Analog Devices ADSP-BF53x Blackfin, Reference

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ADSP-BF53x/BF56x Blackfin Processor Programming Reference 5-13
Address Arithmetic Unit
• The Length (L) register sets the size of the circular buffer and the
address range through which the DAG circulates the I-register.
L is positive and cannot have a value greater than 2
32
– 1. If an
L-register’s value is zero, its circular buffer operation is disabled.
• The Base (B) register or the B-register plus the L-register is the
value with which the DAG compares the modified I-register value
after each access.
To address a circular buffer, the DAG steps the Index pointer (I-register)
through the buffer values, post-modifying and updating the index on each
access with a positive or negative modify value from the M-register.
If the Index pointer falls outside the buffer range, the DAG subtracts the
length of the buffer (L-register) from the value or adds the length of the
buffer to the value, wrapping the Index pointer back to a point inside the
buffer.
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 circular buffers that contains 8-bit data. Circular buffers that
contain 16- and 32-bit data must be 16-bit aligned and 32-bit aligned,
respectively. Exceptions can be made for video operations.
For more infor-
mation, see “Memory Address Alignment” on page 5-16. Circular
buffering uses post-modify addressing.