Remove N extension

[Timmmm]

This extension was never ratified, has been removed from the ISA manual and apparently would need significant changes if it were to ever come back.

Therefore it makes sense to remove it from the model in order to simplify the code.

This was written by @KotorinMinami in #531, I then made some minor formatting changes and some refactoring of the interrupt handling in this commit, and finally rebased it.

[github-actions]

Test Results

396 tests  ±0   396 ✅ ±0   0s ⏱️ ±0s
  4 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit eac9139. ± Comparison against base commit 2b65a0d.

♻️ This comment has been updated with latest results.

[Alasdair]

This was one of the two awkward extensions that made my modularisation attempt tricky, so I'm not too sad that it's gone

[Timmmm]

Yeah it's making writing CLIC a pain too, e.g. it's difficult to make a function get_xcause(p : Privilege) -> xlenbits due to the compilation order and I thought about making it a scattered function but I decided getting this merged would be better.

[jordancarlin]

I was looking at moving the CSRs to actually be scattered, and this was causing compilation order issues for that too. Will be great to get rid of it since it touches so many parts of the model.

[KotorinMinami]

@Timmmm There's absolutely nothing wrong with anything you've done. On the contrary, I'm quite pleased and grateful that you've managed to help resolve the conflicts when merge into master. Similarly, I do hope this PR can be merged and make a valuable contribution to our gold model.

[Timmmm]

@billmcspadden-riscv any objection to merging this?

[Timmmm]

I think we've got good agreement on this anyway and it will make Alasdair and Jordan's lives easier so I'll merge it on Thursday if nobody objects before then.

[billmcspadden-riscv]

LGTM.

However, I would not be surprised if this extension does not raise its head again.
It's been kicked around for at least 5 years and at one point, someone important swore that it would be ratified "real soon now".

[billmcspadden-riscv]

Looks like there are some conflicts that must be resolved. Bill Mc.

…

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