NEC Network Controller uPD98502 - Manual (Page 250)

CHAPTER 4 ATM CELL PROCESSOR
250 Preliminary User’s Manual S15543EJ1V0UM
4.5.1.2 Tx buffer directory
Tx buffer directory contains some buffer descriptors, up to 255, and a link pointer. Its address is word aligned. The
end of buffer directory must be a link pointer. Buffer descriptors must be read and served from the top in a sequential
manner.
4.5.1.3 Tx buffer descriptor
Both a Tx buffer descriptor and a Tx 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). In the Tx buffer descriptor, L bit, bit 31,
indicates that the buffer pointed by this descriptor contains the last portion of a packet.
A Tx link pointer is shown as Figure 4-11. L bit, bit 31, is fixed to zero. If there is no buffer directory to be linked,
directory address of link pointer must be zero, as a null pointer.
Figure 4-11. Tx Buffer Descriptor/Link Pointer
Tx buffer directory Address
-Tx link pointer
Attribute
-Tx buffer descriptor
31 30 0 16 15
Attribute Size L
0
Buffer Address
31 0
DL
31 30
0
DL
4.5.1.4 Data buffer
Data buffer contains actual packet data to be sent. Size of a buffer can vary from 1 byte to 64 Kbytes. Its address is
byte aligned.
4.5.2 Rx pool structure
Rx buffer structure is defined as a pool. Eight pools are supported. A pool is composed of chain of Rx buffer
directories. Each Rx buffer directory has some buffer descriptors and a link pointer. Each buffer descriptor points a
buffer string of received cell data. A link pointer has an address to a next Rx buffer directory.
V R 4120A will create up to 8 pools and give them to ATM Cell Processor.