Program Execution Flow

Unconditional Branching
1 JMP:
 * Analogous to goto statement in C program
 * Syntax - JMP label
 * Short - JMP within +or-128 bytes
 * Near - Is jumping between both, Short & Far
 * Far - JMP from one segment to another segment

2 Call
 * Just like calling a function in C
 * Syntax - Call location
 * There is an associate RET statement with every call
 * Call pushes the next instruction address onto the stack

Program Code starts here

.data
    HelloWorld:
        .asciz "Hello Earth!"
    CallDemo:
        .asciz "CallDemo got called!"
.text
    .globl _start
    _start:
        nop
        #JMP ExitProgram #This to be enabled for JMP eg
        call Callme
        # Write HelloWorld
        movl $4, %eax
        movl $1, %ebx
        movl $HelloWorld, %ecx
        movl $12, %edx
        int $0x80
    ExitProgram:
        # Exit the program
        movl $1, %eax
        movl $10, %ebx
        int $0x80
    Callme:
        # Write CallDemo
        movl $4, %eax
        movl $1, %ebx
        movl $CallDemo, %ecx
        movl $20, %edx
        int $0x80
        ret


JUMP Example
==========

(gdb) break *_start+1

Breakpoint 2 at 0x4000b1: file ProgramExecutionFlow.s, line 10.

(gdb) info break

Num     Type           Disp Enb Address            What

2       breakpoint     keep y   0x00000000004000b1 (gdb) run

Starting program: /home/bala/ASM/ProgramExecFlow

Breakpoint 2, _start () at ProgramExecutionFlow.s:10

10            JMP ExitProgram

(gdb) print /x $rbp

$2 = 0x0

(gdb) print /x $rip

$3 = 0x4000b1

(gdb) disas _start

Dump of assembler code for function _start:

   0x00000000004000b0 <+0>:    nop

=> 0x00000000004000b1 <+1>:    jmp    0x4000c9
   0x00000000004000b3 <+3>:    mov    $0x4,%eax
   0x00000000004000b8 <+8>:    mov    $0x1,%ebx
   0x00000000004000bd <+13>:    mov    $0x6000ec,%ecx
   0x00000000004000c2 <+18>:    mov    $0xc,%edx
   0x00000000004000c7 <+23>:    int    $0x80
End of assembler dump.
(gdb) disas ExitProgram
Dump of assembler code for function ExitProgram:
   0x00000000004000c9 <+0>:    mov    $0x1,%eax
   0x00000000004000ce <+5>:    mov    $0xa,%ebx
   0x00000000004000d3 <+10>:    int    $0x80
End of assembler dump.
(gdb) s
ExitProgram () at ProgramExecutionFlow.s:19
19            movl $1, %eax
(gdb) print /x $rip
$4 = 0x4000c9
(gdb)

CALL Example
==========
bala@bala-laptop:~/ASM$ as -ggstabs -o ProgramExecFlow.o ProgramExecutionFlow.s 
bala@bala-laptop:~/ASM$ ld -o ProgramExecFlow ProgramExecFlow.o
bala@bala-laptop:~/ASM$ ./ProgramExecFlow
CallDemo got called!Hello Earth!bala@bala-laptop:~/ASM$ gdb ./ProgramExecFlow
(gdb) list
8        _start:
9            nop
10            #JMP ExitProgram #This to be enabled for JMP eg
(gdb) break *_start+1
Breakpoint 1 at 0x4000b1: file ProgramExecutionFlow.s, line 11.
(gdb) run
Starting program: /home/bala/ASM/ProgramExecFlow
Breakpoint 1, _start () at ProgramExecutionFlow.s:11
11            call Callme
(gdb) disassemble _start
Dump of assembler code for function _start:
   0x00000000004000b0 <+0>:    nop
=> 0x00000000004000b1 <+1>:    callq  0x4000d8
   0x00000000004000b6 <+6>:    mov    $0x4,%eax
   0x00000000004000bb <+11>:    mov    $0x1,%ebx
   0x00000000004000c0 <+16>:    mov    $0x6000f0,%ecx
   0x00000000004000c5 <+21>:    mov    $0xc,%edx
   0x00000000004000ca <+26>:    int    $0x80
End of assembler dump.
(gdb) disas Callme
Dump of assembler code for function Callme:
   0x00000000004000d8 <+0>:    mov    $0x4,%eax
   0x00000000004000dd <+5>:    mov    $0x1,%ebx
   0x00000000004000e2 <+10>:    mov    $0x6000fd,%ecx
   0x00000000004000e7 <+15>:    mov    $0x14,%edx
   0x00000000004000ec <+20>:    int    $0x80
   0x00000000004000ee <+22>:    retq  
End of assembler dump.
(gdb) print /x $rsp
$2 = 0x7fffffffe3b0
(gdb) print /x $rip
$3 = 0x4000b1
(gdb) s
Callme () at ProgramExecutionFlow.s:25
25            movl $4, %eax
(gdb) print /x $rip
$4 = 0x4000d8
(gdb) print /x $rsp
$5 = 0x7fffffffe3a8
(gdb) x/1xx 0x7fffffffe3a80x7fffffffe3a8:    0x004000b6
(gdb) s
26            movl $1, %ebx
(gdb) s
27            movl $CallDemo, %ecx
(gdb) s
28            movl $20, %edx
(gdb) s
29            int $0x80
(gdb) s
CallDemo got called!Callme () at ProgramExecutionFlow.s:30
30            ret
(gdb) s
_start () at ProgramExecutionFlow.s:13
13            movl $4, %eax
(gdb) s
14            movl $1, %ebx
(gdb) s
15            movl $HelloWorld, %ecx
(gdb) s
16            movl $12, %edx
(gdb) s
17            int $0x80
(gdb) s
Hello Earth!ExitProgram () at ProgramExecutionFlow.s:20
20            movl $1, %eax
(gdb) s
21            movl $10, %ebx
(gdb) s
22            int $0x80
(gdb) s
Program exited with code 012.
(gdb)