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Chapter 10. Synchronization Requirements for Context
Alterations
Changing the contents of certain System Registers, the
contents of SLB entries, or the contents of other system
resources that control the context in which a program
executes can have the side effect of altering the context
in which data addresses and instruction addresses
are interpreted, and in which instructions are executed
and data accesses are performed. For example,
changing MSR IR from 0 to 1 has the side effect of
enabling translation of instruction addresses. These
side effects need not occur in program order, and
therefore may require explicit synchronization by software.
(Program order is defined in Book II.)
An instruction that alters the context in which data
addresses or instruction addresses are interpreted, or
in which instructions are executed or data accesses are
performed, is called a context-altering instruction. This
chapter covers all the context-altering instructions. The
software synchronization required for them is shown in
Table 1 (for data access) and Table 2 (for instruction
fetch and execution).
The notation “CSI” in the tables means any context
synchronizing instruction (e.g., sc, isync, or rfid). A
context synchronizing interrupt (i.e., any interrupt
except non-recoverable System Reset or non-recoverable
Machine Check) can be used instead of a context
synchronizing instruction. If it is, phrases like “the synchronizing
instruction”, below, should be interpreted as
meaning the instruction at which the interrupt occurs. If
no software synchronization is required before (after) a
context-altering instruction, “the synchronizing instruction
before (after) the context-altering instruction”
should be interpreted as meaning the context-altering
instruction itself.
If a sequence of instructions contains context-altering
instructions and contains no instructions that are
affected by any of the context alterations, no software
synchronization is required within the sequence.
Programming Note
Sometimes advantage can be taken of the fact that
certain events, such as interrupts, and certain
instructions that occur naturally in the program,
such as the rfid that returns from an interrupt handler,
provide the required synchronization.
No software synchronization is required before or after
a context-altering instruction that is also context synchronizing
or when altering the MSR in most cases
(see the tables). No software synchronization is
required before most of the other alterations shown in
Table 2, because all instructions preceding the contextaltering
instruction are fetched and decoded before the
context-altering instruction is executed (the processor
must determine whether any of these preceding
instructions are context synchronizing).
Unless otherwise stated, the material in this chapter
assumes a uniprocessor environment.
The synchronizing instruction before the context-altering
instruction ensures that all instructions up to and
including that synchronizing instruction are fetched and
executed in the context that existed before the alteration.
The synchronizing instruction after the contextaltering
instruction ensures that all instructions after
that synchronizing instruction are fetched and executed
in the context established by the alteration. Instructions
after the first synchronizing instruction, up to and
including the second synchronizing instruction, may be
fetched or executed in either context.
Chapter 10. Synchronization Requirements for Context Alterations
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