Example Collection of Assembly Language Sub Routine

Posted by Suraj Singh on December 20, 2018 · 15 mins read
Welcome Again readers,



Well, have You Ever Tried To Write Any Program, In Assembly. No! Don't Worry Today, I am going to share my simple program, written to represent as an example, to my blog viewer. so, that they can follow my step. It's really very fun.


Well, Basically Today In This Example Codes, You Will See, how We Can Use Sub Routine To Do Various Functions, Operation, And How We Can Use Registers To Perform Various Task.

Let's Start By Examples

Collection Of Examples Sub Routine In Assembly Language.


To Find String Length

; Routine To Find String length
_strlen:
; Input
; EAX = address of string
; Output
; EAX = Length of String

; Initialize Registers
push ebx ; push ebx value into stack
mov ebx, 0 ; Set EBX = 0


; iteration Routine
_iterate:
cmp byte [eax], 0 ; Compare Last bit, if true, Set Zero flag = 1 Else Zero flag = 0
jz _return ; If Zero Flag = 1, Then Call function else pass
inc eax ; Move Memory Address Pointer
inc ebx ; Keep Record of Iteration
jmp _iterate ; Repeat

; Return String Calculated Length
_return:
mov eax, ebx ; Set Return Value EAX = EBX
pop ebx ; POP EBX Value
ret ; Return

To Exit

; Exit Routine
_exit:
mov eax, 1 ; Invoke SYS_EXIT
mov ebx, 0 ; set Return Value Zero
int 80h ; interpt kernel

To Print Line (Without New Line End Character)

; Print Line
_print:
; Input
; EAX = addressof(string)
; Output
; Nothing

push ebx ; Store Registers Into Stack
push ecx
push edx
push eax

call _strlen ; Find String Len
mov edx, eax ; Set EDX= EAX {String Length}
mov eax, 4 ; invoke SYS_WRITE
mov ebx, 1 ; STDOUT
pop ecx ; POP String Address
int 80h ; interpt Kernel
pop edx ; POP all registers previous values
pop ecx ;
pop ebx ;
ret ; return

To Print Line (With New Line Character At End)

; PrintLine 
_printline:
; Input
; EAX = addressof(string)
; Output
; nothing

call _print ; Print String

;
mov eax, 0 ; SET EAX = 0
push eax ; PUSH EAX Into Stack To make Sure; String End Character is 0

mov eax, 0AH ; New Line ASCII Code
push eax ; Push into The Stack

;
; Now Our Stack Look Like This
; -> -> -> -> ->
; ---------------------------------------------------
; | EAX[0AH] | EAX [0] | previous data |
; ---------------------------------------------------
; ESP EBP
;

mov eax, esp ; Set EAX = ESP
call _print ; call _print routine
pop eax ; Remove not useful pushed values from stack
pop eax ;
ret ; Return


To Print User Provided Arguments

; print all arguments
_argument:
; Input
; EAX = ESP
; Output
; nothing
;
; Argument Available At The Start of Stack
;

push ebx ; PUSH EBX Values Into The STack
mov ebx, esp ; Point To Starting of Stack
add ebx, 8 ; because of calling _argument, _start function, 12 Bytes Are Already Added To Stack

; +++++++++++++ First Line
; print string without new line character
mov eax, string5 ; EXA = string5
call _print ;

; print script execution command
mov eax, [ebx+4] ; Load memory Address Content
call _printline ; call _printline

; +++++++++++ Second Line ++++++++++++++++++++
; print string without new line character
mov eax, string6 ; EXA = string5
call _print ;

; print script execution command
mov eax, [ebx+8] ; Load memory Address Content
call _printline ; call _printline


; +++++++++++ Third Line +++++++++++++++++++++++++++
; print string without new line character
mov eax, string7 ; EXA = string5
call _print ;

; print script execution command
mov eax, [ebx+12] ; Load memory Address Content
call _printline ; call _printline



pop ebx ; POP Ebx Value
ret


Simple Trigger Function Concept

_trigger:
mov eax, string0 ; EAX = addressof(string)
call _printline ; Call _print

mov eax, string1 ; Second String
call _printline

mov eax, string2 ; Third String
call _printline

mov eax, string3 ; Fourth String
call _printline

mov eax, string4 ; Fifth String
call _printline

ret

_start:
call _argument ; Print Arguments
call _trigger ; Perform All Function
jmp _exit ; EXIT Routine


Assemble All In One To Run It

; Assembly Includable File
; author : suraj singh bisht
; Email : surajsinghbisht054@gmail.com
;
;





; Routine To Find String length
_strlen:
; Input
; EAX = address of string
; Output
; EAX = Length of String

; Initialize Registers
push ebx ; push ebx value into stack
mov ebx, 0 ; Set EBX = 0


