Intel 80287, 80286 manual Inter, Handling Numeric Processing Exceptions

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SYSTEM-LEVEL NUMERIC PROGRAMMING

As described previously, whenever the 80286 CPU encounters an ESC instruction, and its MP and EM status bits are set appropriately (MP=O, EM = I), the 80286 will automatically trap to interrupt #7, the Processor Extension Not Available exception. The return link stored on the stack points to the first byte of the ESC instruction, including the prefix byte(s), if any. The exception handler can use this return link to examine the ESC instruction and proceed to emulate the numeric instruction in software.

The emulator must step the return pointer so that, upon return from the exception handler, execution can resume at the first instruction following the ESC instruction.

To an application program, execution on an 80286 system with 80287 emulation is almost indistin- guishable from execution on an 80287 system, except for the difference in execution speeds.

There are several important considerations when using emulation on an 80286 system:

When operating in Protected-Address mode, numeric applications using the emulator must be executed in execute-readable code segments. Numeric software cannot be emulated if it is executed in execute-only code segments. This is because the emulator must be able to examine the particular numeric instruction that caused the Emulation trap.

Only privileged tasks can place the 80286 in emulation mode. The instructions necessary to place the 80286 in Emulatio,n mode are privileged instructions, and are not typically accessible to an

application.\

An emulator package (E80287) that runs on 80286 systems is available from Intel in the 8086 Software Toolbox, Order Number 122203. This emulation package operates in both Real and Protected mode, providing a complete functional equivalent for the 80287 emulated in software.

When using the E80287 emulator, writers of numeric exception handlers should be aware of one slight difference between the emulated 80287 and the 80287 hardware:

On the 80287 hardware, exception handlers are invoked by the 80286 at the first WAIT or ESC instruction following the instruction causing the exception. The return link, stored on the 80286 stack, points to this second WAIT or ESC instruction where execution will resume following a

retu~n from the exception handler.

Using the E80287 emulator, numeric exception handlers are invoked from within the emulator itself. The return link stored on the stack when the exception handler is invoked will therefore point back to the E80287 emulator, rather than to the program code actually being executed (emulated). An IRET return from the exception handler returns to the emulator, which then returns immediately to the emulated program. This added layer of indirection should not cause confusion, however, because the instruction causing the exception can always be identified from the 80287's instruction and data pointers.

Handling Numeric Processing Exceptions

Once the 80287 has been initialized and normal execution of applications has been commenced, the 80287 NPX may occasionally require attention in order to recover from numeric processing errors. This section provides details for writing software exception handlers for numeric exceptions. Numeric processing exceptions have already been introduced in previous sections of this manual.

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