ARM Cortex R4F Manuel d'utilisateur télécharger pdf (Page 31)

SSAT: Saturate Instruction

• Consider two numbers 0xFFFF FFFE and 0×0000 0002. A 32-bit mathematical

addition would result in 0×1 0000 0001 which contain 9 hex digits or 33 binary

bits. If the same arithmetic is done in a 32 bit processor ideally the carry flag will

be set and the result in the register will be 0×0000 0001.

• If the operation was done by any comparison instruction this would not cause any

harm but during any addition operation this may lead to unpredictable results if the

code is not designed to handle such operations. Saturate arithmetic says that when

the result crosses the extreme limit the value should be maintained at the respective

maximum/minimum (in our case result will be maintained at 0xFFFF FFFF which

is the largest 32-bit number).

• Saturate instructions are very useful in implementing certain DSP algorithms like

audio processing where we have a cutoff high in the amplitude. For instance the

highest amplitude is expressed by a 32-bit value and if my audio filter gives an

output more than this I need not programatically monitor the result. Rather the

value automatically saturates to the max limit.

• Also a new flag field called ‘Q’ has been added to the ARM processor to show us

if there had been any such saturation taken place or the natural result itself was the

maximum

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ARM Cortex R4F Manuel d'utilisateur Page 31