Assembly Language Primer For Hackers (Part 7) Working With Strings

Video Link - http://videos.securitytube.net/Assembly-...trings.mp4
& Here - http://www.securitytube.net/video/225

Description: This is Part 7 of the "Assembly Language Primer for Hackers" video series. Please begin here, by watching Part 1, if you have not already done so. In this video we will look at how to work with strings in Assembly. We will demonstrate how we can move strings from one memory location to the other using the MOVS instruction set, discuss the concept of the Direction Flag (DF) and how to set and clear it using STD and CLD, how to execute multiple string copy instructions using the REP instruction, how to load strings from memory into the EAX register using the LODS instruction set, how to store strings from the EAX register back into memory using the STOS instruction set and finally we shall look at how to compare strings using the CMPS instruction set.

Please download StringBasics.s to try out the various exercises discussed in this video.

Code:

.data
    HelloWorldString:
        .asciz "Hello World of Assembly!"
    H3110:
        .asciz "H3110"
.bss

    .lcomm Destination, 100
    .lcomm DestinationUsingRep, 100
    .lcomm DestinationUsingStos, 100

.text

    .globl _start

    _start:
        nop
        # 1. Simple copying using movsb, movsw, movsl

        movl $HelloWorldString, %esi
        movl $Destination, %edi

        movsb
        movsw
        movsl
      
        # 2. Setting / Clearing the DF flag

        std # set the DF flag
        cld # clear the DF flag
  
        # 3. Using Rep
      
        movl $HelloWorldString, %esi
        movl $DestinationUsingRep, %edi
        movl $25, %ecx # set the string length in ECX
        cld # clear the DF
        rep movsb
        std
      
        # 4. Loading string from memory into EAX register
      
        cld
        leal HelloWorldString, %esi
        lodsb
        movb $0, %al
      
        dec %esi
        lodsw
        movw $0, %ax

        subl $2, %esi # Make ESI point back to the original string
        lodsl

        # 5. Storing strings from EAX to memory

        leal DestinationUsingStos, %edi
        stosb
        stosw
        stosl

        # 6. Comparing Strings
      
        cld
        leal HelloWorldString, %esi
        leal H3110, %edi
        cmpsb
      
        dec %esi
        dec %edi
        cmpsw

        subl $2, %esi
        subl $2, %edi
        cmpsl


        # The exit() routine

        movl $1, %eax
        movl $10, %ebx
        int $0x80

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