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SoftFPU Or No SoftFPU?

Vojin Vidanovic

Posts 770
19 Jun 2017 11:32


Dear Gunnar and team,

What I dont undetstand from "advanced 080 FPU quest" is will :

a)6881 compatibility be abandoned and only new FPU with optimized software will be used
(like AMMX instr. unique to 080 in m68k world)

b)we will softemulate needed 881 first by CPU, later by that advanced FPU. I have seen some softFPU results EXTERNAL LINK and while they are bad, can we at least have such tool at courtesy running the FPU only software?

c) Both "new FPU" and compatibility to 8882 will be maintained, but after SAGA over DIGITAL-VIDEO, Pamela and few other features are planned?

If c) is goal can we have softFPU b) as "stopgap" solution?

A roadmap of what you are doing/planning for next two core releases or so, would be anyway most welcome

FAQ states a new, modified line, which states fully compatibile 060 FPU as goal (in past it said not all FPU instructions are to be included):
"Do Vampires have a FPU ?
    A: The implemented FPU will be a 68060 compatible FPU. FPU is currently disabled and is a WIP."



Gunnar von Boehn
(Apollo Team Member)
Posts 6207
19 Jun 2017 11:34


Vojin Vidanovic wrote:

Dear Gunnar and team,
   
What I dont undestand from "advanced 080 FPU quest" is will :
   
a)6881 compatibility be abandoned and only new FPU with optimized software will be used
(like AMMX instr. unique to 080 in m68k world)

   
No no, this was never said.
This is also not the goal.
 
This is _NOT_ about NEW instructions.
The whole topic is about using the old, normal 68K FPU instructions in a "modern way" of writing code.
"Modern" means in this regards how the whole world writes software since the last 20 years.
 
The old existing 68K FPUs simple have a _very_ old design.
They are not Pipelined.
They are also not Super-Scalar.
Therefore all existing old software is not scheduled for a pipelined, super scalar design.
 
Please understand that this has nothing to do with SIMD or SSE.
We talk here about starting to write FPU code like x86 or PPC software is written since the 90th.
This means ORDERING the FPU instructions in state of the art fashion. Its not about using new instructions.


Vojin Vidanovic

Posts 770
19 Jun 2017 20:54


Thank you for the clarification.
So, like SAGA features, it will be:

a) Fully 882 compatibile
b) and way faster and advanced, if code is "new FPU" optimized

So essentially, once core enables FPU, all the "Vampire has no penis" (FPU) arguments, fall down :-)

posts 3