http://lists.openrisc.net/pipermail/openrisc/2011-December/000533.html where I first propose changing the ISA as well as the implementation.
---Matthew Hicks On Tue, Mar 20, 2012 at 2:48 PM, Stefan Kristiansson < [email protected]> wrote: > On 03/20/2012 09:14 PM, Matthew Hicks wrote: > > > > On Tue, Mar 20, 2012 at 12:52 PM, Jonas Bonn <[email protected]> wrote: > >> >> On Tue, 2012-03-20 at 19:43 +0200, Stefan Kristiansson wrote: >> > On 03/20/2012 03:50 PM, Jonas Bonn wrote: >> > > >> > > I don't agree... in the same way that you don't have to respect any >> ABI >> > > as long as you aren't calling into "external" code, you are free to >> > > fiddle with r0 as much as you want. "r0 contains 0" is ABI, it is >> not a >> > > hardware detail. A bit weird, I agree, but not fundamentally >> incorrect. >> > > >> > > The real problem is that r0 isn't _necessarily_ writable... so there's >> > > no guarantee that you can actually run a program that changes r0 on an >> > > arbitrary implementation. >> > > >> > > >> > >> > No, the real problem is that the specification is not clear about >> > whether it should be possible to do a write to r0 without any side >> effects >> > (regardless if the value actually will be written or not to r0). >> > >> > Like it is now it's freely up to the reader to do an implementation >> > that locks up the cpu when someone tries to write 0 to r0. >> > Although it perhaps wouldn't be a very good implementation ;) >> > >> >> Yeah, fair enough. I guess the ambiguity kills whatever advantage the >> flexibility may give you (dubious though it may be). >> >> R Diez's text is fine... might be even better to go a step further and >> be explicit about the requirements/limitations of r0 (i.e. writing to r0 >> is a no-op; r0 is not guaranteed to contain 0 at startup; etc...) >> > > We had a discussion about this in December 2011. I submitted a patch > that would basically treat any write to r0 as if the value to be written is > 0. Most felt that changing the ISA would be an unacceptable ABI break and > the patch was not accepted. > > > > That's a truth with modification, isn't it? > You proposed changing an implementation (or1200) not the ISA, > and most felt it wasn't worth it since it could as well (and should) be > handled in software. > I don't think there was any discussion about unacceptable ABI breaks and > changing ISAs. > > Stefan > >
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