DescriptionQ795
Q795ArchNVIDIAASIC interview problem
Check per-channel link credits
TechniquesArchMicroarchitectureInterconnectCheckcredits
DifficultyHard
TopicComputer Architecture
LanguageSystemVerilog
Requirements4 checkpoints
01
Problem
A two-channel link starts with four transmit credits per channel. Sending consumes one credit; a returned credit restores one. Write a SystemVerilog checker for credit accounting and protocol errors.
Starting declarationSystemVerilog
input logic clk, rst_n;
input logic tx_fire, tx_ch;
input logic cr_valid, cr_ch;
output logic error;Example input and output
Use this case to check your interpretationInput
Channel 1 has two credits and observes tx_fire and cr_valid for channel 1 on the same edge.Output
Channel 1 remains at two credits and error remains clear.Explanation
At a non-boundary count, the legal send consumes exactly the credit restored by the same-channel return.
02
Requirements (4)
- Track an integer available[2], initialized to four for each channel on reset and constrained to 0..4.
- A tx_fire consumes one from tx_ch; if its pre-edge count is zero, leave the count unchanged and set sticky error.
- A return without a same-channel send adds one; if its pre-edge count is four, leave that count unchanged and set sticky error.
- For a same-channel send and return: counts 1..3 stay unchanged; at 0 flag the send and apply the return to reach 1; at 4 flag the return and apply the send to reach 3.