Generating unique addresses for a write sequence

[ssingh]

While generating constrained random data, how can we ensure that a given value is not generated more than once? I want to generate unique addresses for my write sequence to memory.

[whiteriver]

Try reading about randc in systemverilog documentation.

I never had to use, but I guess you could use something like:

<inside sequence class>

randc bit [7:0] address;

virtual task body();

<loop>

void'(this.randomize());

`uvm_do_with(req, {req.addr == address;})

</loop>

...

the randc generates a random value sequence without repeating any value. After all possible values are met, it generates a new random sequence.

Edited October 10, 2012 by whiteriver

[ssingh]

1. randc has a limitation of 16 bits.

2. randc does not generate same data in one cyclic iteration between lower bound and upper bound of the address. But if the simulator leaps by big margin and finishes one cycle, it can generate the same value in next iteration.

[whiteriver]

to the limitation 1: then, what about using the randc as a seed to each value generated? it would be something like:

<inside sequence class>

randc bit [15:0] addr_seed;

bit [31:0] addr_to_send;

virtual task body();

<loop>

void'(this.randomize());

addr_to_send = $random(addr_seed);

`uvm_do_with(req, {req.addr == addr_to_send;})

</loop>

...

it will generate 32-bit values, the only implication is that it is limited to 2^16 possible values. Which brings to your second statement. I don't know how to solve it. But you can hack and create a sequence of seeds:

int seeds = 0;

<outer repeat loop>

<inner repeat loop 2^16 -1 times>

addr_to_send = $random((addr_seed<<16*seeds));

`uvm_do_with...

</inner repeat>

seeds+1;

</outer repeat>

which then has the limitation of 2^17 possible values (2x outer loop).

[ssingh]

I don't want to be limited to 2^16 possible values. I got one more possible solution on another forum:

You can have a queue of addresses that you have written to, and have a constraint

rand unint64 address;

uint64 list[$];

constraint not_in_list {!(address inside {list};}

function void post_randomize()

list.push_back(address);

endfunction

Thank you for your suggestion.

[whiteriver]

that is a great approach. I will add this to my notes. OTOH, the bigger the list, more time the simulator takes to resolve the constraint.