Atm Buffer Descriptors (Bds) - Freescale Semiconductor MPC850 User Manual

Chapter 37
Buffer Descriptors and Connection
Tables
The communications processor module (CPM) manages ATM traffic through the UTOPIA
and serial interfaces by means of transmit and receive buffer descriptors (BDs) and transmit
and receive connection tables (TCTs and RCTs). The BDs are grouped into circular tables
of pointers into the data buffer space in external memory. The following sections describe
the structure and configuration of the BDs, TCTs, and RCTs.

37.1 ATM Buffer Descriptors (BDs)

ATM segmentation and reassembly (SAR) operates as a multi-channel protocol,
segmenting and reassembling each channel's transmit and receive data to and from different
sets of memory buffers simultaneously. The buffer descriptor (BD) implementation for
ATM operation builds on the traditional SCC buffering method in which each controller has
one pair of BD tables for receiving and transmitting. To accommodate multiple ATM
channels, each channel number is given its own pair of BD tables located in external
memory. The base pointers to a channel's BD tables are programmed as part of the
channel-specific information in the channel's RCT and TCT; see Section 37.2, "Receive
and Transmit Connection Tables (RCTs and TCTs)."
Each transmit channel has a separate TxBD table and a TCT which holds the TxBD pointers
(TBASE and TBD_PTR). Likewise, each receive channel has a separate RxBD table and a
RCT holding the RxBD pointers (RBASE and RBD_PTR). The global parameters
TBDBASE and RBDBASE, located in the ATM-specific SCC (serial and UTOPIA)
parameter RAM (see Chapter 38, "ATM Parameter RAM"), define the base addresses for
the blocks of external memory containing all the channels' BD tables. The BD base offset
pointers (TBASE and RBASE) point to the beginning of a given channel's BD tables. The
BD offset pointers (TBD_PTR and RBD_PTR) specify the next (or currently active) BD.
Figure 37-1 shows TxBD tables and buffers and their associated pointers for two example
transmit channels, channel 1 and channel 4. (The RxBD tables and buffers for receive
channels have the same structure.)
Chapter 37. Buffer Descriptors and Connection Tables