[J-core] Pondering j-core kickstarter ideas.

Rob Landley rob at landley.net
Thu Oct 26 21:16:53 EDT 2017

Oops, I never hit "send" on this one. :)

On 10/19/2017 04:24 PM, Ken Phillis Jr wrote:
> On Thu, Oct 19, 2017 at 11:02 AM, mike maloney <mikerm610 at gmail.com> wrote:
>> What about porting or extending something similar to
>> http://belogic.com/uzebox/index.asp
>> It's open source console running on a microcontroller. The j2 should be able
>> to handle it. Though an extended library to support 3d on the j3+ should be
>> doable. Just a thought.
>> Mike
> I stated this before, but the J2 Core is capable of handling 3D Math.
> The trick is to use Fixed Point Arithmetic[1] to perform calculations.
> This is a common method for providing a limited form of decimal math
> on systems lacking an hardware Floating Point Unit. Fixed Point
> arithmetic is what allowed the original Dos version of Id Software's
> Doom game to be playable on Intel 386 and Intel 486sx processors.

We're using fixed point all over the place in our GPS implementation,
that's how it keeps up with realtime signal processing on a 32 bit
single issue processor running at less than 100 mhz.

> Step 1: Add Hardware Acceleration for Fixed Point Calculations
> For the Most part, this is adding instructions for the various
> calculations found in Libfixmath [6]. The accelerated functions an be
> done as a Co-Processor, but in general this is making modifications to
> the Integer ALU.

What modifications? You follow multiplying by a shift to move the
decimal point back (or just adjust your expected representation).
Presumably you'd shift before dividing, but it didn't come up in the
code I was looking at.

I'm not against adding VHDL and porting libraries, but the stuff we
implemented so far was all driven by existing use cases, not
infrastructure in search of a user...


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