Motorola MPC8260 - Manual (Page 794)

29-14 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA
Part IV. Communications Processor Module
29.4 VCI/VPI Address Lookup Mechanism
The MPC8260 supports two ways to look up addresses for incoming cells:
¥ External CAM lookup
¥ Address compression
Writing to GMODE[ALM] (address-lookup-mechanism bit) in the parameter RAM selects
the mechanism. Both mechanisms are described in the following sections.
29.4.1 External CAM Lookup
An external CAM is usually used when the range of VCI/VPI values varies widely or is
unknown. Clearing GMODE[ALM] selects the external CAM address lookup mechanism.
If there is no match in the external CAM, the cell is considered a misinserted cell. The
external CAM can point to internal or external channels (channels whose connection table
resides in external memory). The CAM input, shown in Figure 29-3, is the 32-bit cell
address: PHY address, GFC + VPI, and VCI.
The output of the CAM, shown in Figure 29-4, is a 32-bit entry (16-bit channel code and a
match-status bit).
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
PHY Addr
(MPHY)
GFC + VPI VCI
Figure 29-3. External CAM Data Input Fields
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
MS Ñ Channel Code
Figure 29-4. External CAM Output Fields