CSC231 Loop Offset is +127/-128 bytes

--D. Thiebaut 09:35, 15 October 2012 (EDT)

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• The LOOP instruction can only take you to a label that is +127 or -128 bytes away!

     1                                  ;;; printStars.asm
     2                                  ;;; D. Thiebaut
     3                                  ;;;
     4                                  ;;; 
     5                                  ;;;
     6                                  ;;; To assemble, link, and run:
     7                                  ;;;     nasm -f elf -F stabs printStars.asm
     8                                  ;;;     ld -melf_i386 -o printStars printStars.o
     9                                  ;;;     ./printStars
    10                                  ;;;
    11                                  
    12                                                  section .data
    13                                  
    14 00000000 0A2A2A2A2A2A2A2A2A-     stars           db      0x0a, "******************************"
    15 00000009 2A2A2A2A2A2A2A2A2A-
    16 00000012 2A2A2A2A2A2A2A2A2A-
    17 0000001B 2A2A2A2A           
    18                                  starsLen        equ     $-stars
    19                                          
    20 0000001F [00000000]              addr            dd      stars
    21 00000023 1F000000                count           dd      starsLen
    22                                          
    23                                                  section .text
    24                                                  global  _start
    25                                  _start: 
    26 00000000 8B0D[23000000]                          mov     ecx, dword[count]
    27                                          
    28 00000006 890D[23000000]          for:            mov     dword[count], ecx                       
    29 0000000C B804000000                              mov     eax, 4
    30 00000011 BB01000000                              mov     ebx, 1
    31 00000016 B9[00000000]                            mov     ecx, stars
    32 0000001B 8B15[23000000]                          mov     edx, dword[count]
    33 00000021 CD80                                    int     0x80
    34                                  
    35 00000023 8B0D[23000000]                          mov     ecx, dword[count]
    36 00000029 E2DB                                    loop    for
    37                                          
    38                                  ;;; exit
    39 0000002B BB00000000                              mov     ebx, 0
    40 00000030 B801000000                              mov     eax, 1
    41 00000035 CD80                                    int     0x80
    42                                  
    43

If you want to loop farther away, then you need to use JMP!