~ National ~ Semiconductor - Al Kossow's Bitsavers

6.0 Direct Memory Access Control (DMA) (Continuedl
L...--r--.... REMOTE ADD
Dual Bus System
64K BUFFER
MEMORY
MAIN CPU
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HANDSHAKE
SIGNALS
LOCAL
MICROPROCESSOR
SYSTEM
DMA
CONTROLLER
SYSTEM
ADDRESS
SYSTEM DATA
BLOCK DATA
TRANSFERS
SYSTEM
I/O PORT
MAIN
MEMORY
DP8390
32-Bit DMA Operation
~ DATA D
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REt.lOT; DMA
LOCAL OMA I-A_DD_R_ES_S..:.{3_2_-B_IT",-l_.
LOCAL BUS
HOST
MEMORY
TL/F/8582-6
BUS
7.0 Packet Reception
TL/F/8582-55
The Local DMA receive channel uses a Buffer Ring Structure
comprised of a series of contiguous fixed length 256
byte (128 wordl buffers for storage of received packets. The
location of the Receive Buffer Ring is programmed in two
registers, a Page Start and a Page Stop Register. Ethernet
packets consist of a distribution of shorter link control packets
and longer data packets, the 256 byte buffer length provides
a good compromise between short packets and longer
packets to most efficiently use memory. In addition these
buffers provide memory resources for storage of back-toback
packets in loaded networks. The assignment of buffers
BUFFER RAM
IUP TO 64 KBYTES)
NIC Receive Buffer Ring
BUFFER #1
BUFFER #2
BUFFER #3
BUFFER N
TL/F/8582-7
1-9