needed for the drive to copy data from buffer number 2
to the application buffer space. Note that the time
needed for the copies performed by the driver depends
upon the sizes of the second and third buffers, and that
the sizes of the second and third buffers need to be set
according to the time needed for the data copy opera-
tions. This means that an iterative self-adjusting mech-
anism needs to be placed into the software to
determine the correct buffer sizing for optimal opera-
tion. Fixed values for buffer sizes may be used; in such
a case, the LAPP method will still provide a significant
performance increase, but the performance increase
will not be maximized.
LAPP Software Requirements
Software needs to set up a receive ring with descriptors
formed into groups of three. The first descriptor of each
group should have OWN = 1 and STP = 1, the second
descriptor of each group should have OWN = 1 and
STP = 0. The third descriptor of each group should
have OWN = 0 and STP = 0. The size of the first buffer
(as indicated in the first descriptor) should be at least
equal to the largest expected header size; however, for
maximum efficiency of CPU utilization, the first buffer
size should be larger than the header size. It should be
equal to the expected number of message bytes,
minus the time needed for interrupt latency and minus
the application call latency, minus the time needed for
the driver to write to the third descriptor, minus the time
Figure B-2 illustrates this setup for a receive ring size
of 9.
OWN = 1 STP = 1
SIZE = A-(S1+S2+S3+S4+S6)
Descriptor
#1
OWN = 1 STP = 0
SIZE = S1+S2+S3+S4
Descriptor
#2
A = Expected message size in bytes
S1 = Interrupt latency
S2 = Application call latency
S3 = Time needed for driver to write
to third descriptor
S4 = Time needed for driver to copy
data from buffer #1 to
Descriptor
#3
OWN = 0 STP = 0
SIZE = S6
Descriptor
#4
OWN = 1 STP = 1
SIZE = A-(S1+S2+S3+S4+S6)
OWN = 1 STP = 0
SIZE = S1+S2+S3+S4
application buffer space
S6 = Time needed for driver to copy
data from buffer #2 to
Descriptor
#5
OWN = 0 STP = 0
SIZE = S6
Descriptor
#6
application buffer space
OWN = 1 STP = 1
SIZE = A-(S1+S2+S3+S4+S6)
Descriptor
#7
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.
OWN = 1 STP = 0
SIZE = S1+S2+S3+S4
Descriptor
#8
OWN = 0 STP = 0
SIZE = S6
Descriptor
#9
22206B-B2
Figure B-2. LAPP 3 Buffer Grouping
n 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 during this
search.
LAPP Rules for Parsing Descriptors
When using the LAPP method, software must use a
modified form of descriptor parsing as follows:
n Software will examine OWN and STP to determine
where an RCV frame begins. RCV frames will only
begin in buffers that have OWN = 0 and STP = 1.
n Software cannot change an STP value in the receive
descriptor ring after the initial setup of the ring is
complete, even if software has ownership of the STP
n Software shall assume that a frame continues until
it finds either ENP = 1 or ERR = 1.
Am79C978
B-5
AMD [ AMD ]
