Hardware interview practice

Constrain exactly five changed bit positions

EasyConstrained RandomSystemVerilog

Two randomized bytes must differ in exactly five bit locations. Write the single constraint that expresses that Hamming-distance relation.

Starting point

Question code

class distance_pair;
  rand bit [7:0] a;
  rand bit [7:0] b;
endclass

Reviewed example

Input

a=8'h00, b=8'h1F

Expected output

legal: $countones(a ^ b) = 5

a XOR b equals 8'b0001_1111, which contains exactly five set bits.

What to cover

Count differing positions rather than counting ones in each operand independently.
Require exactly five differing bits.
Allow every pair that satisfies the relation.
Check whether randomize() succeeds.

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