RTL Design #26 · 2026-10-19 · Priority encoders · Easy · about 10 min

Lowest request wins

ffs is a find-first-set block for eight request lines. idx is the index of the lowest-numbered bit of req that is 1, and valid is 1 whenever any bit of req is set. With no request, valid is 0 and idx is 0. It's purely combinational.

For example, req = 8'b0001_0100 gives idx = 2 and valid = 1.

The current version doesn't report the right bit for some inputs, and valid is wrong for some too. Fix ffs.sv.

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