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Going round in circles, quickly

Posted by Steve Revill Sat, 10 Jul 2021 11:15:00 GMT

There are a few areas where the potential for speed-ups are huge yet to date RISC OS has been unable to harness them. One such area is where the more complicated floating point mathematical operations are used, like sine and cosine, rather than simple add/subtract/multiply/divide – but that has all changed in our nightly RISC OS 5 ROMs from July 2021 onwards.

Origins

The Arm instruction set design includes a chunk of spare space to communicate with coprocessors. Coprocessors are add-on areas of the silicon chip which handle whatever specialism those spare instructions are defined to do. One such add-on is a maths accelerator.

The original math accelerator was unimaginatively called the Floating Point Accelerator, or FPA for short. Even though the last time an FPA was available on chip was the 7500FE used in the A7000+ back in 1997, RISC OS still faithfully supports those instructions through emulation with the FPEmulator module. By modern standards, this can be quite slow, particularly for the more complex mathematical operations.

In steps the Vector Floating Point unit, or VFP for short. Introduced in 1998 the VFP replaced FPA entirely, bringing with is a larger bank of working registers, but VFP wasn’t instruction compatible with FPA. For those platforms sporting Vector Floating Point units, which is now a vast majority, the potential for high speed computations using the VFP unit is great.

Support in a roundabout way

RISC OS now supports a set of 11 hardware-accelerated functions:

• sine and arcsine
• cosine and arcsine
• tangent, arctangent and atan2 (a variant of artangent)
• logarithm base e and base 10
• exponent
• power

A bezier curve uses several of these complex mathematical functions

The VFPSupport module in RISC OS 5 is responsible for looking after the VFP coprocessor, so was the natural home for these new functions. By having them available centrally it’s possible to use any of them from any language that might offer them, or even directly from an application. For example, BASIC provides the keywords SIN and COS which map directly to sine and cosine:

10 start%=TIME
20 FOR rad = 0 TO 2*PI STEP 0.000001
30   x = COS(rad) : y = SIN(rad)
40 NEXT
50 PRINT TIME-start%;" centiseconds"

This works out over 6 million coordinates of a unit circle, so it should be possible to see the difference in speed when run on integer BASIC V and floating point BASIC VI. Try it!

And to round off, a word about making it happen

We’d like to make a special mention of RISC OS FR who were a key sponsor and tester for this development work. There’s a big resource of BASIC-related material on their site, and of course there’s still time to bag a copy of the BASIC-only DDE if you’d like to explore programming in BASIC at no cost.

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