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‫Welcome to the barrel shifter explained lecture in this lecture I'll explain what Abair software is

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‫and how we can implement one in VHDL barrel shift register Bearse a regular shift register a regular

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‫shift register only shifts one bit per clock cycle and a center shift register will only shift left

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‫or right.

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‫You can also make a universal shift register where you can shift left or right based upon the input

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‫and you can also have like a parallel load shift register where you can load all the bits at once or

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‫a serial load shift register where you shift all the bits in.

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‫The main thing to note with a standard shift register is that you can only every clock cycle you'll

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‫make one shift whether it's left or right.

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‫Now if we compare that to a barrel shifter Barish after allows you to shift an number of bits on a single

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‫clock cycle and this end means that it's a configurable input on one clock cycle.

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‫I can shift one bit.

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‫I can shift two I can shift three for any arbitrary number of bits as long as However wide my register

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‫is.

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‫And this will happen on one clock cycle and you can also like to shift to the left or right just the

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‫same.

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‫Now the downside of the barrel shifter versus the regular shift register is that it will require slightly

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‫more resources that moment however what you will gain is a speed factor if you want to shift say 8 bits

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‫to the right with a center shift register.

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‫It's going to take you eight clock cycles to get there with the barrel shift or you can do it in a single

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‫clock cycle.

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‫So in theory here and in reality are eight times faster.

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‫And so that's where the advantage of the barrel shifter is a standard shift register comes in.

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‫Now eight minutes you probably might not notice that.

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‫But if you have a really large register a 64 bit 128 that registers even larger and you can perform

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‫a 128 bit shift on one single clock cycle.

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‫And so that's greatly useful and this is very powerful Let's take a look at an example of we have an

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‫8 bit barrel shifter.

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‫You'll see that we have our input which is seven down is zero meaning that it's eight minutes wide.

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‫We have our select one down to zero two bits wide.

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‫And then we have our shift to down to zero reset and clock line and then we have output.

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‫So we look at a table we have the different time steps.

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‫So if we want to go our time zero we have our input.

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‫A certain bit.

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‫8 bits wide and our select is equal to 3 which means that we want to do a load operation.

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‫So we're going to take the bits that are on our input and put them in the register.

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‫And these are register is what we're going to see on the output of our shift register.

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‫And if we keep A.M.A. the time steps you can see that will do different operations on our on our select

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‫input.

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‫So our select is a zero.

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‫That is a hold.

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‫So on the whole we're not going to do anything we're just going to keep the same same there so you can

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‫say a register doesn't change it just now has a value that was on our template and then our select becomes

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‫a 2.

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‫Now our register is shifted left to the value whatever shift value is which is the value of three minute

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‫shift left three three bits and if you just follow through you'll see our next one we're going to shift

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‫right.

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‫The amount that is on our shift input which is a 4.

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‫So we're going to shift right for bits and you'll see that the register on the next step has that implementation

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‫has been shifted right for bits and if he falls the time steps are we have a shift right by one bit

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‫shift left by two bits and then our select is a zero.

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‫And our shift is zero which is once a zero we know the hold.

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‫So now we're just holding that output.

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‫And that's kind of the huge thing with the barrel shifter is that if I wanted to do any one of these

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‫other operations where on time to I have a shift of three it's going to take three clock cycles to do

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‫that.

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‫We're on the barrel shifter one clock cycle.

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‫I now have that output.

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‫And so you gain a lot of speed but the con is that it takes a few more resources to implement but you

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‫do gain a lot more in speed.

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‫So let's take a look at how would you design the barrel shifter.

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‫If we take a block diagram just to show kind of how the barrel shifter could be design you would have

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‫a process we would call a shift left and a process will of course shift right.

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‫And then our process would point output.

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‫So every time the shift amount changes you are going to shift left and shift right that amount and that

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‫process is going to continue to be happening so soon that changed my shift about my shift left and my

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‫shift right.

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‫We're going to change.

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‫And then my output when I see a clock based on my select if I'm selecting my shift right or shift left

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‫I'm just going to select the output that s r red or s l Reg value.

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‫And so in theory what we're doing every single clock cycle before that clock cycle occurs.

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‫We're shifting left and shifting right at the same exact time based upon the shift amount input and

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‫then the clock cycle occurs.

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‫We're going to make the decision with the mux inside of our process or out the process and decide am

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‫my -- the shift left register or the shift right register and that's how we can make it happen

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‫on a single clock cycle before that clock cycle occurs.

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‫We're already doing the left shifting and the right shifting and based on the input we're giving it

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‫we're going to say OK we're selecting a shift left or shift right because both those outputs have already

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‫been shifted left or right the number of bits that we have selected.

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‫Now the understanding of how the barracks filter works let's get started implementing one on your development

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‫board.