Programming instructions

Programming the Status Register System
Accessing Status Register Information
Chapter 3 107
Deciding How to Monitor
You can use either of two methods described below to access the information in status
registers (both methods allow you to monitor one or more conditions).
• The polling method
In the polling method, the signal generator has a passive role. It tells the controller that
conditions have changed only when the controller asks the right question. This is
accomplished by a program loop that continually sends a query.
The polling method works well if you do not need to know about changes the moment they
occur. Use polling in the following situations:
— when you use a programming language/development environment or I/O interface that
does not support SRQ interrupts
— when you want to write a simple, single-purpose program and don’t want the added
complexity of setting up an SRQ handler
• The service request (SRQ) method
In the SRQ method (described in the following section), the signal generator takes a more
active role. It tells the controller when there has been a condition change without the
controller asking.
Use the SRQ method if you must know immediately when a condition changes. (To detect a
change using the polling method, the program must repeatedly read the registers.) Use the
SRQ method in the following situations:
— when you need time-critical notification of changes
— when you are monitoring more than one device that supports SRQs
— when you need to have the controller do something else while waiting
— when you can’t afford the performance penalty inherent to polling