4
FNOP
No Operation
FNOP
Operation:
None
Assembler
Syntax:
FNOP
Attributes:
Unsized
Description:
This instruction does not perform any explicit operation. However, it is
useful to force synchronization of the FPCP with a main processor, or to force proc-
essing of pending exceptions. The synchronization function is inherent in the way that
the FPCP uses the M68000 Family coprocessor interface. For most FPCP instructions,
the main processor is allowed to continue with the execution of the next instruction
once the FPCP has any operands needed for an operation, thus supporting concurrent
execution of floating-point and integer instructions. However, if the main processor
attempts to initiate the execution of a new floating-point instruction in the MC68881
before the previous one is completed, the main processor is forced to wait until that
instruction execution is finished before proceeding with the new instruction. FNOP is
treated in the same way as other instructions and thus cannot be executed until the
previous floating-point instruction is completed, and the main processor is synchro-
nized with the MC68881.
The MC68882 may not wait to begin execution of another floating-point instruction
until it has completed execution of the current instruction. However, the FNOP instruc-
tion synchronizes the coprocessor and MPU by causing the MPU to wait until the
current instruction (or both instructions) have completed.
The FNOP instruction also forces the processing of exceptions pending from the
execution of previous instructions. This is also inherent in the way that the FPCP utilizes
the M68000 Family coprocessor interface. Once the FPCP has received the input operand
for an arithmetic instruction, it always releases the main processor to execute the next
instruction (regardless of whether or not concurrent execution is prevented for the
instruction due to tracing) without reporting the exception during the execution of
that instruction, Then, when the main processor attempts to initiate the execution of
the next FPCP instruction, a pre-instruction exception may be reported to initiate
exception processing for an exception that occurred during a previous instruction. By
using the FNOP instruction, the user can force any pending exceptions to be processed
without performing any other operations.
Status Register:
Not Affected
FREESCALE
4-84
MC68881/MC68882 USER'S MANUAL
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