
IBM2520L8767
IBM Processor for ATM Resources
atmrm.chapt03.01
08/27/99
Packet Header
Page 39 of 553
Transmit Descriptor Data Structures
Logical Channel Data Structure
on page 38 and
Scheduling Portion of a Transmit Descriptor
on page 40
show the layout of the transmit portion of a Logical Channel Descriptor. When initializing an LCD, any loca-
tions that are not written to a specific value should be initialized to zeroes. Fields that typically need to be ini-
tialized to a non-zero value are flagged with a # in the structure below.
Note:
This is only one possible layout of the transmit portion of the LCD. Some field locations vary and are
further defined later in this section.
Care must be taken when updating fields in the LCD and then immediatly causing the updated fields to be
accessed by other IBM2520L8767 entities. For example, it is possible, although not likely, under the right
conditions, for a normal LCD update followed by a SEGBF cell enqueue operation to actually execute in
reverse order. This is due to IBM2520L8767 internal priority levels and could result in SEGBF fetching the
LCD data before it has been updated to the new value. For this reason, it is highly advisable to use the LCD
update mechanism in RAALL (
RAALL LCD Update Data Registers
on page 295,
RAALL LCD Update Mask
Registers
on page 295, and
RAALL LCD Update Op Register
on page 296) to guarantee that any LCD
update operation completes before any subsequent ops can execute.
The transmit portion of the LCD can be subdivided into three distinct parts based on which chip functions or
entities access that particular part of the LCD. The first three 64-bit words are scheduling related and are
accessed only by CSKED. The next two 64-bit words are related to both scheduling and segmentation and
are accessed and shared by both CSKED and SEGBF. The words following these shared locations are
related only to segmentation and are accessed only by SEGBF. The number of 64-bit words in this portion of
the LCD can vary from zero to three. The actual number being used in an LCD is determined by the AAL type
bits.
The three 64-bit words containing CSKED scheduling information can have four different formats depending
upon whether scheduling is based on virtual path parameters or is configured for ABR. These four formats
are shown in
Scheduling Portion of a Transmit Descriptor
on page 40: