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342
March 2002 Release
1-800-255-7778
Virtex-II Pro™ Platform FPGA Documentation
Chapter 2:
Operational Concepts
R
Synchronization Operations
Various forms of synchronizing operations can be used by programs executing on the
PPC405 processor to control the behavior of instruction execution and memory accesses.
Synchronizing operations fall into the following three categories:
•
Context synchronization
•
Execution synchronization
•
Storage synchronization
Each synchronization category is described in the following sections. Instructions
provided by the PowerPC architecture for synchronization purposes are described on
Context Synchronization
The state of the execution environment (privilege level, translation mode, and memory
protection) defines a program’s context. An instruction or event is
context synchronizing
if
the operation satisfies all of the following conditions:
•
Instruction dispatch is halted when the operation is recognized by the processor. This
means the instruction-fetch mechanism stops issuing (sending) instructions to the
execution units.
•
The operation is not initiated (for instructions, this means dispatched) until all prior
instructions complete execution to a point where they report any exceptions they
cause to occur. In the case of an instruction-synchronize (
isync
) instruction, the
isync
does not complete execution until all prior instructions complete execution to a point
where they report any exceptions they cause to occur.
•
All instructions that precede the operation complete execution in the context they
were initiated. This includes privilege level, translation mode, and memory
protection.
•
All instructions following the operation complete execution in the new context
established by the operation.
•
If the operation is an exception, or directly causes an exception to occur (for example,
the
sc
instruction causes a system-call exception), the operation is not initiated until
all higher-priority exceptions are recognized by the exception mechanism.
The system-call instruction (
sc
), return-from-interrupt instructions (
rfi
and
rfci
), and most
exceptions are examples of context-synchronizing operations.
Context-synchronizing operations do not guarantee that subsequent memory accesses are
performed using the memory context established by previous instructions. When
memory-access ordering must be enforced, storage-synchronizing instructions are
required.
Execution Synchronization
An instruction is
execution synchronizing
if it satisfies the conditions of the first two items
(as described above) for context synchronization:
•
Instruction dispatch is halted when the operation is recognized by the processor. This
means the instruction-fetch mechanism stops issuing (sending) instructions to the
execution units.
•
The operation is not initiated until all instructions in execution complete to a point
where they report any exceptions they cause to occur. In the case of a synchronize
(
sync
) instruction, the
sync
does not complete execution until all prior instructions
complete execution to a point where they report any exceptions they cause to occur.
The
sync
and
move-to machine-state register
(
mtmsr
) instructions are examples of execution-
synchronizing instructions.
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