Jameco Electronics Rabbit 3000 - Manual (Page 292)

User’s Manual 283
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.