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PRELIMINARY
DS3 UNI FOR ATM
XRT7245
REV. 1.03
261
Filter Mask Header Byte” registers. The role of these
registers in “User Cell Filtering” is illustrated in the
example below.
Example—User Cell Filtering
For example, header byte 1 of a given incoming user
cell will be subjected to a bit-by-bit comparison to the
contents of the “Rx CP User Cell Filter Pattern Header
Byte-1” register (Address = 58h). However, the con-
tents of the “Rx CP User Cell Filter Mask Header
Byte-1” register (Address = 5Ch) also plays a role in
this comparison process. For example, if bit-field “0”
within the “Rx CP User Cell Filter Mask Header Byte-1”
register contains a “1”, then the Receive Cell Processor
will perform the comparison operation between bit-
field “0” within the “Rx CP User Cell Filter Pattern
Header Byte-1” register; and bit-field “0” within header
byte 1 of the newly received user cell. Conversely, if
bit-field ‘0’ within the “Rx CP User Cell Filter Mask
Header Byte-1” register contains a ‘0’, then this com-
parison will not be made and bit-field ‘0’ will be treated
as a ‘don’t care’. The role of these two read/write reg-
isters in these comparison operations is more clearly
defined in Table 58, on following page.
Based upon these register settings, any cell containing
values in the range of A0h–AFh are considered to be
matching, at the first byte. This cell will be subjected to
three (3) more tests (e.g., one for each of the remain-
ing header bytes.)
After all of these comparison tests have been
performed, a given user cell will be deemed either
“matching” or “not matching” the settings of the User
Cell Filter. Once the cell has been classified into one
of these two categories, its disposition (or fate) is
dependent upon the content of bit-field 5 (User Cell
Filter Discard) within the “Rx CP Additional Configu-
ration Register (Address = 4Dh). If this bit-field is ‘0’,
then only-matching cells will be retained, and written
into the RxFIFO. All remaining User Cells will be
discarded. Conversely, it this bit-field is ‘1’, then only
‘non-matching’ User Cells will be retained and written
T
ABLE
58: I
LLUSTRATION
OF
THE
R
OLE
OF
THE
“R
X
CP U
SER
C
ELL
F
ILTER
P
ATTERN
H
EADER
B
YTE
”
REGISTER
AND
THE
“R
X
CP U
SER
C
ELL
F
ILTER
M
ASK
H
EADER
B
YTE
”
REGISTER
.
Content of Header Byte-1 (of Incoming User Cell)
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
1
0
1
0
0
1
0
1
Content of “Rx CP User Cell Filter Mask Header Byte-1 Register
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
1
1
1
1
0
0
0
0
Content of “Rx CP User Cell Filter Pattern Header Byte-1 Register
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
1
1
1
1
0
0
0
0
Comments
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
Comparison is Forced (by the “1s” in the Rx CP User Cell Fil-
ter Mask Header Byte-1 Register)
Don’t Care
Don’t Care
Don’t Care
Don’t Care
Resulting “User Cell Fiilter” Pattern for Header Byte-1
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
1
0
1
0
x
x
x
x