User`s guide

76 DC 900-1339H
FMP Programmer’s Guide
Data Message Types: Only data buffers are packed. If a non-data buffer (such as a data
acknowledgment or report) is placed on the waiting queue, data packing is temporarily
interrupted. Data packing resumes when two sequential data type buffers appear on the
queue. The buffer retains the command code of the first message in a packed buffer. For
example, if an ETB block (
DLI_PROT_SEND_NORM_DATA) was on the queue and an ETX
block (
DLI_PROT_SEND_NORM_DATA_EOM) was packed into it, the packed buffer would
retain the ETB command code (
DLI_PROT_SEND_NORM_DATA).
Buffer Reporting: The link buffer request (Section 3.4.2.1 on page 53) reports the
number of buffers in the ICP waiting queue (HOSTOQ) at any one time. For best
results on an active link, the buffer request should be issued through the control access
mode (Section 2.2 on page 32). Because the ICP always posts two writes to the Freeway
server, data packing normally does not start until after the third buffer arrives on the
queue.
Sequence Number Field: When the data packing option enabled, the Simpact assigned
sequence numbers for packed buffers become meaningless for obvious reasons. As
such, the sequence number field in the optional arguments structure (Section 3.3.1 on
page 44) is used for a different purpose when data packing is enabled. With data packing
enabled, the sequence number field contains the number of messages (whole buffers)
that were packed into this buffer. Incoming data buffers that do not remain in ICP
memory long enough to pack have a 1 in the sequence number field. On Freeway record
types with a 5-byte header, (see Section 4.13.4 on page 87 and Section 4.13.5 on
page 88), the sequence number appears in bytes 4 and 5 of the record header even
though packing is enabled.
4.9.2 Data Packing Examples
Figure 4–1 through Figure 4–5 give examples of how data is packed for the various Mes-
sage Blocking options (Section 4.13 on page 81). Compare these with the examples of
unpacked data in Figure 4–7 on page 83 through Figure 4–14 on page 89. Both series of