Intel 64 and IA-32 Architectures Software Developers Manual Volume 1, Basic Architecture

Vol. 1 D-19
GUIDELINES FOR WRITING X87 FPU EXCEPTION HANDLERS
epilogue must not load an unmasked exception flag into the x87 FPU or another
exception will be requested immediately.
The following code examples show the ASM386/486 coding of three skeleton excep-
tion handlers, with the save spaces given as correct for 32-bit protected mode. They
show how prologues and epilogues can be written for various situations, but the
application-dependent exception handling body is just indicated by comments
showing where it should be placed.
The first two are very similar; their only substantial difference is their choice of
instructions to save and restore the x87 FPU. The trade-off here is between the
increased diagnostic information provided by FNSAVE and the faster execution of
FNSTENV. (Also, after saving the original contents, FNSAVE re-initializes the x87 FPU,
while FNSTENV only masks all x87 FPU exceptions.) For applications that are sensi-
tive to interrupt latency or that do not need to examine register contents, FNSTENV
reduces the duration of the “critical region,” during which the processor does not
recognize another interrupt request. (See the Section 8.1.10, “Saving the x87 FPU’s
State with FSTENV/FNSTENV and FSAVE/FNSAVE,” for a complete description of the
x87 FPU save image.) If the processor supports Streaming SIMD Extensions and the
operating system supports it, the FXSAVE instruction should be used instead of
FNSAVE. If the FXSAVE instruction is used, the save area should be increased to 512
bytes and aligned to 16 bytes to save the entire state. These steps will ensure that
the complete context is saved.
After the exception handler body, the epilogues prepare the processor to resume
execution from the point of interruption (for example, the instruction following the
one that generated the unmasked exception). Notice that the exception flags in the
memory image that is loaded into the x87 FPU are cleared to zero prior to reloading
(in fact, in these examples, the entire status word image is cleared).
Example D-3 and Example D-4 assume that the exception handler itself will not
cause an unmasked exception. Where this is a possibility, the general approach
shown in Example D-5 can be employed. The basic technique is to save the full x87
FPU state and then to load a new control word in the prologue. Note that considerable
care should be taken when designing an exception handler of this type to prevent the
handler from being reentered endlessly.