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27. S
YSTEM
/U
SER
M
ODE
27.1 Overview
The Rabbit 4000 provides support for two tiers of control in the processor:
System Mode
,
which provides full access to all processor resources; and
User Mode
, a more restricted
mode. Table 27-1 describes the essential differences between the System Mode and the
User Mode. The System Mode is essentially the same as the normal operation when the
System/User Mode is disabled.
The main intent of the System/User Mode is to protect critical code (for example, code
that performs remote firmware updates), data, and the current processor state (memory
setup, peripheral control, etc.) from inadvertent changes by the user’s standard code. By
removing access to the processor’s I/O registers and preventing memory writes to critical
regions, the user’s code can run without the danger of locking up the processor to the point
where it cannot be restarted remotely and/or new code uploaded.
Table 27-1. Differences Between System Mode and User Mode
System Mode
User Mode
All peripherals accessible.
No peripherals accessible by default.
All processor control registers available.
No processor control registers available.
All interrupt priorities available.
Interrupt Priority 3 not allowed.
IDET instruction has no effect.
IDET instruction causes Priority 3 “System
mode violation” interrupt.
No write protection when 0x00 is written to
WPCR (write protection in User mode only)
Write to protected segment causes Priority 3
“write protection violation” interrupt.
Easy to enter user mode (SETUSR instruction).
Difficult to enter system mode (requires
interrupt, SYSCALL, or RST instruction).
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