Re: [boost] Fixed point integer proposal

Subject: Re: [boost] Fixed point integer proposal
From: Ravi (lists_ravi_at_[hidden])
Date: 2009-07-01 09:20:44

On Thursday 25 June 2009 18:41:43 Christian Schladetsch wrote:
> In fact, fixed point is not faster on many architectures than floating
> point. One assumes that it is mainly PSP, DS and Gamecube guys that want
> fixed-point just for speed these days (I'm looking at you, Kenny ;).

My work requires fixed-point types for modeling cheap embedded microprocessors 
and other digital signal processing circuits. After using home-made code, code 
based on Williams' DDJ article and SystemC fixed-point types, I have finally 
settled on the AlgorithmicC datatypes from Mentor Graphics. A boost 
implementation would be extremely useful for DSP modeling.

For DSP modeling, N-bit x N-bit = N-bit will not suffice. Typically, hardware 
implementation in critical paths of DSP chips have code equivalent to the 
following:
  s[3:-3] x u[3:-4] = s[4:-3]
where s[4:-3] indicates an 9-bit signed fixed-point number with bits 
representing: sign, 2^4, 2^2, ..., 2^--3. In this case, the general-purpose 
fixed-point library does the following:
  8-bit signed * 8-bit unsigned = 16-bit signed -> 9-bit signed
where the 7 bits to be removed are found from the specification above.

Regards,
Ravi