AM79C970 datasheet(159/168 Pages) AMD

159 / 168 page

AMD

1-1026

Am79C970

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-

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:

18220A-56

= 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 dur-

ing 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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