Hardware interview practice
Summarize one 32-bit scan-unload mismatch
A scan diagnostic compares one expected unload word against the observed word and needs both a count and the first failing chain position. Implement the combinational mismatch summarizer.
Starting point
Question code
input logic [31:0] expected, observed;
output logic mismatch;
output logic [4:0] first_index;
output logic [5:0] mismatch_count;Reviewed example
Work through one case
Input
expected=32'h0000_0000, observed=32'h0000_0028.Expected output
mismatch=1, first_index=3, mismatch_count=2.The XOR difference mask has bits 3 and 5 set, so there are two mismatches and bit 3 is the lowest failing position.
What to cover
Requirements
- Inputs contain only known bits. Define difference bit i as expected[i] XOR observed[i].
- mismatch is 1 if at least one difference bit is set, and mismatch_count is the exact population count from 0 through 32.
- When mismatch=1, first_index is the lowest-numbered differing bit; when mismatch=0, first_index is 0.
- Use complete combinational RTL with a loop whose bounds are constant and synthesizable.