ADSP-BF53x/BF56x Blackfin Processor Programming Reference
2-23
Computational Units
The
RND_MOD
bit in the
ASTAT
register enables biased rounding. When the
RND_MOD
bit is cleared, the
RND
option in multiplier instructions uses the
normal, unbiased rounding operation, as discussed in
“Unbiased Round-
ing” on page 2-20
.
When the
RND_MOD
bit is set (=1), the processor uses biased rounding
instead of unbiased rounding. When operating in biased rounding mode,
all rounding operations with
A0.L/A1.L
set to 0x8000 round up, rather
than only rounding odd values up. For an example of biased rounding, see
Table 2-7
.
Biased rounding affects the result only when the
A0.L/A1.L
register con-
tains 0x8000; all other rounding operations work normally. This mode
allows more efficient implementation of bit specified algorithms that use
biased rounding (for example, the Global System for Mobile Communica-
tions (GSM) speech compression routines).
Truncation
Another common way to reduce the significant bits representing a number
is to simply mask off the N – M lower bits. This process is known as trun-
cation and results in a relatively large bias. Instructions that do not
support rounding revert to truncation. The
RND_MOD
bit in
ASTAT
has no
effect on truncation.
Table 2-7. Biased Rounding in Multiplier Operation
A0/A1 Before RND
Biased RND Result
Unbiased RND Result
0x00 0000 8000
0x00 0001 8000
0x00 0000 0000
0x00 0001 8000
0x00 0002 0000
0x00 0002 0000
0x00 0000 8001 0x00 0001 0001
0x00 0001 0001
0x00 0001 8001
0x00 0002 0001
0x00 0002 0001
0x00 0000 7FFF 0x00 0000 FFFF
0x00 0000 FFFF
0x00 0001 7FFF
0x00 0001 FFFF
0x00 0001 FFFF
Содержание ADSP-BF53x Blackfin
Страница 38: ...Conventions xxxviii ADSP BF53x BF56x Blackfin Processor Programming Reference...
Страница 134: ...System Reset and Powerup 3 18 ADSP BF53x BF56x Blackfin Processor Programming Reference...
Страница 324: ...Instruction Overview 7 20 ADSP BF53x BF56x Blackfin Processor Programming Reference...
Страница 486: ...Instruction Overview 13 28 ADSP BF53x BF56x Blackfin Processor Programming Reference...
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Страница 688: ...Instruction Overview 18 48 ADSP BF53x BF56x Blackfin Processor Programming Reference...
Страница 742: ...Instruction Overview 19 54 ADSP BF53x BF56x Blackfin Processor Programming Reference...
Страница 752: ...Examples 20 10 ADSP BF53x BF56x Blackfin Processor Programming Reference...
Страница 780: ...Product Identification Register 21 28 ADSP BF53x BF56x Blackfin Processor Programming Reference...
Страница 790: ...ADSP BF535 Flags A 10 ADSP BF53x BF56x Blackfin Processor Programming Reference...
Страница 800: ...Performance Monitor Registers B 10 ADSP BF53x BF56x Blackfin Processor Programming Reference...
Страница 994: ...Instructions Listed By Operation Code C 194 ADSP BF53x BF56x Blackfin Processor Programming Reference...
Страница 1042: ...Index I 40 ADSP BF53x BF56x Blackfin Processor Programming Reference...