AM79C970 datasheet(164/168 Pages) AMD

164 / 168 page

AMD

1-1031

Am79C970

Figure D4 shows the buffer sizing for the two-interrupt

method. Note that the second buffer size will be about

the same for each method.

18220A-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 = HEADER_SIZE (minimum 64 bytes)

Descriptor

OWN = 1

STP = 0

SIZE = S1+S2+S3+S4

Descriptor

OWN = 0

STP = 0

SIZE = 1518 - (S1+S2+S3+S4+HEADER_SIZE)

Descriptor

OWN = 1

STP = 1

Descriptor

SIZE = HEADER_SIZE (minimum 64 bytes)

OWN = 1

Descriptor

STP = 0

SIZE = S1+S2+S3+S4

Descriptor

OWN = 0

STP = 0

SIZE = 1518 - (S1+S2+S3+S4+HEADER_SIZE)

OWN = 1

STP = 1

Descriptor

SIZE = HEADER_SIZE (minimum 64 bytes)

OWN = 1

Descriptor

STP = 0

SIZE = S1+S2+S3+S4

Descriptor

OWN = 0

STP = 0

SIZE = 1518 - (S1+S2+S3+S4+HEADER_SIZE)

Figure D4. LAPP 3 Buffer Grouping for TWO-INTERRUPT Method

There is another alternative which is a marriage of the

two previous methods. This third possibility would use

the buffer sizes set by the two-interrupt method, but

would use the polling method of determining frame end.

This will give good frame latency but at the price of very

high CPU utilization.

And still, there are even more compromise positions that

use various fixed buffer sizes and effectively, the flow of

the one-interrupt method. All of these compromises will

reduce the complexity of the one-interrupt method by re-

moving the heuristic buffer sizing code, but they all

become less efficient than heuristic code would allow.

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