Address Mapping; Internal Look-Up Mechanism (Srstate[Ext] = 0) - Freescale Semiconductor MPC850 User Manual

Chapter 39
ATM Controller
This chapter describes the address mapping mechanisms of the ATM controller to support
connection tables for both single- and multi-PHY interfaces, and the commands provided
to control ATM transmit and receive operations on a channel-by-channel basis.

39.1 Address Mapping

Three methods for mapping incoming cell header addresses to local ATM channel numbers
are available. The first method is based on an internal look-up table and is used when the
maximum number of ATM receive connections can be handled by the internal channels.
When more than 31 receive connections are required (extended channel mode), two
methods for address mapping are supported: address compression and the use of content
addressable memory (CAM).
Only one address mapping method can be used at a time, and it should be selected in the
parameter RAM (see the EXT and ACP field descriptions in Section 38.2, "SAR Receive
State Register (SRSTATE)") during system start-up. The following sections describe each
address mapping method.

39.1.1 Internal Look-up Mechanism (SRSTATE[EXT] = 0)

The internal look-up mechanism maps the address fields in the header of incoming cells to
internal channel numbers. This mapping mechanism uses two tables: an address matching
table and a pointing table. The matching table contains up to thirty-one 32-bit address
(GFC/VPI/VCI/PTI/CLP) entries and one empty entry at the base of the table reserved for
the raw cell queue. (Generally, channel number 0 is used for the raw cell queue as a
convention.) The pointing table contains thirty-two 16-bit RCT base address pointers
corresponding one-to-one with the address matching table entries.
When a cell is received, the cell header is masked by performing bitwise AND logic with
HMASK in the parameter RAM (see Table 38-8). The resulting masked header is then
compared with each entry in the matching table starting from the top (AMEND). When an
entry matches, the parallel entry in the pointing table is then read to get the RCTn base
address of the local channel n assigned to the incoming cell's address. If, however, the
Chapter 39. ATM Controller