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How are bit shifts implemented at the hardware level when the number to shift by is unknown?
I can't imagine that there would be a separate circuit for each number you can shift by (that would 64 shift circuits on a 64-bit machine), nor can I imagine that it would be a loop of shifts by one (that would take up to 64 shift cycles on a 64-bit machine). Is it some sort of compromise between the two or is there some clever trick?
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Bit shifting means moving the position of the bits in the left direction or in the right direction. For example, if we write in binary form the decimal number 23 , we get 00010111 . If we shift all bits left with one position, we'll get 00101110 . Bit 7 (MSB) was replaced by bit 6, bit 6 by bit 5 and so on.
Shifters and rotators move bits and multiply or divide by powers of 2. As the name implies, a shifter shifts a binary number left or right by a specified number of positions. Several kinds of commonly used shifters exist: Logical shifter—shifts the number to the left or right and fills empty spots with 0's.
A barrel shifter is a digital circuit that can shift a data word by a specified number of bits without the use of any sequential logic, only pure combinational logic, i.e. it inherently provides a binary operation.
In general just one. But the correct answer is it depends of the CPU architecture, specifically of its arithmetic and logical unit. Then, if the ALU implements a sequential shift register, then it will take so cycles as bits in the register.
The circuit is called a "barrel shifter" - it's a load of multiplexers basically. It has a layer per address-bit-of-shift-required, so an 8-bit barrel shifter needs three bits to say "how much to shift by" and hence 3 layers of muxes.
Here's a picture of an 8-bit one from http://www.globalspec.com/reference/55806/203279/chapter-9-additional-circuit-designs:
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