AM79C978VCSLASHW datasheet(245/261 Pages) AMD

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Am79C978

B-5

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

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.

Figure B-2 illustrates this setup for a receive ring size

of 9.

Figure B-2.

LAPP 3 Buffer Grouping

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 shall assume that a frame continues until

it finds either ENP = 1 or ERR = 1.

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.

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

Descriptor

OWN = 1 STP = 1

SIZE = A-(S1+S2+S3+S4+S6)

OWN = 1 STP = 0

SIZE = S1+S2+S3+S4

OWN = 0 STP = 0

SIZE = S6

OWN = 1 STP = 1

SIZE = A-(S1+S2+S3+S4+S6)

OWN = 1 STP = 0

SIZE = S1+S2+S3+S4

OWN = 0 STP = 0

SIZE = S6

OWN = 1 STP = 1

SIZE = A-(S1+S2+S3+S4+S6)

OWN = 1 STP = 0

SIZE = S1+S2+S3+S4

OWN = 0 STP = 0

SIZE = S6

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

application buffer space

S6 = Time needed for driver to copy

data from buffer #2 to

application buffer space

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.

22206B-B2

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