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Best way to dynamically select static length part of a bus

Altera_Forum
Honored Contributor II
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Hello, 

 

I am trying to figure out how to select a w-bit(w is some parameter, shorter than the bus) part of a bus based on an incoming variable x with the smallest resource footprint possible, in one clock tick. I have tried two methods so far. 

The first uses a shifter: 

 

... 

parameter bw=96 

parameter w=8 

... 

input wire [$clog2(bw):0] x, 

input wire [bw-1:0] inputWire, 

output reg [w-1:0] out 

... 

always@(posedge clk) out=inputWire>>x;  

 

At the second I use a for loop and multiplexers: 

 

 

integer i; 

always@(posedge clk)begin 

for (i =0; i<bw-w;i=i+1) begin:myforloop 

if(x==i) out=inputWire[i+:w]; 

end 

end  

 

I was quite surprised that the second method used much less resources(about half) than the first. Is there any reason for this? The only one I can come up with is that the second one covers a smaller range, but it still does not explain why the difference is this large. Also a single instance of the shifter used around 110 ALMs which is much more than I anticipated. Simpler adders take up like 4; even if the input is large like 96 it shouldn't take up so much space, should it? 

Is there any better/more compact method to dynamically select part of a bus?
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