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So I know I can bit shift a mask of 0xFFFFFFFF by say 8 for example to get 0xFFFFFF00 & it with an address and get everything but the last 8 bits, but I would like to go the other way so I can grab the bottom end of an address with a mask like 0x000000FF, however bit shifting right wont work obviously. Any ideas?

Here is the code for the first type bit shift I mentioned.

public int Block_tag(int address, int block_size, int max_address) { int bit_shift = (int)Math.Log(block_size, 2); int bit_mask = max_address << bit_shift; return (address & bit_mask); }

I could do it by hand with about 12 "If" statements but that isn't very clean.

Thanks!

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  • Why do you need to shift? Just use the literal mask: address & 0x000000FF Commented Apr 1, 2013 at 4:17
  • Well I would like to shift the address of all F's so I can use the method modularly without having to hard code every shift. Commented Apr 1, 2013 at 4:22

2 Answers 2

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To simply invert the mask use the ~ operator

int not_mask = ~bit_mask;

For example, ~0xFFFFFF00 == 0x000000FF.

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Wow, I didn't know that's how that operator was used. Works like a charm thanks!
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You can right shift it by (32 - number of mask bits you need). So if you need lower 8 bits, you can right shift by (32-8 = 24) bits.

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I thought that too, but since I convert my strings of hex to integers for other reasons they are negative, and it causes problems. Thanks though!

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