Hardware interview practice
Verify a tagged grid-path counter
Verify an accelerator with up to four tagged, outstanding requests that counts right-or-down paths through a rectangular grid and may return responses out of order.
Starting point
Question code
interface paths_if(input logic clk);
logic rst_n, req_valid, req_ready;
logic [3:0] req_id;
logic [4:0] rows, cols;
logic rsp_valid, rsp_ready;
logic [3:0] rsp_id;
logic rsp_error;
logic [63:0] path_count;
endinterfaceReviewed example
Work through one case
Input
accept id2 grid=2x3 and id7 grid=3x3; responses return id7 then id2Expected output
id7 path_count=6 PASS; id2 path_count=3 PASSThe 64-bit dynamic-programming oracle is keyed by request ID, so reversed response order remains legal.
What to cover
Requirements
- For dimensions 1 through 16, predict in 64 bits with one on the top and left edges and upper-plus-left in every interior cell.
- Track at most four unique outstanding req_id values and compare each response by ID rather than acceptance order.
- Unknown, repeated, or stale responses fail; invalid dimensions return an error and zero path count.
- Bound response latency, hold stalled responses stable, cancel all outstanding work on reset, and delay old-ID reuse long enough to expose late replies.