Advanced CA IDMS Programming Topics › Basic Mode

Basic Mode

In basic mode, DC performs device-dependent data transfers between your program and the terminal. Your program must format the data and supply device-control characters based on the type of terminal in use; DC inserts the necessary line control information. For example, with 3270-type devices, you must send and receive data with device-control information that includes write control characters, orders, and buffer addresses.

The figure below shows a basic mode data transfer. DC appends framing characters to the input data stream and performs the required I/O.

Data stream as built by the user
┌───────────────────────────────────────┐
│  Data and device-control information  │
└───────────────────────────────────────┘

Data stream as passed by basic mode request
┌────────────────┬───────────────────────────────────────┬────────────────┐
│  LINE CONTROL  │  Data and device-control information  │  LINE CONTROL  │
└────────────────┴───────────────────────────────────────┴────────────────┘

For information on using basic mode to support System Network Architecture (SNA) protocols, see CA IDMS DML Reference Guide for Assembler.

I/O Requests Under Basic Mode

Basic mode supports synchronous and asynchronous read and write requests. The terms synchronous and asynchronous do not refer to line protocol for data transmission but rather to task processing during I/O operations. Synchronous and asynchronous I/O requests function in the following manner:

• Following a synchronous I/O request, control returns to DC, which places the issuing task in an inactive state. When the requested I/O operation is complete, DC places the task in a ready state and the task resumes processing according to its established dispatching priority.
• Following an asynchronous I/O request, the issuing task continues executing.

DC assumes that all I/O requests are synchronous unless a program explicitly requests asynchronous processing.

What You Can Do in Basic Mode

Using basic mode terminal management, you can perform the following functions:

• Read data-You can transfer data from the terminal to program variable storage.
• Write data-You can transfer data from program variable storage to the terminal.
• Determine if I/O is complete-You can check to determine if a previously issued asynchronous request is complete.