Intel 80287, 80286 manual Finitifninit

PROGRAMMING NUMERIC APPLICATIONS

It should also be noted that the 8087 instructions FENI and FDISI perform no function in the 80287. If these opcodes are detected in an 80286/80287 instruction stream, the 80287 will perform no specific operation and no internal states will be affected. For programmers interested in porting numeric software from 8087 environments to the 80286, however, it should be noted that program sections containing these exception-handling instructions are not likely to be completely portable to the 80287. Appendix B contains a more complete description of the differences between the 80287 and the 8087 NPX.

FINITIFNINIT

FINIT/FNINIT (initialize processor) sets the 80287 NPX into a known state, unaffected by any previous activity. The no-wait form of this instruction will cause the 80287 to abort any previous numeric operations currently executing in the NEU. This instruction performs the functional equivalent of a hardware RESET, with one exception; FINIT/FNINIT does not affect the current 80287 operating mode (either Real-Address mode or Protected mode). FINIT checks for unmasked numeric exceptions, FNINIT does not.

Note that if FNINIT is executed while a previous 80287 memory-referencing instruction is running, 80287 bus cycles in progress will be aborted. This instruction may be necessary to clear the 80287 if a Processor Extension Segment Overrun Exception (Interrupt 9) is detected by the CPU.

FSETPM

FSETPM (set Protected mode) sets the operating mode of the 80287 to Protected Virtual-Address mode. When the 80287 is first initialized following hardware RESET, it operates in Real-Address mode, just as does the 80286 CPU. Once the 80287 NPX has been set into Protected mode, only a hardware RESET can return the NPX to operation in Real-Address mode.

When the 80287 operates in Protected mode, the NPX exception pointers are represented differently than they are in Real-Address mode (see the FSAVE and FSTENV instructions that follow). This distinction is evident primarily to writers of numeric exception handlers, however. Forgeneral appli-

cation programmers, the operating mode of the 80287 need not be a concern. .

FLDCW source

FLDCW (load control word) replaces the current processor control word with the .word defined by the source operand. This instruction is typically used to establish or change the·80287's mode of operation. Note that if an exception bit in the status word is set, loading a new control word that unmasks that exception and clears the interrupt enable mask will generate an immediate interrupt request before the ne~t instr!!o::ti0!! is ~x~~nt""L When changing modes, the recomme\1ded procedure is to first clear any exceptions and then load the new control word.

FSTCW/FNSTCW destination

FSTCW/FNSTCW (store control word) writes the current processor control word to the memory location defined by the destination. FSTCW checks for unmasked numeric exceptions, FNSTCW does not.

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