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2-6 Optimizing DSP56300/DSP56600 Applications MOTOROLA

Data Operations

16-bit Arithmetic Mode (DSP56300 Only)

2.2 16-BIT ARITHMETIC MODE (DSP56300 ONLY)

The 16-bit Arithmetic mode causes the Data ALU to use only 16 bits

of the 24-bit data in transfers and calculations, allowing use of the

DSP56300 as a 16-bit data processor. The 16-bit data is right aligned

in the memory, but left aligned in data registers (in order to comply

with the fractional numerical representation convention). The

hardware makes the proper alignments and shifts in data transfers

and operations, so the user does not have any overhead when using

this mode. This includes the accumulators, which in the 16-bit mode

are 40-bits wide (in accumulator A, for example, A0 and A1 are 16

bits each, and the extension A2 remains 8 bits wide). All data

operations are properly defined to give correct 16-bit arithmetic

results. For more information on the 16-bit Arithmetic mode, see

Section 3.4 in the

DSP56300 Family Manual

for a general

description, and Appendix A in the same manual (Instruction set)

for a detailed description on the functionality of each instruction

affected by this mode.

Using the 16-bit Arithmetic mode may give many advantages from

a general system point of view:

• Ability to implement a 16-bit exact algorithms. The DSP56300

also supports saturation arithmetic and flexible rounding

modes required by some standards.

• 16-bit exact algorithms exact algorithms could be integrated

easily into a software system that also includes 24-bit exact

routines. Changing of the arithmetic mode can be done “on

the fly”.

The 16-bit Arithmetic mode is activated by setting the SA bit in the

Status Register (SR).

Note: This is not the same as the 16-bit Compatibility mode

(activated by setting the SC bit in the Status Register). The

16-bit Compatibility mode affects address registers and

address calculations and enables object code compatibility

with the DSP56000 family (which uses 16-bit wide

addresses).

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Motorola DSP56305 Manuale Utente Pagina 21