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Set the first N bits of a DWORD

Created by: Parkotron at: 3/25/2009 8:14 PM (4 Replies)

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3/25/2009 8:14 PM	Rate (0)
Parkotron Posts: 13 Rating: (1)	Hello folks, I'm trying to write an FC that accepts an interger (N) between 0 and 32 and returns a DWORD with the first N bits true and the rest false. I've written a couple solutions, but they seem unnecessarily complex. This seems like it should be doable with a simple straight forward solution. So I guess this is more a challenge than a question. What the most elegant solution you can come up with in STL.
Parkotron Posts: 13 Rating: (1)
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3/25/2009 9:30 PM	Rate (0)
Parkotron Posts: 13 Rating: (1)	Never mind. I found the elegant solution. [code]L 32 L #bitsToFill -I L L#-1 SRD T #mask[/code]
Parkotron Posts: 13 Rating: (1)
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3/26/2009 6:05 AM	Rate (0)
Aret Posts: 2348 Rating: (265)	L#bitsToFill L 1 SLD + L#-1 T #mask Who gives less?:^)
Aret Posts: 2348 Rating: (265)
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3/26/2009 1:20 PM	Rate (0)
Parkotron Posts: 13 Rating: (1)	Darn, beaten by one instruction! Obviously I should have said "I found an elegant solution." There is, however, a difference between our solutions. If #bitsToFill is outside the range of 0 to 32 my solution returns 0x0 while your solution returns 0xffffffff. I guess an ideal solution would return 0x0 for negative numbers and 0xffffffff for numbers greater than 32. That would be easy enough to do by just restricting #bitsToFill to be inside that range, but that would definitely increase the instruction count! :)
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3/27/2009 7:08 AM	Rate (0)
Aret Posts: 2348 Rating: (265)	I like such kind of quiz:^) About zero - you're absolutely right, it have to be checked.... I'll think about that %))
Aret Posts: 2348 Rating: (265)	Last edited by: Aret at: 3/27/2009 7:09 AM
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