Intel 80287, 80286 Dedicated and Reserved 1/0 Locations, Processor Initialization and Control

SYSTEM-LEVEL NUMERIC PROGRAMMING

Dedicated and Reserved 1/0 Locations

The 80287 NPX does not require that any memory addresses be set aside for special purposes. The 80287 does make use of 1/0 port addresses in the range 00F8H through OOFFH, although these 1/0 operations are completely transparent to the 80286 software. 80286 programs must not reference these reserved 1/0 addresses directly.

To prevent any accidental misuse or other tampering with numeric instructions in the 80287, the 80286's 1/0 Privilege Level (IOPL) should be used in multiuser reprogrammable environments to restrict application program access to the 1/0 address space and so guarantee the integrity of 80287 compu- tations. Chapter Eight of the 80286 Operating System Writer's Guide contains more details regarding the use of the 1/0 Privilege Level.

PROCESSOR INITIALIZATION AND CONTROL

One of the principal responsibilities of systems software is the initialization, monitoring, and control of the hardware and software resources of the system, including the 80287 NPX. In this section, issues related to system initialization and control are described, including recognition of the NPX, emulation of the 80287 NPX in software if the hardware is not available, and the handling of exceptions that may occur during the execution of the 80287.

System Initialization

During initialization of an 80286 system, systems software must

•Recognize the presence or absence of the NPX

•Set flags in the 80286 MSW to reflect the state of the numeric environment

If an 80287 NPX is present in the system, the NPX must be

•Initialized

•Switched into Prolecled mode (if desired)

All of these activities can be quickly and easily performed as part of the overall system initialization.

Recognizing. the 80287 NPX

figure 3-1shuVv's ali cAulliplc cf:(l reccgr..iti~n rc!!ti~e th9..t det~-!'!!line.5 Ulh~thp.r:!n NPX is present~ and

distinguishes between the 80387 and the 8087/80287. This routine can be executed on any 80386, 80286, or 8086 hardware configuration that has an NPX socket.

The example guards against the possibility of accidentally reading an expected value from a floating data bus when no NPX is present. Data read from a floating bus is undefined. By expecting to read a specific bit pattern from the NPX, the routine protects itself from the indeterminate state of the bus. The example also avoids depending on any values in: reserved bits, thereby maintaining compatibility with future numerics coprocessors.

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