Motorola M68000 User Manual download pdf (Page 113)

Floating Point Instructions

5-102 M68000 FAMILY PROGRAMMER’S REFERENCE MANUAL MOTOROLA

FNOP No Operation FNOP

(MC6888X, MC68040)

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 ﬂoating- point unit with an integer unit or to force

processing of pending exceptions. For most ﬂoating-point instructions, the integer unit

is allowed to continue with the execution of the next instruction once the ﬂoating-point

unit has any operands needed for an operation, thus supporting concurrent execution

of ﬂoating-point and integer instructions. The FNOP instruction synchronizes the

ﬂoating-point unit and the integer unit by causing the integer unit to wait until all

previous ﬂoating-point instructions have completed. Execution of FNOP also forces

any exceptions pending from the execution of a previous ﬂoating-point instruction to be

processed as a preinstruction exception.

The MC68882 may not wait to begin execution of another floating- point instruction until

it has completed execution of the current instruction. The FNOP instruction synchro-

nizes the coprocessor and microprocessor unit by causing the microprocessor unit to

wait until the current instruction (or both instructions) have completed.

The FNOP instruction also forces the processing of exceptions pending from the exe-

cution of previous instructions. This is also inherent in the way that the floating-point

coprocessor utilizes the M68000 family coprocessor interface. Once the floating-point

coprocessor 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 report-

ing the exception during the execution of that instruction. Then, when the main proces-

sor attempts to initiate the execution of the next floating-point coprocessor instruction,

a preinstruction 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.

Floating-Point Status Register: Not Affected.

1 2 ... 108 109 110 111 112 113 114 115 116 117 118 ... 356 357

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Motorola M68000 User Manual Page 113