SNAX/APC Application Programming Manual

Using the Application Program Interface

SNAX/APC Application Programming Manual—138786

1-27

Basic Conversations

Basic conversations (those using basic verbs) define a logical record as the elementary

unit of information transferred between TPs. A logical record consists of the two-byte

length prefix (LL) followed by up to 32,767 bytes of data.

A pair of LU 6.2 LUs transmit and receive data as General Data Stream (GDS)

variables.

GDS variables consist of a four-byte GDS header, followed by the data. The GDS

header consists of a two-byte length field (LL, which contains the length of the GDS

variable, including the GDS header) and a two-byte identification field (ID). The GDS

variable can be of any length. However, if the length exceeds 32,765 bytes, the record is

divided into multiple segments, each with its own length field; only the first segment of

a multisegment GDS variable contains an ID field. The high-order bit (bit 0 of byte 0)

in the length field is a continuation indicator. When the bit is set, the associated segment

is followed by another segment.

Thus, a GDS variable segment is also a logical record. A logical record does not need

the ID field required in GDS variables; however the ID can be included as part of the

data. If an ID is included, the TP assigns and interprets the ID field. Note that the

inclusion or exclusion of the ID field is application-dependent; if the local TP is

communicating with a remote TP that uses mapped verbs, the local TP must be prepared

to unpack the GDS variables.

Buffering Transmitted Data

SNAX/APC stores the data from a SEND-DATA verb in a send buffer. SNAX/APC

transmits the data when its send buffer overflows or when SNAX/APC receives a verb

from the local TP that explicitly flushes the buffer (using CONFIRM, DEALLOCATE,

FLUSH, PREPARE-TO-RECEIVE, RECEIVE-AND-WAIT, or SEND-ERROR). The

size of the send buffer is determined by the MAXAPPLIOSIZE parameter.

(MAXAPPLIOSIZE specifies the maximum number of bytes that an application can

send to or receive from the SNAX/APC process.) Thus, the TP can issue several SEND-

DATA requests before the data is actually transmitted by SNAX/APC.

Note. If you ar using basic verbs and the local TP is communicating with a computer

produced by a vendor other than Tandem, the local TP must translate data from ASCII to

the representation used in that remote system. Thus, if a local TP communicates with an

IBM System 38, the local TP must translate data from ASCII to EBCDIC before

sending the data (and EBCDIC to ASCII when receiving data). The SNAX/APN or

SNAX/XF (or SNAX/CDF) lines and LUs (SNAX/CDF APPL objects) cannot be

configured to translate data because the translation would also affect the GDS headers

and FMH data.

If you are using mapped verbs, you can use character mapping support feature of

SNAX/APC to have SNAX/APC do the character translation automatically.