How to count zeros in registers?

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davyzhu

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Hi all,

reg[7:0] register; (Verilog)
The register contains data like
[0 0 0 1 0 1 0 1]
And I want to know the number of the zeros before the first 1
(in this example is 3 zeros).

How to do this in a combinational logic?

All the logic have be done in Verilog

Best regards,
Davy
 

Cannot undersand what are you exactly trying to do!! You can put 7 LED if its a parallel output register.....if its serial you need to convert it into parallel first.
 

Just in case this is a homework problem, I'll give you only a hint. You can use a priority encoder.
 

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    davyzhu

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search of "normalization" and "VHDL". You will find the info you need.

Also this functionality is natively supported in some modern microprocessors. In their case, this operation is called: "FIND FIRST ONE".

the_penetrator©
 

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    davyzhu

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Hi,

It's a interview problem

I have solve it in priority encoder. But the circuit seems not balanced.

All the best,
Davy
 

haha is it an interview problem? i had it as a homework in my old logic class, except our assignment was find the number of zeros before the first one. i think your teacher just inverted my old homework.
 

shift the register contents and xor the previous output with the present one.
count the number of clock pulses elapsed till the output of xor is 1.
the number of clock pulses id ur answer.
 

The XOR technique sounds good to me. For visual and interactive circuits go here: **broken link removed**. It may help.
 

Hi,

You can find the answer at the first section in chapter 5 of Digital Design by Mano

You won't reach your target unless you consider a sequential circuit design steps. the first thing is you should is to draw a state diagram.
 

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