In the intel486 pentium and p6 processor family and

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Unformatted text preview: an x87 FPU exception. In this case, if the x87 FPU exception handler ever did get invoked, it could not determine which instruction caused the exception. To properly manage the interface between the processor's FERR# output, its IGNNE# input, and the IRQ13 input of the PIC, additional external hardware is needed. A recommended configuration is described in the following section. D.2.1.2 Recommended External Hardware to Support the MS-DOS Compatibility Sub-mode Figure D-1 provides an external circuit that will assure proper handling of FERR# and IGNNE# when an x87 FPU exception occurs. In particular, it assures that IGNNE# will be active only inside the x87 FPU exception handler without depending on the order of actions by the exception handler. Some hardware implementations have been less robust because they have depended on the exception handler to clear the x87 FPU exception interrupt request to the PIC (FP_IRQ signal) before the handler causes FERR# to be de-asserted by clearing the exception from the x87 FPU itself. Figure D-2 shows the details of how IGNNE# will behave when the circuit in Vol. 1 D-5 GUIDELINES FOR WRITING X87 FPU EXCEPTION HANDLERS Figure D-1 is implemented. The temporal regions within the x87 FPU exception handler activity are described as follows: 1. The FERR# signal is activated by an x87 FPU exception and sends an interrupt request through the PIC to the processor's INTR pin. 2. During the x87 FPU interrupt service routine (exception handler) the processor will need to clear the interrupt request latch (Flip Flop #1). It may also want to execute non-control x87 FPU instructions before the exception is cleared from the x87 FPU. For this purpose the IGNNE# must be driven low. Typically in the PC environment an I/O access to Port 0F0H clears the external x87 FPU exception interrupt request (FP_IRQ). In the recommended circuit, this access also is used to activate IGNNE#. With IGNNE# active, the x87 FPU exception handler may execute any x87 FPU instruction without being blocked by an active x87 FPU exception. 3. Clearing the exception within the x87 FPU will cause the FERR# signal to be deactivated and then there is no further need for IGNNE# to be active. In the recommended circuit, the deactivation of FERR# is used to deactivate IGNNE#. If another circuit is used, the software and circuit together must assure that IGNNE# is deactivated no later than the exit from the x87 FPU exception handler. D-6 Vol. 1 GUIDELINES FOR WRITING X87 FPU EXCEPTION HANDLERS Figure D-1. Recommended Circuit for MS-DOS Compatibility x87 FPU Exception Handling In the circuit in Figure D-1, when the x87 FPU exception handler accesses I/O port 0F0H it clears the IRQ13 interrupt request output from Flip Flop #1 and also clocks out the IGNNE# signal (active) from Flip Flop #2. So the handler can activate IGNNE#, if needed, by doing this 0F0H access before clearing the x87 FPU exception condition (which de-asserts FERR#). However, the circuit does not...
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