; iteration Routine
_iterate:
cmp byte [eax], 0 ; Compare Last bit, if true, Set Zero flag = 1 Else Zero flag = 0
jz _return ; If Zero Flag = 1, Then Call function else pass
inc eax ; Move Memory Address Pointer
inc ebx ; Keep Record of Iteration
jmp _iterate ; Repeat

; Return String Calculated Length
_return:
mov eax, ebx ; Set Return Value EAX = EBX
pop ebx ; POP EBX Value
ret ; Return

; Exit Routine
_exit:
mov eax, 1 ; Invoke SYS_EXIT
mov ebx, 0 ; set Return Value Zero
int 80h ; interpt kernel


; Print Line
_print:
; Input
; EAX = addressof(string)
; Output
; Nothing

push ebx ; Store Registers Into Stack
push ecx
push edx
push eax

call _strlen ; Find String Len
mov edx, eax ; Set EDX= EAX {String Length}
mov eax, 4 ; invoke SYS_WRITE
mov ebx, 1 ; STDOUT
pop ecx ; POP String Address
int 80h ; interpt Kernel
pop edx ; POP all registers previous values
pop ecx ;
pop ebx ;
ret ; return

; PrintLine
_printline:
; Input
; EAX = addressof(string)
; Output
; nothing

call _print ; Print String

;
mov eax, 0 ; SET EAX = 0
push eax ; PUSH EAX Into Stack To make Sure; String End Character is 0

mov eax, 0AH ; New Line ASCII Code
push eax ; Push into The Stack

;
; Now Our Stack Look Like This
; -> -> -> -> ->
; ---------------------------------------------------
; | EAX[0AH] | EAX [0] | previous data |
; ---------------------------------------------------
; ESP EBP
;

mov eax, esp ; Set EAX = ESP
call _print ; call _print routine
pop eax ; Remove not useful pushed values from stack
pop eax ;
ret ; Return

; bss section
SECTION .bss


; data section
SECTION .data
string0 db "[+] Hello Guys, How are You? ", 0h ; Decleare String
string1 db "[+] Nice ! ", 0h ; Decleate String
string2 db "[+] Welcome To Our Blog, ", 0h ;
string3 db "[+] My name is suraj singh ", 0h ;
string4 db "[+] EMail : surajsinghbisht054@gmail.com", 0h ;
string5 db "[+] Your Program Executing Command : ", 0h ;
string6 db "[+] Your First Provide Argument : ", 0h ;
string7 db "[+] Your Second Provide Argument : ", 0h ;

; text section
SECTION .text
global _start ; Trigger

; print all arguments
_argument:
; Input
; EAX = ESP
; Output
; nothing
;
; Argument Available At The Start of Stack
;

push ebx ; PUSH EBX Values Into The STack
mov ebx, esp ; Point To Starting of Stack
add ebx, 8 ; because of calling _argument, _start function, 12 Bytes Are Already Added To Stack

; +++++++++++++ First Line
; print string without new line character
mov eax, string5 ; EXA = string5
call _print ;

; print script execution command
mov eax, [ebx+4] ; Load memory Address Content
call _printline ; call _printline

; +++++++++++ Second Line ++++++++++++++++++++
; print string without new line character
mov eax, string6 ; EXA = string5
call _print ;

; print script execution command
mov eax, [ebx+8] ; Load memory Address Content
call _printline ; call _printline


; +++++++++++ Third Line +++++++++++++++++++++++++++
; print string without new line character
mov eax, string7 ; EXA = string5
call _print ;

; print script execution command
mov eax, [ebx+12] ; Load memory Address Content
call _printline ; call _printline



pop ebx ; POP Ebx Value
ret

_trigger:
mov eax, string0 ; EAX = addressof(string)
call _printline ; Call _print

mov eax, string1 ; Second String
call _printline

mov eax, string2 ; Third String
call _printline

mov eax, string3 ; Fourth String
call _printline

mov eax, string4 ; Fifth String
call _printline

ret

_start:
call _argument ; Print Arguments
call _trigger ; Perform All Function
jmp _exit ; EXIT Routine


Compile

; Compile :
;
; $ mkdir bin
; $ mkdir output
; $ nano includable_functions.asm
; $ nasm -f elf includable_functions.asm -o bin/includable_function.o
; $ ld -m elf_i386 bin/includable_function.o -o output/includable_function
; $ ./output/includable_function aaaa bbbb
;


Run

; Run
; :~$ ./output/includable_function aaaa bbbb
; [+] Your Program Executing Command : ./output/includable_function
; [+] Your First Provide Argument : aaaa
; [+] Your Second Provide Argument : bbbb
; [+] Hello Guys, How are You?
; [+] Nice !
; [+] Welcome To Our Blog,
; [+] My name is suraj singh
; [+] EMail : surajsinghbisht054@gmail.com
;
;


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