AM79C970A Advanced Micro Devices, AM79C970A Datasheet - Page 214

• Manufacturer Part Number: AM79C970A
• Description: PCnet-PCI II Single-Chip Full-Duplex Ethernet Controller for PCI Local Bus Product
• Manufacturer: Advanced Micro Devices

Figure D4 shows the buffer sizing for the two-interrupt
method. Note that the second buffer size will be about
the same for each method.
There is another alternative which is a marriage of the
two previous methods. This third possibility would use
the buffer sizes set by the two-interrupt method, but
would use the polling method of determining frame end.
This will give good frame latency but at the price of very
high CPU utilization.
D-10
AMD
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
#1
#2
#3
#4
#5
#6
#7
#8
#9
SIZE = HEADER_SIZE (minimum 64 bytes)
OWN = 1
OWN = 1
OWN = 0
OWN = 1
OWN = 1
OWN = 0
OWN = 1
OWN = 1
OWN = 0
SIZE = S1+S2+S3+S4
SIZE = 1518 - (S1+S2+S3+S4+HEADER_SIZE)
SIZE = S1+S2+S3+S4
SIZE = 1518 - (S1+S2+S3+S4+HEADER_SIZE)
SIZE = S1+S2+S3+S4
SIZE = 1518 - (S1+S2+S3+S4+HEADER_SIZE)
SIZE = HEADER_SIZE (minimum 64 bytes)
SIZE = HEADER_SIZE (minimum 64 bytes)
Figure D4. LAPP 3 Buffer Grouping for TWO-INTERRUPT Method
STP = 1
STP = 0
STP = 0
STP = 1
STP = 0
STP = 0
STP = 1
STP = 0
STP = 0
Am79C970A
And still, there are even more compromise positions that
use various fixed buffer sizes and effectively, the flow of
the one-interrupt method. All of these compromises will
reduce the complexity of the one-interrupt method by
removing the heuristic buffer sizing code, but they all
become less efficient than heuristic code would allow.
A
S1 = Interrupt latency
S2 = Application call latency
S3 = Time needed for driver to write
S4 = Time needed for driver to copy
S6 = Time needed for driver to copy
Note that the times needed for tasks S1,
S2, S3, S4, and S6 should be divided by
0.8 microseconds to yield an equivalent
number of network byte times before
subtracting these quantities from the
expected message size A.
= Expected message size in bytes
to third descriptor
data from buffer #1 to
data from buffer #2 to
application buffer space
application buffer space
19436A-60