22Implementation

Grant the lowest request

Problem

Intended behavior. LowestRequest serves four requesters. Bit i of req is requester i, and data(i) is that requester's 8-bit value. When one or more requests are high, out carries the data of the lowest-numbered requester. When no request is high, out is 0.

Current behavior. The starter drives out with a constant 0, so every request pattern prints 0.

Task. Replace the constant with a selection that implements both rules. Keep the module name and ports; the req Bools already split from io.req are there for you to use.

Choose the selector shape

Before writing code, describe the selectors you have. Several requests can be high at once, so the conditions overlap. Some order must decide between them, and the case where no condition is true has its own required output. Go through the utilities in the Learn chapter's contract table and ask, for each one, what it does with overlapping selectors and what it returns when none is true.

Predict before running

The public run sets data to 11, 22, 33, 44 and prints req with the most significant bit first. Predict out for 0100, 0110, 1010, 1000, and 0000 before running your change. Choose one row with overlapping requests and the idle row to distinguish a correct design from two tempting wrong ones.

Run and explain

Replace the constant, run, and compare all five rows with your prediction. For the two rows you marked, state what a selector that ORs overlapping data would print, and what a selector that falls through to its last value would print. Those differences are the reason your choice is correct, not luck on the sample.

Completion check

Each of the 16 request patterns selects the lowest set bit's data.
With no request, out is 0, never one of the data inputs.
The selection follows data for any values, not only the four printed ones.
Two simultaneous requests return one requester's data, never a mixture.

Progressive hints

Hint 1

The idle row rules out one utility, and the rows with two requests rule out another. Which utilities remain?

Hint 2

PriorityMux has the right priority but no default argument. A utility with an explicit default and first-listed-wins priority needs no extra logic.

Hint 3

The condition list can be built during elaboration with Seq.tabulate, pairing req(i) with io.data(i). The list order decides who wins.

Concept

Selection utilities differ in their assumptions, not in their values on easy inputs. Mux1H requires exactly one true selector; other cases have unspecified results. Its current multi-input UInt implementation ORs the selected data, which does not implement priority. PriorityMux resolves overlaps but falls through to its last value when nothing is selected. MuxCase resolves overlaps by list order and returns an explicit default, which matches both rules here.

Extension experiment

Change the rule so the highest-numbered requester wins. Predict the five public rows, and identify which part of your code is the only part that has to change.