Supporting external writes to registers

[rachitnigam]

Support adding @external to std_reg. However, I'm holding off the merge because it requires changing what register's encoding looks like which might make synthesis tools freak out. @andrewb1999 thoughts on this new encoding of registers?

Without this, $readmemh and $writememh do not actually work since they expect memories.

[andrewb1999]

This looks like it will work fine. I don't think there will be any difference after synthesis because essentially all you're doing is making the register explicit instead of using the system verilog feature that allows output logics to be promoted to registers.

[sampsyo]

Agreed; this hopefully should not affect what synthesis tools do. I guess I have learned never to trust synthesis tools to do anything, however, so it would of course be even nicer to check and see what Vivado does, for example… but that seems like a hassle and probably unnecessary here.

It might be nice to have a small test that uses an @external std_reg?

[andrewb1999]

Can we take another look at this soon? Would be a useful feature to have.

[rachitnigam]

Yeah, something is borked with the CI that needs to be fixed...

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