[libre-riscv-dev] [Bug 325] create POWER9 TRAP pipeline

[bugzilla-daemon@libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=325

--- Comment #77 from Cole Poirier <colepoirier@gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #73)
> will explain tomorrow when it is not 4am.

You are very bad at taking your own advice :) Thank you for explaining... the
entire process I should follow to gain MUCH greater understanding of the SOC
and how it works... all at 4 am!!

> briefyly: every Record has a dictionary called fields.  by walking this
> dictionary we can cheat and copy *only* a subset of fields... as long as the
> names all match up.
> 
> see eq_from_execute1 and grep source code for where it is used.
> 
> 
> > One difference between OP_TRAP and OP_SC is that the end of the OP_TRAP
> > function is:
> > 
> > ```
> > # take a copy of the current PC in SRR0
> > comb += srr0_o.data.eq(cia_i)   # old PC
> > comb += srr0_o.ok.eq(1)
> > ```
> > While the end of the OP_SC function is:
> > 
> > ```
> > # and store the (next-after-return) PC in SRR0
> > comb += srr0_o.data.eq(cia_i+4) # addr to begin from on return
> > comb += srr0_o.ok.eq(1)
> > ```
> > 
> > So should the function be:
> > 
> > ```
> > def trap(self, m, addr, nia_o, srr0_o, trap_addr):
> >     comb = m.d.comb
> >     comb += nia_o.data.eq(trap_addr)
> >     comb += nia_o.ok.eq(1)
> > 
> >     comb += srr0_o.data.eq(addr) # addr to begin from on return
> >     comb += srr0_o.ok.eq(1)
> 
> yes! well done.  ok i would cut out srr0_o from those args and just redo
> them inside the fn.

Thanks! Will do.

> so now also cookie cut srr0_0 and nia_o from lines 89 and 90 "convenience"
> variables.  see how that works?
> 
> so OP_SC should call self.trap(0xc00, cia_i+4) and OP_TRAP should be obvious.
> 
> 
> > ```
> > 
> > The below is where my shakiness on the CompXOpSubset and the decoder starts
> > to confuse me. If you can briefly explain that to me here, I can use this
> > when doing the documentation after the tape-out deadline.
> 
> tomorrow.
> 
> late.

Hehe :)

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