"Draw it for me" is the fastest way for an interviewer to see whether you understand a mechanism. A good drawing is small, labelled, shows time running in one direction, and includes the corner case that makes the mechanism necessary. Draw each of these from memory in under three minutes before looking.
Can you draw the core mechanisms from memory, with the case that matters?
Draw clk, valid, ready and data for three transfers, with ready low for two cycles during the second.
Transfers only on cycles where valid and ready are both high; during the stall, valid stays high and data stays unchanged; valid rose without waiting for ready. Label which cycles transfer.
Initial credits equal to buffer slots; decrement per flit; increment per returned credit; the round trip from sending a flit to getting its credit back. Say: full throughput needs buffer ≥ rate × round trip.
Sequence numbers, the replay buffer holding unacknowledged packets, discard of out-of-sequence packets, cumulative ACK, and the replay timer for lost ACKs. Say the window limit: 2^n − 1 for go-back-N.
Draw ARVALID, ARREADY, the address, RVALID, RREADY, RDATA and RLAST for a 4-beat burst with one stall on R.
One address handshake for the whole burst; four data beats, each on an RVALID and RREADY cycle; the stalled beat held stable; RLAST on beat 4; the read ID on each beat if asked about multiple outstanding.
Draw four requesters, the grant sequence when requesters 0, 2 and 3 request continuously, and the priority pointer.
Grants 0, 2, 3, 0, 2, 3...; the pointer moving to just after the last winner; requester 1 skipped because it is not requesting; the bound: each requester waits at most three other grants.
Time in one direction (left to right for waveforms, top to bottom for sequences).
Label every signal and every arrow; number the cycles you talk about.
Include the corner case: the stall, the error, the wrap-around, the empty pool.
Say the number that sizes the mechanism while you draw.
Draw the five core mechanisms from memory: handshake, credits, replay, burst, arbitration.
Every drawing includes the corner case that makes the mechanism necessary.
State the sizing number (round trip, window, wait bound) while drawing.