UVM RAL: Randomizing registers in a register model

Question

I have a large register map modelled with RAL and I would like to randomize some of the registers. If I want to constrain the registers seperately then this is simple:

reg_model.register_a.randomize() with {value > 5;}
reg_model.register_b.randomize() with {value < 2;}
reg_model.update(status);

However, if I want a relationship between the two values written I think I have to add a constraint to the whole register model:

reg_model.randomize() with {register_a.value > register_b.value;}
reg_model.register_a.update(status);
reg_model.register_b.update(status);

The problem here is that the other 254 registers in the model will also get randomized. I could just update the two registers that I want randomized, but then the mirror will not match the hardware. If I had backdoor access working I could refresh the mirror, but I don't and I certainly don't want to read back 254 registers through the front door.

Is there a way to randomize just those two registers yet still have the constraint solver maintain a relationship between them?

Answer 1

You could do

reg_model.randomize(register_a,register_b) with {register_a.value > register_b.value;}

Then only registers a and b will get randomized.

Answer 2

This can't possibly work because registers don't have a "value" variable, only fields have the "value" member variable.

This should work:

reg_model.randomize(register_a,register_b) with {register_a.get() > register_b.get();}

Answer 3

Here my two cents, Here all registers and all fields are constrained to zero.

top_regm.get_registers(regs, UVM_HIER);      //NOW take ALL registers (regs) inside the block
regs.sort with (item.get_address());   //sort ascending by address         
for (int unsigned r=0; r<regs.size(); r++) begin                    
    assert(regs[r].randomize with {    
         regs[r].get() == 0;     //constraint with function is  heavy consumming               
    } );
    reg_value_q.push_back(regs[r].get()); //store all the constrained values in a queue
end

Answer 4

I don't think

reg_model.randomize(register_a,register_b) with {register_a.get() > register_b.get();}

will work, becasue regmodel.randomize will only randomize the class member 'value' whose value will be copied to m_desired only in post_randomization.

If you want to randomize whole register, I believe you should randomize two intermediate variabls, then pass the variabls into regmodel.

int a_value,b_value;
std::randomize(a_value,b_value) with {a_value > b_value};
regmodel.register_a.set(a_value);
regmodel.register_b.set(b_value);
regmodel.register_a.update(status);
regmodel.register_b.update(status);