Freescale Semiconductor StarCore SC140 - Manual (Page 49)

DALU
SC140 DSP Core Reference Manual 2-17
The following table (Table 2-11) shows the arithmetic saturation and rounding operations for the four
possible cases of scaling, no scaling, and arithmetic saturation mode on/off.
2.2.2 DALU Arithmetic and Rounding
The following paragraphs describe the DALU data representation, rounding modes, and arithmetic
methods.
2.2.2.1 Data Representation
The SC140 core uses either a fractional or integer two’s complement data representation for all DALU
operations. The main difference between fractional and integer representations is the location of the
decimal (or binary) point. For fractional arithmetic, the decimal (or binary) point is always located
immediately to the right of the most significant bit of the high portion. For integer values, it is always
located immediately to the right of the least significant bit (LSB) of the value. Figure 2-3 shows the
location of the decimal point (binary point) bit weighting and operand alignment for different fractional
and integer representations supported on the SC140 architecture.
Table 2-11. Saturation and Rounding Interactions
Scaling
Selected
Arithmetic
Saturation
Mode
Arithmetic Saturation
Rounding
Saturable DALU
Instructions
Special Six Instructions
None Off None None Rounding with no scaling
Up/down Off None None Rounding with scaling
considered
None On Saturation can occur None Rounding with no scaling
Up/down On Saturation can occur, no
scaling considered
None Rounding with no scaling