DAGs, Registers, and Memory
4-22
ADSP-2126x SHARC Processor Hardware Reference
because the same bus is needed by both operations in the same cycle.
Therefore, the second operation must be delayed. The following example
causes a delay because it exhibits a write/read dependency in which
I0
is
written in one cycle. The results of that register write are not available to a
register read for one cycle. Note that if either instruction had specified
I1
,
the stall occurs only if the first instruction performs a long word (LW)
access. The DAG detects write/read dependencies with a register pair
granularity:
I0 = 8;
DM(I0,M1) = R1;
Certain sequences of instructions cause incorrect results on the DSP and
are flagged as errors by the DSP assembler software. The following types
of instructions can execute on the processor, but cause incorrect results.
• An instruction that stores a DAG register in memory using indirect
addressing from the same DAG, with or without an update of the
index register. The instruction writes the wrong data to memory or
updates the wrong index register.
Do not try these:
DM(M2,I1) = I0;
or
DM(I1,M2) = I0;
These example instructions do not work because
I0
and
I1
are both
DAG1 registers.
• An instruction that loads a DAG register from memory using indi-
rect addressing from the same DAG, with an update of the index
register. The instruction either loads the DAG register or updates
the index register, but not both.
Do not try this:
L2 = DM(I1,M0);
This example instruction does not work because
L2
and
I1
are both
DAG1 registers.
1
DAG registers are accessible in pair granularity for single cycle access. The pairings are odd-even. For
example I0 and I1 are a pair, and I2 and I3 are a pair.
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 ...