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asm-shift

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Apr 29th, 2013
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  1. .data
  2. taps:
  3. .long 1
  4. .long 2
  5. .long 9
  6. .long 15
  7. .long 25
  8. .text
  9. .globl main
  10. main:
  11. movl 0x0831423, %eax
  12. call createKeystream
  13. ret
  14.  
  15. createKeystream:
  16. movl $32, %edx g
  17. lfsr:
  18. movl $taps, %esi
  19. movl $5, %ecx # array size
  20. movl %eax, %ebx # seed -> temp
  21. xorl %edi, %edi # zero out index
  22.  
  23. part1: cmpl %edi, %ecx
  24. je part2
  25. shrl (%esi,%edi,4), %ebx # temp >> taps[i] <-------- PROBLEM
  26. xorl %eax, %ebx # seed ^ temp > temp
  27. incl %edi
  28. jmp part1
  29.  
  30. part2: andl $1, %ebx # temp &= 1
  31. shl $31, %ebx # temp << 31
  32. shr $1, %eax # seed >> 1
  33. or %ebx, %eax # temp | seed -> seed
  34. dec %edx
  35. jnz lfsr
  36. ret
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