am79c970 Advanced Micro Devices, am79c970 Datasheet - Page 159

• Manufacturer Part Number: am79c970
• Description: Pcnettm-pci Single-chip Ethernet Controller For Pci Local Bus
• Manufacturer: Advanced Micro Devices

tion call latency, minus the time needed for the driver to
write to the third descriptor, minus the time needed for
the driver to copy data from buffer #1 to the application
buffer space, and minus the time needed for the driver to
copy data from buffer #2 to the application buffer space.
Note that the time needed for the copies performed by
the driver depends upon the sizes of the 2nd and 3rd
buffers, and that the sizes of the second and third buff-
ers need to be set according to the time needed for the
data copy operations! This means that an iterative self-
LAPP Rules for Parsing of Descriptors
When using the LAPP method, software must use a
modified form of descriptor parsing as follows:
1-1026
Software will examine OWN and STP to determine
where a RCV frame begins. RCV frames will only
begin in buffers that have OWN = 0 and STP = 1.
Software shall assume that a frame continues until
it finds either ENP = 1 or ERR= 1.
Software must discard all descriptors with OWN =
0 and STP = 0 and move to the next descriptor
when searching for the beginning of a new frame;
ENP and ERR should be ignored by software dur-
ing this search.
Software cannot change an STP value in the re-
ceive descriptor ring after the initial setup of the
AMD
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
Descriptor
#1
#2
#3
#4
#5
#6
#7
#8
#9
SIZE = A-(S1+S2+S3+S4+S6)
SIZE = A-(S1+S2+S3+S4+S6)
SIZE = A-(S1+S2+S3+S4+S6)
OWN = 1 STP = 1
OWN = 1 STP = 0
OWN = 0 STP = 0
OWN = 1 STP = 1
OWN = 1
OWN = 0 STP = 0
OWN = 1 STP = 1
OWN = 1
OWN = 0 STP = 0
SIZE = S1+S2+S3+S4
SIZE = S1+S2+S3+S4
SIZE = S1+S2+S3+S4
SIZE = S6
SIZE = S6
SIZE = S6
STP = 0
STP = 0
Figure D2. LAPP 3 Buffer Grouping
Am79C970
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.
adjusting mechanism needs to be placed into the soft-
ware to determine the correct buffer sizing for optimal
operation. Fixed values for buffer sizes may be used; in
such a case, the LAPP method will still provide a signifi-
cant performance increase, but the performance in-
crease will not be maximized.
The following diagram illustrates this setup for a receive
ring size of 9:
When LAPPEN = 1, then hardware will use a modified
form of descriptor parsing as follows:
= Expected message size in bytes
ring is complete, even if software has ownership of
the STP descriptor unless the previous STP de-
scriptor in the ring is also OWNED by the software.
The controller will examine OWN and STP to de-
termine where to begin placing a RCV frame. A
new RCV frame will only begin in a buffer that has
OWN = 1 and STP = 1.
The controller will always obey the OWN bit for
determining whether or not it may use the next
buffer for a chain.
The controller will always mark the end of a frame
with either ENP = 1 or ERR= 1.
to third descriptor
data from buffer #1 to
data from buffer #2 to
application buffer space
application buffer space
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