Intel ARCHITECTURE IA-32 User Manual download pdf (Page 232)

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INTERRUPT AND EXCEPTION HANDLING

Prior to executing a waiting x87 FPU instruction or the WAIT/FWAIT instruction, the x87 FPU

checks for pending x87 FPU floating-point exceptions (as described in step 2 above). Pending

x87 FPU floating-point exceptions are ignored for “non-waiting” x87 FPU instructions, which

include the FNINIT, FNCLEX, FNSTSW, FNSTSW AX, FNSTCW, FNSTENV, and FNSAVE

instructions. Pending x87 FPU exceptions are also ignored when executing the state manage-

ment instructions FXSAVE and FXRSTOR.

All of the x87 FPU floating-point error conditions can be recovered from. The x87 FPU floating-

point-error exception handler can determine the error condition that caused the exception from

the settings of the flags in the x87 FPU status word. See “Software Exception Handling” in

Chapter 8, “Programming with the x87 FPU,” in the IA-32 Intel® Architecture Software Devel-

oper’s Manual, Volume 1, for more information on handling x87 FPU floating-point exceptions.

Exception Error Code

None. The x87 FPU provides its own error information.

Saved Instruction Pointer

The saved contents of CS and EIP registers point to the floating-point or WAIT/FWAIT instruc-

tion that was about to be executed when the floating-point-error exception was generated. This

is not the faulting instruction in which the error condition was detected. The address of the

faulting instruction is contained in the x87 FPU instruction pointer register. See “x87 FPU

Instruction and Operand (Data) Pointers” in Chapter 8, “Programming with the x87 FPU,” in the

IA-32 Intel® Architecture Software Developer’s Manual, Volume 1, for more information about

information the FPU saves for use in handling floating-point-error exceptions.

Program State Change

A program-state change generally accompanies an x87 FPU floating-point exception because

the handling of the exception is delayed until the next waiting x87 FPU floating-point or

WAIT/FWAIT instruction following the faulting instruction. The x87 FPU, however, saves

sufficient information about the error condition to allow recovery from the error and re-execu-

tion of the faulting instruction if needed.

In situations where non- x87 FPU floating-point instructions depend on the results of an x87

FPU floating-point instruction, a WAIT or FWAIT instruction can be inserted in front of a

dependent instruction to force a pending x87 FPU floating-point exception to be handled before

the dependent instruction is executed. See “x87 FPU Exception Synchronization” in Chapter 8,

“Programming with the x87 FPU,” in the IA-32 Intel® Architecture Software Developer’s

Manual, Volume 1, for more information about synchronization of x87 floating-point-error

exceptions.

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