B.1.4 System/User Mode

By default, all of the hardware is accessible by the programmer. However, if a control bit in the Enable Dual Mode Register (EDMR) is set to one, two operating modes, System and User, become available. The System mode is just like the normal operating mode, but the User mode restricts program access to the hardware and to the System mode. Individ- ual peripherals may be enabled for User mode access in the User Enable registers listed below. When enabled for User mode access, a peripheral interrupt (if it is capable of gen- erating an interrupt) can only be requested at interrupt priority level -2 or -1, and it is assumed that the interrupt service routine will be executed by User mode code. Note that the processor automatically enters the System mode when entering the ISR area in response to an interrupt, and the User mode must be specifically entered before continuing with the interrupt service routine. The System/User mode is discussed in great detail in Appendix C.

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Jameco Electronics 3000, 2000 manual System/User Mode