ATM Controller
MPC885 PowerQUICC Family Reference Manual, Rev. 2
39-4
Freescale Semiconductor
39.1.2.3
Address Compression Example
Figure 39-2
shows an example of address compression. The first-level mask (FLMASK) selects the third
PTI bit and five VPI bits. Bitwise ANDing of the FLMASK with the GFC, VPI, and PTI bits results in a
6-bit pointer. Pointer1 turns out to equal 0x3, and therefore is pointing to the fourth entry of the FLT. The
entry contains a 16-bit mask (SLMASK) for the VCI field, and the second-level table offset (SLTOFFSET)
pointing to one of the SLTs. The VCI is masked with SLMASK, resulting in a 7-bit pointer to the assigned
channel number in the SLT.
Figure 39-2. Address Compression Example
39.1.2.4
Preventing Channel Aliasing
Reliable one-to-one mapping of VCs to local channel numbers requires that the address bits not taken into
account during the translation have a fixed value (chosen to be zero). Otherwise, multiple VCs could
translate to a single local channel number. The CUMB feature (check unused mask bits) can be used to test
the reliability of the mapping by screening out misinserted cells. When FLMASK[CUMB] is set, address
header bits not used in the first- and second-level address masking procedure are checked for non-zero
values. If a non-zero value is found, the cell is passed to the default raw cell queue. See
Table 38-11
for a
description of the CUMB bit.
32-bit entries
First-level addressing table (FLT)
GFC
0000
Pointer1
FLBASE
SLTOFFSET
SLMASK
VCI
00000111 11110000
Pointer2
16-bit entries
ch#
Second-level addressing tables (SLTs)
SLBASE
FLMASK
PHY#
64-Kbyte
memory space
(If in multi-PHY UTOPIA master mode)
PTI
VPI
00011111
100
Summary of Contents for PowerQUICC MPC870
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Page 1090: ...UTOPIA Interface MPC885 PowerQUICC Family Reference Manual Rev 2 43 8 Freescale Semiconductor ...
Page 1312: ...Byte Ordering MPC885 PowerQUICC Family Reference Manual Rev 2 A 8 Freescale Semiconductor ...
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Page 1370: ...MPC880 MPC885 PowerQUICC Family Reference Manual Rev 2 E 4 Freescale Semiconductor ...
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