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an instruction-caused exception in the exception handler. SRR0 and SRR1 should also be
saved before enabling external interrupts.
The PowerPC architecture supports four types of exceptions:
¥ Synchronous, preciseÑThese are caused by instructions. All instruction-caused
exceptions are handled precisely; that is, the machine state at the time the exception
occurs is known and can be completely restored. This means that (excluding the trap
and system call exceptions) the address of the faulting instruction is provided to the
exception handler and that neither the faulting instruction nor subsequent
instructions in the code stream will complete execution before the exception is
taken. Once the exception is processed, execution resumes at the address of the
faulting instruction (or at an alternate address provided by the exception handler).
When an exception is taken due to a trap or system call instruction, execution
resumes at an address provided by the handler.
¥ Synchronous, impreciseÑThe PowerPC architecture deÞnes two imprecise
ßoating-point exception modes, recoverable and nonrecoverable. These are not
implemented on the MPC8260.
¥ Asynchronous, maskableÑThe external, system management interrupt (SMI), and
decrementer interrupts are maskable asynchronous exceptions. When these
exceptions occur, their handling is postponed until the next instruction and any
exceptions associated with that instruction complete execution. If no instructions are
in the execution units, the exception is taken immediately upon determination of the
correct restart address (for loading SRR0).
¥ Asynchronous, nonmaskableÑThere are two nonmaskable asynchronous
exceptions: system reset and the machine check exception. These exceptions may
not be recoverable, or may provide a limited degree of recoverability. All exceptions
report recoverability through MSR[RI].
2.5.2 MPC8260 Implementation-SpeciÞc Exception Model
As speciÞed by the PowerPC architecture, all processor core exceptions can be described
as either precise or imprecise and either synchronous or asynchronous. Asynchronous
exceptions (some of which are maskable) are caused by events external to the processorÕs
execution. Synchronous exceptions, which are all handled precisely by the processor core,
are caused by instructions. The processor core exception classes are shown in Table 2-4.
MOTOROLA
Chapter 2. PowerPC Processor Core
Part I. Overview
2-23
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