ADSP-2126x SHARC Processor Hardware Reference
4-13
Data Address Generators
updating the index on each access with a positive or negative modify value
(
M
register or immediate value). If the index pointer falls outside the buf-
fer, the DAG subtracts from or adds to the length of the buffer value,
wrapping the index pointer back to the start of the buffer. The DAG’s
support for circular buffer addressing appears in
,
and an example of circular buffer addressing appears in
.
The starting address that the DAG wraps around is called the buffer’s base
address (
B
register). There are no restrictions on the value of the base
address for a circular buffer.
Circular buffering may only use post-modify addressing. The
DAG’s architecture, as shown in
support pre-modify addressing for circular buffering because circu-
lar buffering requires that the index be updated on each access.
It is important to note that the DAGs do not detect memory map over-
flow or underflow. If the address post-modify produces
I + M
> 0xFFFF
FFFF or
I – M
< 0, circular buffering may not function correctly. Also, the
length of a circular buffer should not let the buffer straddle the top of the
memory map. For more information on the DSP’s memory map, see
As shown in
, programs use the following steps to set up a circu-
lar buffer:
1. Enable circular buffering (
BIT SET Mode1 CBUFEN;
). This operation
is only needed once in a program.
2. Load the buffer’s base address into the
B
register. This operation
automatically loads the corresponding
I
register.
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 ...