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3.0 Functional Description
3.4 Segmentation Coprocessor
Bt8230
ATM Segmentation and Reassembly Controller—SAR
N8230DS1F
VCC_CTRL
Segmentation options for the buffer. The VCC_CTRL field controls segmentation options on a per-buffer
basis. These options should only be changed at a cell or CPCS-PDU boundary. The options are:
LENGTH
Number of bytes of data contained in the buffer. For local buffers, if the EOM bit (VCC_CTRL[0]) is not set,
this length must be set to an integral multiple of 4 bytes. Host buffers may be on non-modulo 4 length. This
location is not changed by the Bt8230.
NEXT_PTR
Pointer to next descriptor for the VCC. The two least significant bits of the pointer are assumed to be zero
(word-aligned).
Table 3-6. Buffer Descriptor Format Field Descriptions (2 of 2)
Name
Description
VCC_CTRL[15]
FAST_START—When data is available for VCC, send cell as soon as possible.
This is useful for transmission of latency sensitive traffic such as AAL0.
GCRA mode VCCs will send cell immediately instead of waiting for rate parame-
ter/time to elapse before sending first cell.
UBR VCCs will be placed to the top instead of the bottom of the UBR queue only
if VCC is not currently in queue.
VCC_CTRL[14]
LOCAL—Buffer address is a buffer in Bt8230 local memory instead of host mem-
ory. Local buffers must begin on word-aligned addresses.
VCC_CTRL[13]
SET_CI—Sets the middle bit of the ATM header PTI field for all cells.
VCC_CTRL[12]
WR_GFC—Overwrites the ATM header GFC field for all cells with GFC_DATA.
Global GFC changes (active for all buffers of VCC) can be set in VCC structure
ATM header.
VCC_CTRL[11]
WR_PTI—Overwrites the ATM header PTI field for all cells with PTI_DATA. Used
to generate F5 and RM OAM cells. OAM cells are not included in PM TUC or
BIP16 calculations.
VCC_CTRL[10]
WR_VCI—Sets the ATM header VCI value for all cells to VCI_DATA (Most Signif-
icant Bits [MSBs] of VCI are set to zero). Used to generate F4 OAM cells. OAM
cells are not included in PM TUC or BIP16 calculations.
VCC_CTRL[9]
SET_CLP—Sets the ATM header CLP bit for all cells to 1.
VCC_CTRL[8]
CELL—Read entire 52-octet ATM cell from segmentation buffer. The
ATM_HEADER stored in the VCC structure (Table 3-7) is not used in this mode
VCC_CTRL[7]
ABORT—Generates an abort cell and end processing of descriptor.
VCC_CTRL[6]
STM_MODE—When set high, a SEG_xS_WRITE interrupt occurs on each buffer.
When low, the interrupt occurs on PDU boundaries.
VCC_CTRL[5]
GEN_PDU—Generates AAL3/4 header and trailer and AAL5 trailer. Setting this bit
on an AAL5 cell will also enable the CRC32 calculation.
VCC_CTRL[4]
CRC_10—Overwrites last 10 bits of a cell with CRC10 calculation.
VCC_CTRL[3]
AAL3/4—Selects AAL3/4 operation. AAL5 operation is used if not set. This
option also allows OAM cells by selecting AAL5 operation.
VCC_CTRL[2]
Reserved—Set to zero.
VCC_CTRL[1]
BOM—Buffer contains beginning of message (AAL3/4 only).
VCC_CTRL[0]
EOM—Buffer contains end of message.