posted 12 years ago

Negative numbers in bitwise arithmetic are confusing. You have to work out what the negative numbers look like in binary (two's complement representation). Then the bitwise operation will make sense.

When writing code, I would suggest that you never use negative integer literals (e.g. -2) in a bitwise expression. The clearest representation is usually hexadecimal literals (e.g. 0xFFFFFFFE), because that is very easily translatable to binary in the head of the person reading your code.

When writing code, I would suggest that you never use negative integer literals (e.g. -2) in a bitwise expression. The clearest representation is usually hexadecimal literals (e.g. 0xFFFFFFFE), because that is very easily translatable to binary in the head of the person reading your code.

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posted 12 years ago

The formula is to always convert the number to binary 0's and 1's, perform the operation, then convert it back. For exams or homework problems, you're not likely to find a better way to do it by hand.

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posted 12 years ago

As previously mentioned, negative numbers and bitwise operations are tricky.

I'll use a hit (or nibble...4 bits) to show the steps:

0101 = 5

1010 = ~5 (1s complement - flip all the bits)

1011 = -5 (2s complement - add 1 to 1s complement)

now shift right by 2, carry the sign bit

1101 = shift one

1110 = shift two (result)

so the result is 1110, which is a negative number.

I'll apply 2s complement to translate to an easier to read positive number, keeping in mind that the answer is really negative.

1110 = result

0001 = ~result

0010 = -result = 2 (but we know that it's really negative so: ) -2

I'll use a hit (or nibble...4 bits) to show the steps:

0101 = 5

1010 = ~5 (1s complement - flip all the bits)

1011 = -5 (2s complement - add 1 to 1s complement)

now shift right by 2, carry the sign bit

1101 = shift one

1110 = shift two (result)

so the result is 1110, which is a negative number.

I'll apply 2s complement to translate to an easier to read positive number, keeping in mind that the answer is really negative.

1110 = result

0001 = ~result

0010 = -result = 2 (but we know that it's really negative so: ) -2