Intel 80287, 80286 manual Ffree destination

PROGRAMMING NUMERIC APPLICATIONS

FSTENV /FNSTENV must be allowed to complete before any other 80287 instruction is decoded. When FSTENV is coded, an explicit FWAIT, or assembler-generated WAIT, should precede any subsequent 80287 instruction.

FLDENV source

FLDENV (load environment) reloads the environment from the memory area defined by the source operand. This data should have been written by a previous FSTENV /FNSTENV instruction. CPU instructions (that do not reference the environment image) may immediately follow FLDENV. An FWAIT is not required after FLDENV. FLDENV will automatically wait for all data transfers to complete before executing the next instruction.

Note that loading an environment image that contains an unmasked exception will cause a numeric exception when the next WAIT or error-checking-ESC instruction is executed.

FINCSTP

FINCSTP (increment stack pointer) adds 1 to the stack top pointer (ST) in the status word. It does not alter tags or register contents, nor does it transfer data. It is not equivalent to popping the stack, because it does not set the tag of the previous stack top to empty. Incrementing the stack pointer when ST=7 produces ST=O.

FDECSTP

FDECSTP (decrement stack pointer) subtracts 1 from ST, the stack top pointer in the status word. No tags or registers are altered, nor is any data transferred. Executing FDECSTP when ST=O produces ST=7.

FFREE destination

FFREE (free register) changes the destination register's tag to empty; the content of the register is unaffected.

FNOP

FNOP (no operation) stores the stack top to the stack top (FST ST,ST(O)) and thus effectively performs

no operation. '

FWAIT (CPU INSTRUCTION)

FWAIT is not actually an 80287 instruction, but an alternate mnemonic for the CPU WAIT instruc- tion. The FWAIT or WAIT mnemonic should be coded whenever the programmer wants to synchro- nize the CPU to the NPX, that is, to suspend further instruction decoding until the NPX has completed the current instruction. FWAIT will check for unmasked numeric exceptions.

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