4
FRESTORE
Restore Internal State
(Privileged Instruction}
FRESTORE
Operation:
If in supervisor state
then FPCP State Frame i Internal State
else TRAP
Assembler
Syntax:
FRESTORE <ea>
Attributes:
Unsized, privileged.
Description:
Aborts the execution of any operation in progress, and loads a new internal
state from the state frame located at the effective address. The first word at the
specified address is the format word of the state frame, which specifies the size of
the frame and the revision number of the FPCP that created it. The MPU writes the
first word to the FPCP Restore CIR to initiate the restore operation, and then reads the
response CIR to verify that the FPCP recognizes the format word as valid. If the format
word is invalid for the FPCP (either because the size of the frame is not recognized,
or the revision number does not match the revision of the processor), the MPU is
instructed to take a format exception. The MPU then writes an abort to the control
CIR, and the FPCP enters the IDLE state. If the format word is valid, the appropriate
state frame is loaded, starting at the specified location and proceeding through higher
addresses.
The FRESTORE instruction does not normally affect the programmer's model registers
of the FPCP
(except
for the NULL state
size,
as described below), but is used only to
restore the user invisible portion of the machine. The FRESTORE instruction is used
with the FMOVEM instruction to perform a full context restoration of the FPCP, in-
cluding the floating-point data registers and system control registers. In order to ac-
complish a complete restoration, the FMOVEM instructions are first executed to load
the programmer's model, followed by the FRESTORE instruction to load the internal
state and continue any previously suspended operation. Refer to 6.4 CONTEXT
SWITCHING for more information.
The current implementation of the FPCP supports three state frames. Refer to 6.4.2
State Frames
for the exact format of these state frames.
NULL: This state frame is four bytes long, with a format word of $0000. A FRESTORE
operation with this size state frame is equivalent to a hardware reset of
the FPCP. The programmer's model is set to the reset state, with nonsig-
naling NANs in the floating-point data registers and zeros in the FPCR,
FPSR and FPIAR. (Thus, it is unnecessary to load the programmer's model
before this operation.)
FREESCALE
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MC68881/MC68882 USER'S MANUAL
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