If you are using VTAM®, the CEDA DEFINE TYPETERM IOAREALEN command determines the initial size of the terminal input/output area (TIOA) to be passed onto a transaction for each terminal. The syntax for IOAREALEN is ({0|value1},{0|value2}). This operand is used only for the first input message for all transactions.
One value defining the minimum size is used for non-SNA devices, while two values specifying both the minimum and maximum size are used for SNA devices.
This book does not discuss the performance aspects of the CICS® Front End Programming Interface. See the CICS Front End Programming Interface User’s Guide for more information.
When value1,0 is specified for IOAREALEN, value1 is the minimum size of the terminal input/output area that is passed to an application program when a RECEIVE command is issued. If the size of the input message exceeds value1, the area passed to the application program is the size of the input message.
When value1, value2 is specified, value1 is the minimum size of the terminal input/output area that is passed to an application program when a RECEIVE command is issued. Whenever the size of the input message exceeds value1, CICS will use value2. If the input message size exceeds value2, the node abnormal condition program sends an exception response to the terminal.
If you specify ATI(YES), you must specify an IOAREALEN of at least one byte.
Real storage can be wasted if the IOAREALEN (value1) or TIOAL value is too large for most terminal inputs in the network. If IOAREALEN (value1) or TIOAL is smaller than most initial terminal inputs, excessive GETMAIN requests can occur, resulting in additional processor requirements, unless IOAREALEN(value1) or TIOAL is zero.
IOAREALEN(value1) or TIOAL should be set to a value that is slightly larger than the average input message length for the terminal. The maximum value that may be specified for IOAREALEN/TIOAL is 32767 bytes.
If a value of nonzero is required, the best size to specify is the most commonly encountered input message size. A multiple of 64 bytes minus 21 allows for SAA requirements and ensures good use of operating system pages.
For VTAM, you can specify two values if inbound chaining is used. The first value should be the length of the normal chain size for the terminal, and the second value should be the maximum size of the chain. The length of the TIOA presented to the task depends on the message length and the size specified for the TIOA. (See the example in Figure 54.)
Where x is any number of bytes, the following applies.
Without chain assembly:
If the TIOA size is specified as 20x
and the message length is 15x
then the TIOA acquired is 20x
If the TIOA size is specified as 20x
and the message length is 25x
then the TIOA acquired is 25x
With chain assembly:
If Value1 size is 20x
and Value2 size is 25x, then
if the length of a message is 15x
the TIOA acquired is 20x
and if the message length is 22x
the TIOA acquired is 25x
Avoid specifying too large a value1, for example, by matching it to the size of the terminal display screen. This area is used only as input. If READ with SET is specified, the same pointer is used by applications for an output area.
If too small a value is specified for value1, extra processing time is required for chain assembly, or data is lost if inbound chaining is not used.
In general, a value of zero is best because it causes the optimum use of storage and eliminates the second GETMAIN request. If automatic transaction initiation (ATI) is used for that terminal, a minimum size of one byte is required.
The second value for SNA devices is used to prevent terminal streaming, and so should be slightly larger than the largest possible terminal input in the network. If a message larger than this second value is encountered, a negative response is returned to the terminal, and the terminal message is discarded.
For VTAM, the TIOA value is specified in the CEDA DEFINE TYPETERM IOAREALEN attribute.
RMF™ and NetView® Performance Monitor (NPM) can be used to show storage usage and message size characteristics in the network.