[libre-riscv-dev] [isa-dev] Re: FP transcendentals (trigonometry, root/exp/log) proposal

lkcl luke.leighton at gmail.com
Sat Aug 10 17:03:01 BST 2019

On Friday, August 9, 2019 at 7:12:00 PM UTC+1, Allen Baum wrote:

Well, one way to get at the underlying HW characteristics is an ECALL to a 
> higher priv mode; That will take a little longer, but I don't see why it 
> would be used more than once during the run of an applications, so that 
> should be OK.

i think samuel pointed out that this is (sorely) needed: there was 
something where, at the moment, you have to get at some information right 
at boot time then "hope like hell it never gets changed".  providing a 
mechanism for user-space to call up to higher priv levels would fix that.

> But a similar issue has come up in several forums, and I've proposed a 
> standard discovery mechanism  that will enable all kinds discovery using a 
> CSR interface.


> Writing the CSR initializes a pointer into a discovery data structure, and 
> reading it will return the contents of the structure at that offset (with 
> optional autoincrement)
>  The data structure is a linked list of capabilities (ID, offset of next 
> capability) where the actual capability is implementation dependent (on the 
> ID).
> So, a config string ID, debug capability ID, ISA capability ID can be 
> independently defined to be YAML, binary or ascii formats, as desired.


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