Systemverilog Interview Questions 3

1) What is the difference between mailbox and queue?

Answer:

A queue is a variable-size, ordered collection of homogeneous elements. A Queue is analogous to one dimensional unpacked array that grows and shrinks automatically. Queues can be used to model a last in, first out buffer or first in, first out buffer.

// Other data type as reference
// int q[]; dynamic array
// int q[5]; fixed array
// int q[string]; associate array
// include <
// List#(integer) List1;    //


int q[$] = { 2, 4, 8 };
int p[$];
int e, pos;
e = q[0]; // read the first (leftmost) item
e = q[$]; // read the last (rightmost) item
q[0] = e; // write the first item
p = q; // read and write entire queue (copy)

A mailbox is a communication mechanism that allows messages to be exchanged between processes. Data can be sent to a mailbox by one process and retrieved by another. 


2) What data structure you used to build scoreboard?


Answer:

    In SV, we use mailbox to get data from different modules and compare the result.

class Scoreboard;

mailbox drvr2sb;
mailbox rcvr2sb;

function new(mailbox drvr2sb,mailbox rcvr2sb);
  this.drvr2sb = drvr2sb;
  this.rcvr2sb = rcvr2sb;
endfunction:new


task start();
  packet pkt_rcv,pkt_exp;
  forever
  begin
    rcvr2sb.get(pkt_rcv);
    $display(" %0d : Scorebooard : Scoreboard received a packet from receiver ",$time);
    drvr2sb.get(pkt_exp);
    if(pkt_rcv.compare(pkt_exp))
    $display(" %0d : Scoreboardd :Packet Matched ",$time);
    else
      $root.error++;
  end
endtask : start

endclass


In VMM, we use channels to connect all the modules and compare the result.


class Scoreboard extends vmm_xactor;

   Packet_channel   drvr2sb_chan;
   Packet_channel   rcvr2sb_chan;


function new(string inst = "class",
             int unsigned stream_id = -1,
             Packet_channel   drvr2sb_chan = null,
             Packet_channel   rcvr2sb_chan = null);

      super.new("sb",inst,stream_id);
   
      if(drvr2sb_chan == null)
           `vmm_fatal(this.log,"drvr2sb_channel is not constructed");
      else
           this.drvr2sb_chan = drvr2sb_chan;
     
      if(rcvr2sb_chan == null)
           `vmm_fatal(this.log,"rcvr2sb_channel is not constructed");
      else
           this.rcvr2sb_chan = rcvr2sb_chan;
     
      `vmm_note(log,"Scoreboard created ");

endfunction:new


task main();
  Packet pkt_rcv,pkt_exp;
  string msg;
  super.main();
  forever
  begin
    rcvr2sb_chan.get(pkt_rcv);
    $display(" %0d : Scoreboard : Scoreboard received a packet from receiver ",$time);
    drvr2sb_chan.get(pkt_exp);
    if(pkt_rcv.compare(pkt_exp,msg))
    $display(" %0d : Scoreboard :Packet Matched ",$time);
    else
    `vmm_error(this.log,$psprintf(" Packet MissMatched \n %s ",msg));
  end
endtask : main

endclass


3) What are the advantages of linkedlist over the queue ?
   
 Answer:


 Queue has a certain order. It's hard to insert the data within the queue. But Linkedlist can easily insert the data in any location.


4) What is the use of $cast?


Using Casting one can assign values to variables that might not ordinarily be valid because of differing data type. SystemVerilog adds 2 types of casting. Static casting and dynamic casting.


e.g.  i = int '(10.0-0.1); // static cast convert real to integer


// Dynamic casting
function int $cast( singular dest_var, singular source_exp );
or
task $cast( singular dest_var, singular source_exp );


e.g. $cast( col, 2 + 3 );