FP DSP

Floating Point (FP) DSPs  
[updated nov2015]
The goal of this section is to understand the relevance of FP DSP to the world of DSP today.

Background

[see edward Lee SPmag circa 1990 tutorial for more ]
The first FP DSPs were ATT and OKI (1983),  ADI Sharc was the first successful, but not as much as the TI hype ("I've seen the future and it is floating point" ) introduced in 1987. Anyway by 1991 [ref. 1] each of the 4 DSP stars had a FP architecture:

• TI C3x then C4x
• ADI Sharc
• Moto DSP96000
• ATT DSP32C

That was about the time that DSP people started questioning the wisdom of "old" DSP architectures as opposed to "modern RISC". For instance, the Intel i860 was starting to cut them in pieces in many markets [ref 2,3].

• TBD: We will not go here in details, but the argument was largely biased because DSPs were primarily SOC whereas  the i860 was just .. well it was the i860. Try comparing a Bugatti Veyron with a Porsche Cayenne. And Intel with ADI..

Going back to the DSP mainstream story, the FP DSPs took 4% of the GP DSP market and 20 years later it was still the same. They look largely irrelevant, so why bother?

• Remembering all these years, it is funny to consider the time we spent explaining to people that FP DSPs were not going to replace FXP DSPs. 

Features and recent evolution

Indeed, why bother with FP DSPs?
Firstly they were the first DSP to be architectured in a "modern way".

• For instance the C40 had a register file and emphasis was put on the C compiler. 
• Then its successor, the C67x was VLIW
• which is an extreme CPU technique.. superscalar without safety net... 
• The same thing happened with ADI Sharc (the only competition to TI). The Sharc was turned into a modern CPU (Tiger Sharc). Mind you, they did not go overboard. It was a static superscalar not a VLIW.

Secondly, TI introduced DSP which both execute natively FP and integer.

• The evolution of the low cost C28xx with the C283xx.
• The evolution of the C67x workhorse into the C674x family
• Most interesting is the most advanced DSP core, the C66 which is both natively FP and integer 32 (and 64 bit). It seems to be quite a major statement, that the only "relevant" DSP family has chosen to go the FP way. As described by Gene Frantz [ref. 6], the main reason is matrix computation. which is another way to spell Matlab.

The old arguments turned upside down
The  argument against FP was that the complexity was not worth it. Nowadays, with the matrix problem, this is the other way round. If you use FXP, you must work at least in 32-bit (and you loose cost advantage of data size) , develop much longer algorithm (so you loose the code size advantage and worse the power consumption). All together the system price of a FP datapath is less than a FXP one.
Especially, in CPU architecture a FP unit is just added next to the IP unit so it is easy to figure out the cost.
(Well.. the exception model might suffer a bit too..)

Future of FP DSPs

The future of FP DSPs is not bright. All existing architectures (C67, Sharc) survive in their existing application space (military or audio) but have no serious roadmap. The only recent introductions such as C673x and C66x are the result of convergence more than FP evolution.
Also the C66 is built like a CPU, most striking features come from high end CPU (level 3 interconnect and caches) with a few dsp features left from the past. It stands a good chance in infrastructure (given the poor competition..PPC, Intel) but has no future as the next core for OMAP 7.  One day will come for TI to decide if the C66x support is worth the bother or if A9, A15, A21 or A333 is the best bet.

Lessons for Coprocessing

On the other hand there is no doubt that FP has a good future in Cops and AS-DSPs.

1. Designers have full freedom and can draw from the rich past of FP DSPs. For instance, on the choice of data size, they can use 16+6 (as the first FP DSP from OKI) or 80-bit a la Intel.
2. Matlab Mapping becomes unidimensional.
3. But not all problems can be solved. 
1. FP (single) is not enough. That at least, we've heard it from the audio claque (because linarity is 23-bit).
2. More seriously, we primarily live in digital world (not a numeric world). The numeric values (traditional DSP) are overwhelmed by the packeting and bit stuffing. Take any audio or video codecs.   

References

1. Ray Weiss "32-bit FP DSP processors" , EDN Nov 7 1991
2. Steve Paavola " GP processors target FP DSP" www.edn.mag April 1, 1999
3. Plenty of similar articles in RTC magazine circa 2000 , e.g. from Spectrum Signal Processing
4. The EDN DSP Directories (before it became a joke), had good 1 page description of the 4 FP "old" DSPs (ex: june9,1994) 
5. And so has the BDTI bible version 1995.
6. Gene Frantz White Paper " Where will Floating Point take us?" http://www.ti.com/lit/wp/spry145/spry145.pdf, oct 2010
1. this is the latest on Gene's white papers on FXP vs FP. See also::
2.  Jim Larimer, Daniel Chen "Fixed or floating? ..." EDN 1995