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21.2
Serializing instructions
ARM DDI 0301H
ID012310
A serializing instruction is one that stalls because of activity in the VFP11 pipelines without the
presence of a register or resource hazard. In general, an access to a VFP11 control or status
register is a serializing instruction.
The serializing instructions are FMRX and FMXR, including the FMSTAT instruction.
Serializing instructions stall the VFP11 coprocessor in the Issue stage and the ARM processor
in the Execute 2 stage until:
•
the VFP11 pipeline is past the point of updating either the condition codes or the exception
status
•
a write to a system register can no longer affect the operation of a current or pending
instruction.
An FMRX or FMSTAT instruction stalls until all prior floating-point operations are completed,
and the data to be written by the VFP11 coprocessor is valid. For example, a compare operation
updates the FPSCR register condition codes in the Writeback stage of the compare.
An FMXR instruction stalls until all prior floating-point operations are past the point of being
affected by the instruction. For example, writing to the FPSCR register stalls until the point
when changing the control bits cannot affect any operation currently executing or awaiting
execution. Writing to the FPEXC, FPINST, or FPINST2 register stalls until the pipeline is
completely clear.
Uses of serializing instructions include:
•
capturing condition codes and exception status
•
delineating a block of instructions for execution with the ability to capture the exception
status of that block of instructions
•
modifying the mode of operation of subsequent instructions, such as the rounding mode
or vector length.
While no instruction can change the contents of the FPSID register, you can access the FPSID
register with FMRX or FMXR as a general-purpose serializing operation or to create an
exception boundary.
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VFP Instruction Execution
21-3
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