
MOTOROLA
Chapter 1. Overview
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identically to the precise mode (that is, all enabled floating-point enabled exceptions
are always precise on the 603e). (Note that the EC603e microprocessor does not
support floating-point operations.)
Asynchronous, maskable—The 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, completes execution. If there are no
instructions in the execution units, the exception is taken immediately upon
determination of the correct restart address (for loading SRR0).
Asynchronous, nonmaskable—There 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 the MSR[RI] bit.
1.3.4.2 Implementation-Specific Exception Model
As specified by the PowerPC architecture, all 603e 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 processor’s execution;
synchronous exceptions, which are all handled precisely by the 603e, are caused by
instructions. The 603e exception classes are shown in Figure 1-4.
Although exceptions have other characteristics as well, such as whether they are maskable
or nonmaskable, the distinctions shown in Figure 1-4 define categories of exceptions that
the 603e handles uniquely. Note that Figure 1-4 includes no synchronous imprecise
instructions. While the PowerPC architecture supports imprecise handling of floating-point
exceptions, the 603e, with the exception of the EC603e microprocessor, implements
floating-point exception modes as precise exceptions.
The 603e’s exceptions, and conditions that cause them, are listed in Figure 1-5.
Figure 1-5. Exceptions and Conditions
Figure 1-4. Exception Classifications
Synchronous/Asynchronous
Precise/Imprecise
Exception Type
Asynchronous, nonmaskable
Imprecise
Machine check
System reset
Asynchronous, maskable
Precise
External interrupt
Decrementer
System management interrupt
Synchronous
Precise
Instruction-caused exceptions
Exception
Type
Vector Offset
(hex)
Causing Conditions
Reserved
00000
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