which mask should I use for backdoor access in vr_ad? - specman

Question

I'm trying to understand the backdoor access feature in vr_ad.
Seems there are two different masks:
One is the backdoor_mask field, and the other – get_bd_mask().
What is the difference between the two?
which of them should I use?

Answer 1

When performing backdoor access, the vr_ad sequencer updates the shadow model with the value written/read to/from the DUT. (reminder – in front door access, it's the monitor's responsibility to call update() ).
The exact value of the shadow model after the access considers -
- Was this field accessed? (the register in the RTL was read/written)
- Is this field readable/writable?The two "masks" guide the way the shadow model is updated.
mask_backdoor :
This field controls whether the read and write masks of the register should be applied also in backdoor access. If mask_backdoor is FALSE, the read/write masks of the registers are ignored when updating the shadow model after a backdoor access.
This means, for example, we assume that Read Only fields can be written in backdoor access. If you want read/write masks to be considered in backdoor access (same as they are in front door access) - set mask_backdoor to TRUE. (default of this is FALSE. read/write masks are ignored)
get_bd_mask() :
The backdoor access is performed by the vr_ad, by accessing the path defined with set_backdoor_path() or set_field_backdoor_path().
When performing a backdoor write, the vr_ad writes to the RTL only the fields whose path was set (naturally…). So, when it goes to update the shadow model – it should know which fields were written and hence should be updated in the shadow model, and which – were not.
For example –assume this register definition
reg_def EX_R1 {
reg_fld fld_0 : uint(bits : 8);
reg_fld fld_1 : uint(bits : 8);
set_static_info() is also {
set_field_backdoor_path("fld_1" ,"top.ex_r1[7:0]");
};
};
When you issue backdoor write of this register – vr_ad will write to top.ex_r1[7:0]. Then it will update the shadow model, but it will have to update only fld_1, and not fld_0.
In this case, for this register, get_bd_mask() returns {0x0, 0xf} indicating that the first field is masked (0), not to be updated.
Typically, backdoor access is performed by the vr_ad, so you should do nothing about this method. But – if you override the definition of the *_reg_backdoor methods – you should also extend get_bd_mask(). If in your implementation you access all the fields – then this methods should return a list of 1's. Any bit you do not access – should be masked out.