSetPrint

    FlagNotSetPrint:

        # Write CallDemo

        movl $4, %eax

        movl $1, %ebx

        leal ZeroFlagNotSet, %ecx

        movl $20, %edx

        int $0x80

        jmp ExitProgram

    FlagSetPrint:

        # Write CallDemo

        movl $4, %eax

        movl $1, %ebx

        leal ZeroFlagSet, %ecx

        movl $20, %edx

        int $0x80

        jmp ExitProgram

    ExitProgram:

        # Exit the program

        movl $1, %eax

        movl $10, %ebx

        int $0x80

Jump on Zero Example

================

bala@bala-laptop:~/ASM$ as -ggstabs -o ConditionalBranch.o ConditionalBranch.s

bala@bala-laptop:~/ASM$ ld -o ConditionalBranch ConditionalBranch.o

bala@bala-laptop:~/ASM$ ./ConditionalBranch

Zero Flag was Set!

bala@bala-laptop:~/ASM$ gdb ./ConditionalBranch

(gdb) list 1

8    .text

9        .globl _start

10        _start:

(gdb) break *_start+1

Breakpoint 1 at 0x4000b1: file ConditionalBranch.s, line 12.

(gdb) run

Starting program: /home/bala/ASM/ConditionalBranch

Breakpoint 1, _start () at ConditionalBranch.s:12

12            movl $10, %eax

(gdb) s

13            xorl %eax, %eax  #To set Zero Flag

(gdb) info registers

rbp            0x0    0x0

rsp            0x7fffffffe3b0    0x7fffffffe3b0

rip            0x4000b6    0x4000b6 <_start+6>

eflags         0x202    [ IF ]

cs             0x33    51

ss             0x2b    43

(gdb) s

14            jz FlagSetPrint

(gdb) info registers

rbp            0x0    0x0

rsp            0x7fffffffe3b0    0x7fffffffe3b0

rip            0x4000b8    0x4000b8 <_start+8>

eflags         0x246    [ PF ZF IF ]

cs             0x33    51

ss             0x2b    43

(gdb) s

FlagSetPrint () at ConditionalBranch.s:25

25            movl $4, %eax

(gdb) s

26            movl $1, %ebx

(gdb) s

27            leal ZeroFlagSet, %ecx

(gdb) s

28            movl $20, %edx

(gdb) s

29            int $0x80

(gdb) s

Zero Flag was Set!

30            jmp ExitProgram

LOOP Instruction

 * This is a normal Loop instruction
 * Number of times to Loop given in ECX, i-- automatically
 *

Program Start here

.data

    HelloWorld:

        .asciz "Hello Earth!\n"

.text

    .globl _start

    _start:

        nop

        movl $5, %ecx

    PrintFiveTimes:

        pushq %rcx

        movl $4, %eax

        movl $1, %ebx

        leal HelloWorld, %ecx

        movl $13, %edx

        int $0x80

        popq %rcx

    loop PrintFiveTimes

    jmp ExitProgram

ExitProgram:

        # Exit the program

        movl $1, %eax

        movl $10, %ebx

        int $0x80

bala@bala-laptop:~/ASM$ gdb ./Loop

(gdb) break *_start+1

Breakpoint 1 at 0x4000b1: file Loop.s, line 9.

(gdb) run

Starting program: /home/bala/ASM/Loop

Breakpoint 1, _start () at Loop.s:9

9            movl $5, %ecx

(gdb) s

PrintFiveTimes () at Loop.s:11

11            pushq %rcx

(gdb) print /x $rsp

$1 = 0x7fffffffe3d0

(gdb) x/1xw 0x7fffffffe3d0

0x7fffffffe3d0:    0x00000001

(gdb) s

PrintFiveTimes () at Loop.s:12

12            movl $4, %eax

(gdb) print /x $rsp

$2 = 0x7fffffffe3c8

(gdb) x/1xw 0x7fffffffe3c8

0x7fffffffe3c8:    0x00000005

(gdb) s

13            movl $1, %ebx

(gdb) s

14            leal HelloWorld, %ecx

(gdb) s

15            movl $13, %edx

(gdb) s

16            int $0x80

(gdb) s

Hello Earth!

17            popq %rcx

(gdb) s

PrintFiveTimes () at Loop.s:18

18        loop PrintFiveTimes

(gdb) print /x $rcx

$4 = 0x5

(gdb) s

11            pushq %rcx

(gdb) s

PrintFiveTimes () at Loop.s:12

12            movl $4, %eax

(gdb) s

13            movl $1, %ebx

(gdb) s

14            leal HelloWorld, %ecx

(gdb) s

15            movl $13, %edx

(gdb) s

16            int $0x80

(gdb) s

Hello Earth!

17            popq %rcx

(gdb) print /x $rsp

$5 = 0x7fffffffe3c8

(gdb) x /1xw 0x7fffffffe3c8

0x7fffffffe3c8:    0x00000004

(gdb)