DP83266 datasheet(125/152 Pages) NSC

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70 Control Information (Continued)

Output Data Unit Descriptor (ODUD)

An Output Data Unit Descriptor (ODUD) contains the part byte address and size of an Output Data Unit During Request

operations ODUDs are fetched by the MACSI device from a list in memory using the address in the ODUD List Pointer Register

(in the Pointer RAM)

ODUDFirsts and ODUDMiddles may have a zero byte count which is useful for fixed protocol stacks One layer may be called

and if it has no data to add to the frame it may add an ODUD with a zero byte count to the list ODUDOnlys and ODUDLasts

may not have a zero byte count

The MACSI device checks for the following inconsistencies when an ODUD is loaded from memory

1 ODUDFirst when previous ODUD was not an ODUDLast or ODUDOnly

2 ODUD which is not an ODUDFirst when the previous ODUD was ODUDLast or ODUDOnly

3 ODUDLast or ODUDOnly with a zero byte count

When an inconsistency is detected the MACSI device aborts the Request and reports the exception in the Request Status field

of the CNF Descriptor

The entire ODUD object must contain at least 4 bytes (for short addresses)

RES

CNT

Word 0

F-L

RES

LOC

Word 1

Bit

Symbol

Description

Word 0

D12 – D0

CNT

Byte Count

Number of bytes in the ODU For an ODUDFirst or ODUDMiddle the size may be Zero

which is useful for fixed protocol stacks

D31 – D13

RES

Reserved

Word 1

D27 – D0

LOC

Location

Pointer to the first byte of the corresponding ODU

D29 – D28

RES

Reserved

D31 – D30

F-L

FirstLast Tag

Identifies the Output Data Unit part ie Only First Middle or Last FL e 10 e First

FL e 00 e Middle FL e 01 e Last FL e 11-Only

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DP83266 Datasheet(PDF) 125 Page - National Semiconductor (TI)

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