ia-32_volume1_basic-arch

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Unformatted text preview: verflow or underflow, the x87 FPU carries out one of the following operations: If an inexact result occurs in conjunction with masked overflow or underflow, the OE or UE flag and the PE flag are set and the result is stored as described for the overflow or underflow exceptions (see Section 8.5.4, "Numeric Overflow Exception (#O)," or Section 8.5.5, "Numeric Underflow Exception (#U)"). If the inexact result exception is unmasked, the x87 FPU also invokes a software exception handler. If an inexact result occurs in conjunction with unmasked overflow or underflow and the destination operand is a register, the OE or UE flag and the PE flag are set, the result is stored as described for the overflow or underflow exceptions (see Section 8.5.4, "Numeric Overflow Exception (#O)," or Section 8.5.5, "Numeric Underflow Exception (#U)") and a software exception handler is invoked. If an unmasked numeric overflow or underflow exception occurs and the destination operand is a memory location (which can happen only for a floating-point store), the inexact-result condition is not reported and the C1 flag is cleared. 8.6 X87 FPU EXCEPTION SYNCHRONIZATION Because the integer unit and x87 FPU are separate execution units, it is possible for the processor to execute floating-point, integer, and system instructions concurrently. No special programming techniques are required to gain the advantages of concurrent execution. (Floating-point instructions are placed in the instruction stream along with the integer and system instructions.) However, concurrent execution can cause problems for floating-point exception handlers. This problem is related to the way the x87 FPU signals the existence of unmasked floating-point exceptions. (Special exception synchronization is not required for Vol. 1 8-43 PROGRAMMING WITH THE X87 FPU masked floating-point exceptions, because the x87 FPU always returns a masked result to the destination operand.) When a floating-point exception is unmasked and the exception condition occurs, the x87 FPU stops further execution of the floating-point instruction and signals the exception event. On the next occurrence of a floating-point instruction or a WAIT/FWAIT instruction in the instruction stream, the processor checks the ES flag in the x87 FPU status word for pending floating-point exceptions. If floating-point exceptions are pending, the x87 FPU makes an implicit call (traps) to the floatingpoint software exception handler. The exception handler can then execute recovery procedures for selected or all floating-point exceptions. Synchronization problems occur in the time between the moment when the exception is signaled and when it is actually handled. Because of concurrent execution, integer or system instructions can be executed during this time. It is thus possible for the source or destination operands for a floating-point instruction that faulted to be overwritten in memory, making it impossible for t...
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