X86 assembly – Handling the IDIV instruction

The first part of Mysticials answer is correct, idiv does a 128/64 bit division, so the value of rdx, which holds the upper 64 bit from the dividend must not contain a random value. But a zero extension is the wrong way to go.

As you have signed variables, you need to sign extend rax to rdx:rax. There is a specific instruction for this, cqto (convert quad to oct) in AT&T and cqo in Intel syntax. AFAIK newer versions of gas accept both names.

movq    %rdx, %rbx
cqto                  # sign extend rax to rdx:rax
idivq   %rbx

Related Posts:

  1. need help understanding the movzbl call in this function
  2. Display value found at given address gdb
  3. How to compile makefile using MinGW?
  4. xorl %eax – Instruction set architecture in IA-32
  5. Difference between “move” and “li” in MIPS assembly language
  6. Assembly x86 – “leave” Instruction
  7. How can one see content of stack with GDB?
  8. How do I compile the asm generated by GCC?
  9. MIPS to C Translation
  10. What does the LEAL assembly instruction do?
  11. C compiler for Windows?
  12. What does the MOVZBL instruction do in IA-32 AT&T syntax?
  13. What’s the purpose of the LEA instruction?
  14. How to move ST(0) to EAX?
  15. The point of test %eax %eax
  16. The difference between cmpl and cmp
  17. What is the function of the push / pop instructions used on registers in x86 assembly?
  18. What is callq instruction?
  19. Difference between JA and JG in assembly
  20. IDIV operation in assembly (understanding)
  21. How to do scanf for single char in C
  22. Break statement not within loop or switch in C
  23. How to properly malloc for array of struct in C
  24. Cache Simulator in C
  25. Where is the C auto keyword used?
  26. Convert char array to a int number in C
  27. Bind failed: Address already in use
  28. Warning/error “function declaration isn’t a prototype”
  29. What is the difference between %f and %lf in C?
  30. What is stdin in C language?
  31. How to prevent multiple definitions in C?
  32. How to pass 2D array (matrix) in a function in C?
  33. How can you print multiple variables inside a string using printf?
  34. C dynamically growing array
  35. Why should we check WIFEXITED after wait in order to kill child processes in Linux system call?
  36. x86 Assembly pointers
  37. Mapping a numeric range onto another
  38. Why are hexadecimal numbers prefixed with 0x?
  39. x86 assembly: How does the ‘subl’ command work in AT&T syntax
  40. too many arguments for format [-Wformat-extra-args]
  41. what does the cmpq instruction do?
  42. How to copy a char array in C?
  43. cast to pointer from integer of different size, pthread code
  44. Invalid type argument of unary ‘*’ (have ‘int’) Error in C
  45. Why do I get “cast from pointer to integer of different size” error?
  46. C subscripted value is neither array nor pointer nor vector when assigning an array element value
  47. error: too few arguments to function `printDay’ (C language)
  48. Where to find the complete definition of off_t type?
  49. struct has no member named
  50. printf not printing to screen
  51. Which variable size to use (db, dw, dd) with x86 assembly?
  52. How do AX, AH, AL map onto EAX?
  53. What do numbers using 0x notation mean?
  54. Can I create an Array of Char pointers in C?
  55. Does stack grow upward or downward?
  56. Convert Little Endian to Big Endian
  57. JNZ & CMP Assembly Instructions
  58. Excess elements of scalar initializer for pointer to array of ints
  59. error C2371: ‘functionname’ redefinition: different basic types
  60. How does one represent the empty char?
  61. How to write to a file using open() and printf()?
  62. pop or add esp, 4 ? What is the difference?
  63. note: previous implicit declaration of ‘point_forward’ was here
  64. What is the difference between MOV and LEA?
  65. Can I define a function inside a C structure?
  66. What exactly is meant by “de-referencing a NULL pointer”?
  67. getc() vs fgetc() – What are the major differences?
  68. Can’t understand the working of getint() in C as per K&R
  69. Difference between JE/JNE and JZ/JNZ
  70. Tokenizing strings in C
  71. In C, what exactly happens when you pass a NULL pointer to strcmp()?
  72. “-bash: gcc: command not found” using cygwin when compiling c?
  73. How to format strings using printf() to get equal length in the output
  74. What do \t and \b do?
  75. Why and when to use static structures in C programming?
  76. Where does linux store my syslog?
  77. Example of waitpid() in use?
  78. Can a function return two values?
  79. Assembly Language – How to do Modulo?
  80. C: warning: excess elements in array initializer; near initialization for ‘xxx’ ; expects ‘char *’, but has type ‘int’
  81. write() to stdout and printf output not interleaved?
  82. How to know what the ‘errno’ means?
  83. Split string with multiple delimiters using strtok in C
  84. Printf was not declared in this scope
  85. warning: missing terminating ” character [enabled by default]
  86. How to free memory from char array in C
  87. Can I get Unix’s pthread.h to compile in Windows?
  88. Initializing 2D char array in C
  89. FFT in a single C-file 
  90. Process exited with return value 3221225477
  91. How do I check if a string contains a certain character?
  92. malloc(sizeof(int)) vs malloc(sizeof(int *)) vs (int *)malloc(sizeof(int))
  93. %p Format specifier in c
  94. GDB no such file or directory
  95. Level vs Edge Trigger Network Event Mechanisms
  96. “Nothing to be done for makefile” message
  97. Convert Char to String in C
  98. [Binary Bomb – Phase 4
  99. What is the difference between signed and unsigned int
  100. `js` and `jb` instructions in assembly

Leave a Comment