
RCPU
REFERENCE MANUAL
EXCEPTIONS
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MOTOROLA
6-5
6.1.3.1 Asynchronous, Maskable Exceptions
The RCPU supports the following asynchronous, maskable exceptions: external
interrupts, decrementer interrupts, and maskable internal and external breakpoint
exceptions.
External and decrementer interrupts are masked by the external interrupt enable
(EE) bit in the MSR. When MSR[EE] = 0, these exception conditions are latched
and are not recognized until MSR[EE] is set. MSR[EE] is cleared automatically
when an exception is taken to delay recognition of external and decrementer inter-
rupts.
Maskable internal or external breakpoint exceptions are recognized only when the
RI (recoverable exception) bit in the MSR = 1. This ensures that (with proper soft-
ware safeguards) the processor can always recover from one of these exceptions.
Refer to
SECTION 8 DEVELOPMENT SUPPORT
for details on maskable and
non-maskable internal and external breakpoints.
6.1.3.2 Asynchronous, Non-Maskable Exceptions
Asynchronous, non-maskable exceptions include reset and non-maskable internal
and external breakpoint exceptions. These exceptions have the highest priority
and can occur while other exceptions are being processed. Because these excep-
tions are non-maskable, they are never delayed; therefore, if an asynchronous,
non-maskable exception occurs immediately after another exception, the state in-
formation saved by the first exception may be overwritten when the second excep-
tion occurs.
For additional information, refer to
6.5.2 Recovery from Unordered Exceptions
.
Refer to
SECTION 8 DEVELOPMENT SUPPORT
for details on maskable and
non-maskable internal and external breakpoints.
6.2 Exception Vector Table
The setting of the exception prefix (IP) bit in the MSR determines how exceptions
are vectored. If the bit is cleared, exceptions are vectored to the physical address
0x0000 0000 plus the vector offset; if IP is set, exceptions are vectored to the phys-
ical address 0xFFF0 0000 plus the vector offset.
Table 6-3
shows the exception
vector offset of the first instruction of the exception handler routine for each excep-
tion type.
NOTE
The exception vectors shown in
Table 6-3
, up to and including the
floating-point assist exception (vector offset 0x00E00), are defined
by the PowerPC architecture. Exception vectors beginning with offset
0x01000 (software emulation exception in the RCPU) are reserved in
the PowerPC architecture for implementation-specific exceptions.
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