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asic.fyi

Hardware interview practice

Predict and safely read a build-tag prefix service

MediumHardware-Software IntegrationSystemVerilog

Write the reference predictor and safe firmware transaction sequence for a memory-mapped I/O (MMIO) service that returns the longest common byte prefix of up to eight bounded build tags.

Starting point

Question code

typedef enum { LCP_OK, LCP_BAD_ARG, LCP_HW_ERROR, LCP_TIMEOUT } lcp_status_t;
lcp_status_t lcp_run(const char tag[8][16], const uint8_t len[8],
                     uint8_t count, char out[17], uint8_t *out_len);
// TAG_SEL, TAG_DATA, TAG_LEN[0:7], COUNT
// CMD{START,ABORT}, STATUS{BUSY,DONE,ERR}, RESULT_LEN, RESULT_DATA
Reviewed example

Work through one case

Input
tags=["asic-fyi","asic-flow","asic"], lengths=[8,9,4]
Expected output
RESULT_LEN=4; firmware output="asic\0"

All three byte strings match through index 3 and the shortest tag ends there; only firmware appends the terminator.

What to cover

Requirements

  1. Validate pointers, count 1 through 8, and active lengths 0 through 16 before any register access.
  2. Write each selected tag and length in order; START must snapshot the programmed inputs.
  3. Read exactly RESULT_LEN bytes, reject a result length above 16, and append one firmware-only terminator.
  4. Give ERR priority over DONE and preserve both caller outputs after hardware error, timeout, abort, or reset.
Continue practicing
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