
ADSP-2126x SHARC Processor Hardware Reference
2-15
Processing Elements
32-Bit Fixed-Point Format
The DSP always represents fixed-point numbers in 32 bits, occupying the
32 MSBs in 40-bit data registers. Fixed-point data may be fractional or
integer numbers and unsigned or twos-complement. Each computational
unit has its own limitations on how these formats may be mixed for a
given operation. All computational units read the upper 32 bits of data
(inputs, operands) from the 40-bit registers (ignoring the eight LSBs) and
write results to the upper 32 bits (zeroing the eight LSBs).
Rounding Mode
The
TRUNC
bit in the
MODE1
register determines the rounding mode for all
ALU operations, all floating-point multiplies, and fixed-point multiplies
of fractional data. The DSP supports two modes of rounding:
round-toward-zero and round-toward-nearest. The rounding modes com-
ply with the IEEE 754 standard and have the following definitions:
• Round-toward-zero (
TRUNC
bit=1). If the result before rounding is
not exactly representable in the destination format, the rounded
result is the number that is nearer to zero. This is equivalent to
truncation.
• Round-toward-nearest (
TRUNC
bit=0). If the result before rounding
is not exactly representable in the destination format, the rounded
result is the number that is nearer to the result before rounding. If
the result before rounding is exactly halfway between two numbers
in the destination format (differing by an LSB), the rounded result
is the number that has an LSB equal to zero.
Statistically, rounding up occurs as often as rounding down, so there is no
large sample bias. Because the maximum floating-point value is one LSB
less than the value that represents infinity, a result that is halfway between
the maximum floating-point value and Infinity rounds to Infinity in this
mode.
Summary of Contents for ADSP-21261 SHARC
Page 30: ...Contents xxx ADSP 2126x SHARC Processor Hardware Reference ...
Page 40: ...Register Diagram Conventions xl ADSP 2126x SHARC Processor Hardware Reference ...
Page 58: ...Differences From Previous SHARCs 1 18 ADSP 2126x SHARC Processor Hardware Reference ...
Page 112: ...Secondary Processing Element PEy 2 54 ADSP 2126x SHARC Processor Hardware Reference ...
Page 178: ...Summary 3 66 ADSP 2126x SHARC Processor Hardware Reference ...
Page 204: ...DAG Instruction Summary 4 26 ADSP 2126x SHARC Processor Hardware Reference ...
Page 322: ...Setting Up DMA 7 32 ADSP 2126x SHARC Processor Hardware Reference ...
Page 436: ...SPORT Programming Examples 9 86 ADSP 2126x SHARC Processor Hardware Reference ...
Page 521: ...ADSP 2126x SHARC Processor Hardware Reference 11 31 Input Data Port rts IDP_ISR end ...
Page 522: ...Input Data Port Programming Example 11 32 ADSP 2126x SHARC Processor Hardware Reference ...
Page 590: ...Timer Programming Examples 14 20 ADSP 2126x SHARC Processor Hardware Reference ...
Page 796: ...I O Processor Registers A 174 ADSP 2126x SHARC Processor Hardware Reference ...
Page 800: ...B 4 ADSP 2126x SHARC Processor Core Manual ...
Page 846: ...Index I 36 ADSP 2126x SHARC Processor Hardware Reference ...