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wu :: forums    riddles    microsoft (Moderators: Eigenray, SMQ, towr, william wu, ThudnBlunder, Icarus, Grimbal)    ASM question « Previous topic | Next topic » Pages: 1  Reply Notify of replies Send Topic Print    Author  Topic: ASM question  (Read 7594 times) vishal_changrani Newbie     Posts: 4 ASM question   « on: Feb 15th, 2006, 12:07pm » Quote Modify Hi, I am back with one more question.....supposedly asked  in an MS interview...   Q. The following asm block performs a common math function, what is it? CWD XOR AX,DX SUB AX,DX   My guess earlier was it is exchangin AX and DX but doesnt seemt to be that. Also is CWD carried out first or XOR? in the first line   any and all replies would be appreciated. Thank u Vishal IP Logged vishal_changrani Newbie     Posts: 4 Re: ASM question   « Reply #1 on: Feb 15th, 2006, 12:35pm » Quote Modify Sorry but i think i found the  answer...its just doing a MOD operation   sorry for posting it in the first place -vishal IP Logged Grimbal wu::riddles Moderator Uberpuzzler     Gender: Posts: 7527 Re: ASM question   « Reply #2 on: Feb 16th, 2006, 4:22am » Quote Modify No, it does an ABS(AX)   In fact, it splits AX into sign and absolute value:   DX = sign(AX) AX = abs(AX) IP Logged ruhela_nitk Newbie     Gender: Posts: 1 Re: ASM question   « Reply #3 on: Jul 28th, 2006, 7:48am » Quote Modify Hi   If anyone  can explain the answer , please let me? IP Logged Grimbal wu::riddles Moderator Uberpuzzler     Gender: Posts: 7527 Re: ASM question   « Reply #4 on: Jul 29th, 2006, 11:48am » Quote Modify CWD converts a word to double.  It does so by extending the sign of AX into DX.  Practically, it means it does DX=0 if AX is >=0 or DX=-1 (all bits set) if AX<0.   The XOR and SUB do nothing if DX=0. If DX=-1, the XOR reverses all bits of AX and the SUB adds 1 to AX.  Together, it does     AX = (not AX) + 1 which is exactly how you compute AX=-1 in the 2's complement binary representation of integers.   So, the effect is to set DX=0 or -1 depending on AX's sign, and to set AX = abs(AX). « Last Edit: Jul 29th, 2006, 11:50am by Grimbal » IP Logged Pages: 1  Reply Notify of replies Send Topic Print Forum Jump: ----------------------------- riddles -----------------------------  - easy   - medium   - hard   - what am i   - what happened => microsoft   - cs   - putnam exam (pure math)   - suggestions, help, and FAQ   - general problem-solving / chatting / whatever ----------------------------- general -----------------------------  - guestbook   - truth   - complex analysis   - wanted   - psychology   - chinese « Previous topic | Next topic »

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