NEC Network Controller uPD98502 - Manual (Page 253)

CHAPTER 4 ATM CELL PROCESSOR
Preliminary User’s Manual S15543EJ1V0UM 253
4.5.2.1 Rx pool descriptor
A pool descriptor contains two words shown as Figure 4-14. Its address is word aligned.
Figure 4-14. Rx Pool Descriptor
-R x pool descrip tor
31 30 0 16 15
R x buffer directory A ddress
R em aining # of dir. in the pool
31 0
24 23
0 all 0 A lert le vel
28 27
dir. size in a pool
Table 4-2. List of Rx Pool Attributes
Field Note
Alert level When the remaining number of buffer directories is lower than this number times 4, an alert interrupt will
be issued.
Dir. size in a pool It contains the number of Rx buffer descriptors in a buffer directories in the pool.
Remaining number of
dir. in the pool
It contains the current remaining number of buffer directories.
Directory Address It contains address of the first buffer directory in the pool.
4.5.2.2 Rx buffer directory
Rx buffer directory contains some buffer descriptors, up to 255, and a link pointer. Number of buffer descriptors in
each directory in one pool is identical. Number can vary in different pools.
Address of buffer directory is word aligned. The end of buffer directory must be a link pointer. Buffer descriptor
must be read and used from the top in a sequential manner.
4.5.2.3 Rx buffer descriptor
Both an Rx buffer descriptor and an Rx link pointer consist of 2 words. DL bit, bit 30 of the first word, indicates that
these two words are a buffer descriptor (DL = 1) or a link pointer (DL = 0).
An Rx buffer descriptor and an Rx link pointer are shown as Figure 4-16. Its address is word aligned. L bit indicates
that the buffer pointed by this descriptor contains the last portion of a packet.
If there is no buffer directory to be linked, directory address of link pointer must be zero, as a null pointer.