Motorola PowerQUICC II MPC8280 Series Reference Manual page 1030

VCI/VPI Address Lookup Mechanism
0 3 4
PHY Addr
(MPHY)
Figure 31-3. External CAM Data Input Fields
The output of the CAM, shown in Figure 31-4, is a 32-bit entry (16-bit channel code and a
match-status bit).
0 1
MS
The external CAM fields are described in Table 31-2.
Table 31-2. External CAM Input and Output Field Descriptions
Field
PHY Addr In multiple PHY mode, this field contains the 4 least-significant bits of the current channel's physical
address. Because this CAM comparison field is limited to 4 bits, two CAM devices are needed if using
more than 16 PHYs.The msb of the PHY address (bit 4) selects between the two devices. If the msb is
zero, the CP accesses the CAM whose address is written in the EXT_CAM_BASE parameter in the
parameter RAM; if the msb is set, the CP uses EXT_CAM1_BASE. See Section 16.4.1, "CMX UTOPIA
Address Register (CMXUAR)."
In single PHY mode, clear this field.
GFC+VPI, The GFC, VPI, and VCI of the current channel.
VCI
Ch Code Pointer to internal or external connection table.
— Reserved, should be cleared.
MS Match status.
0 Match was found.
1 Match was not found.
31.4.2 Address Compression
The address compression mechanism uses two levels of address translation to help
minimize the memory space needed to cover the available address range. The first level of
translation (VP-level) uses a look-up table based on the 4-bit PHY address and the 12-bit
virtual path identifier; the second level (VC-level) uses the 16-bit virtual channel identifier.
If there is no match during address compression, the cell is considered a misinserted cell.
During the VP-level translation, VP_MASK in the ATM parameter RAM compresses an
incoming cell's PHY address and VPI to create an index into the VP-level table. The
VP-level table entry consists of another mask (VC_MASK) and a pointer to one of the
VC-level tables (VCOFFSET). Note that the VP table should reside in the dual-port RAM.
31-16
Freescale Semiconductor, Inc.
15 16
GFC + VPI
—
Figure 31-4. External CAM Output Fields
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15 16
Channel Code
Description
VCI
MOTOROLA
31
31