
12-20
Operation
This information is needed to ensure that the APU does not prematurely
insert an EOM identifier into the SAR Headers of AAL 2 and 3/4 cells or
encode an EOM identifier into the PTI fields of the ATM Headers of AAL
5 cells. Therefore, it is important that a flag field be included in the Chan-
nel Parameter Entry that indicates whether the fragment represents end of
CS-PDUorifmorefragmentsexistfortheCS-PDU.Firmwarerunningon
theAPUmustcheckthisconditionduringthesegmentationprocess.Since
the APU must check the resulting byte count each time it decrements it, it
is possible to signal end-of-CS-PDU by providing a byte count in the
Channel Parameter Entry that will reach exactly zero at the end of a frag-
ment that represents the end of the CS-PDU and one that will produce a
negative result for fragments that are not the last fragment (the byte count
wouldbeatleastonebytelessthantheactualcount).Theseandothertech-
niques can be employed to dramatically reduce the number of APU
instructions required to generate (or process) a cell and shall be expanded
upon later in the section on programming the APU.
As mentioned previously, the APU may choose to generate more than one
cell from a given CS-PDU before proceeding onto the nextCS-PDU. This
choice is up to the user but it is important to understand that generating
multiple cells per CS-PDU reduces the number of APU cycles required to
build a cell. The APU cycles required to retrieve and restore the Channel
Parameter Entry for the CS-PDU can be amortized over the number of
cells generated.
Once the cell generation routine has been entered, cell generation involves
the APU retrieving a Channel Parameter Entry (from the internal VCR or
externally), using the DMA Address to initiate a Memory Read operation
to retrieve the SAR-SDU (size dependent on AAL type and on Gather
algorithm employed), retrieving the ATM Header from the Channel
Parameter Entry, modifying certain fields (GFC, PTI, CLP) if necessary,
and writing the Header into the appropriate location in the VCR (just in
frontofwheretheDMAControllerwasinstructedtowritetheSAR-SDU).
IfthecellisanAAL3/4cell,theAPUmustalsoretrievethepreviousSAR
Header and use it the previous sequence number to generate the current
SAR Header. The APU must also set the LO Field in the VCR by writing
ittotheendofwheretheDMAControllerwasinstructedtowritetheSAR-
SDU after the SAR-SDU retrieval has completed (since the DMAC does
not clip VCR or memory write operations on the tail end of the last word).
Finally, the APU must queue the cell for transmission by writing its VCR
addressintotheCellAddressFIFOintheACITransmitter.AAL5cellsdo