MMACs

Fixed point (part two)

Fixed point.

Please read this article before you read the below content…

However , Fixed point arithmetic is popular in embedded system or the high performance system but low power consumption (system that desiged with the
limitation in budget).The primary advantage of fixed point system is “in fixed point , the calculation process of rational number is done in the same way as
calculation process of integer number”. So , it has advantage over floating point in that it require only Arithmetic Logic Unit or ALU (ALU is only the
unit that exist in almost CPU in the world) in its computation process. On the contrary , floating point arithmetic require both ALU and FPU (FPU is hardware
processing unit that used for handle the floating point calculation process). Please look at this page for more information about FPU Design.

You may find out some digital signal processing (DSP) chips don’t have the FPU part , that’s not strange because fixed point can be used in DSP tasks too.
The DSP chip without FPU make its cost cheaper, furthermore it consume less energy when compare to DSP chip with FPU.
Blackfin of Analog Devices or TMS320C5x series of Texas Instruments is an example of Fixed-point DSP chip.

Due to the operation speed of Fixed point is faster than floating point, so in some kind of tasks that need to complete calculation process
within the limit of time, in this case Fixed point DSP processor is more applicable than floating point DSP processor.

The efficiency of DSP processor can be measured by “the capability in multiply and accumulate per second”.The official technical word call
it “Millions of Multiply Accumulates (MMAC) per second” or “MMAC per second” or MMACs.
The MMACs value of fixed point DSP processor is higher than floating point DSP processor.
You can easily compare DSP chip from Texus Instruments in these two series , TMS320C67x (Floating point processor)
and TMS320C64x (Fixed point processor) , please click the link under the chip series and you will know that the peak MMACs of TMS320C64x series
is much higher than TMS320C67x.

So , I can summarize that high performance DSP chip mostly use Fixed point arithmetic. Task that suitable for using high MMACs Fixed-point DSP chip
is the task that include heavily mathematical calculation such as Audio Coding or Graphic Engine in video games box etc…

The next article is the last article relate to fixed point. It will about Q-format.

Thank you,
Jitkasem

Reference : Semiconductor Electronics Magazine , Thailand (September/2553 issue).