AM79C970A datasheet(209/219 Pages) AMD

209 / 219 page

AMD

D-5

Am79C970A

application 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 buffers need to be set according to the time needed

for the data copy operations! This means that an

iterative self-adjusting mechanism needs to be placed

into the software 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 pro-

vide a significant performance increase, but the per-

formance increase will not be maximized.

The following diagram illustrates this setup for a receive

ring size of 9:

19436A-58

= 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.

OWN = 1 STP = 1

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

Descriptor

OWN = 1 STP = 0

SIZE = S1+S2+S3+S4

Descriptor

OWN = 0 STP = 0

SIZE = S6

Descriptor

OWN = 1 STP = 1

Descriptor

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

OWN = 1

Descriptor

STP = 0

SIZE = S1+S2+S3+S4

Descriptor

OWN = 0 STP = 0

SIZE = S6

OWN = 1 STP = 1

Descriptor

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

OWN = 1

Descriptor

STP = 0

SIZE = S1+S2+S3+S4

Descriptor

OWN = 0 STP = 0

SIZE = S6

Figure D2. LAPP 3 Buffer Grouping

LAPP Rules for Parsing of Descriptors

When using the LAPP method, software must use a

modified form of descriptor

parsing as follows:

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 during this search.

Software cannot change an STP value in the re-

ceive descriptor ring after the initial setup of the

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.

When LAPPEN = 1, then hardware will use a modified

form of descriptor

parsing as follows:

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.

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