Online diagnostics can be run in three modes:
| Concurrent Mode | Allows the normal system functions to continue while selected resources are being checked. |
| Service Mode | Allows checking of most system resources. |
| Maintenance Mode | Allows checking of most system resources. |
Concurrent mode provides a way to run online diagnostics on some of the system resources while the system is running normal system activity. Because the system is running in normal operation, devices such as the following may require additional actions by the user or diagnostic application before testing can be done.
Service mode provides the most complete checkout of the system resources. This mode also requires that no other programs be running on the system. All system resources, except the SCSI adapter and the disk drives used for paging, can be tested. However, note that the memory and processor are only tested during Power On Self Tests (POSTs).
Service Mode is entered by booting the operating system in service mode.
Maintenance mode provides the exact same test coverage as Service Mode. The difference between the two modes is the way they are invoked. Maintenance mode requires that all activity on the operating system be stopped.
The shutdown -m command is used to stop all activity on the operating system and put the operating system into maintenance mode. After setting the terminal type, use the diag command to start Diagnostics.