Reversing : The Hacker's Guide to Reverse Engineering

_alldiv PROC NEAR

push edi

push esi

push ebx

; Set up the local stack and save the index registers. When this is

; done the stack frame will look as follows (assuming that the

; expression a/b will generate a call to lldiv(a, b)):

;

; -----------------

; | |

; |---------------|

; | |

; |--divisor (b)--|

; | |

; |---------------|

; | |

; |--dividend (a)-|

; | |

; |---------------|

; | return addr** |

; |---------------|

; | EDI |

; |---------------|

; | ESI |

; |---------------|

; ESP---->| EBX |

; -----------------

;

DVND equ [esp + 16] ; stack address of dividend (a)

DVSR equ [esp + 24] ; stack address of divisor (b)

; Determine sign of the result (edi = 0 if result is positive, non-zero

; otherwise) and make operands positive.

xor edi,edi ; result sign assumed positive

mov eax,HIWORD(DVND) ; hi word of a

or eax,eax ; test to see if signed

jge short L1 ; skip rest if a is already positive

inc edi ; complement result sign flag

mov edx,LOWORD(DVND) ; lo word of a

neg eax ; make a positive

neg edx

sbb eax,0

Listing B.2 The alldivfunction used for performing 64-bit divisions in code generated

by the Microsoft compilers. (continued)

Understanding Compiled Arithmetic 531

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