Computer Hardware User's Guide

Circular Addressing
6-22
Figure 6–6. Logical and Physical Representation of Circular Buffer after Writing Eight Values
Start End
a) Logical representation
value
6
value
7
value
2
value
6
value
2
value
7
b) Physical representation
value
5
value
3
value
4
value
3
value
4
value
5
Start
End
To implement a circular buffer in the ’C3x, the following criteria must be satis-
fied (more than one circular buffer can be implemented on the ’C3x as long as
the size of the buffers are identical):
Specify the size of the circular buffer (R) by storing the length of the buffer
in the block-size register (BK). The size of the buffer must be less than or
equal to 64K (16 bits).
Align the start of the buffer to a K-bit boundary (that is, the K LSBs of the
starting address of the circular buffer must be 0) by satisfying the following
formula:
2
K
> R
where:
R = length of circular buffer
K = number of 0s in the LSBs of the circular buffer starting address
Example 6–23. Examples of Formula 2
K
> R
Length of Buffer BK Register Value Starting Address of Buffer
31 31 XXXXXXXXXXXXXXXXXXX00000
2
32 32 XXXXXXXXXXXXXXXXXX000000
2
1024 1024 XXXXXXXXXXXXX00000000000
2