Intel 80286, 80287 manual Programming Interface, PASCAL-286 FORTRAN-286

OVERVIEW OF NUMERIC PROCESSING

all 80287 numeric instructions in software. This emulation is completely transparent to the appli- cation software-the same object code may be used by both 80286 and 80287 systems. No relinking or recompiling of application software is necessary; the same code will simply execute faster on the 80287 than on the 80286 system.

To facilitate this design of upgradable 80286 systems, Intel provides a software emulator for the 80287 that provides the functional equivalent of the 80287 hardware, implemented in software on the 80286. Except for timing, the operation of this 80287 emulator (E80287) is the same as for the 80287 NPX hardware. When the emulator is combined as part of the systems software, the 80286 system with 80287 emulation and the 80286 with 80287 hardware are virtually indistinguishable to an application program. This capability makes it easy for software developers to maintain a single: set of programs for both systems. System manufacturers can offer the NPX as a simple plug-in performance option without necessitating any changes in the user's software.

Programming Interface

The 80286/80287 pair is programmed as a single processor; all of the 80287 registers appear to a programmer as extensions of the basic 80286 register set. The 80286 has a class of instructions known as ESCAPE instructions, all having a common format. These ESC instructions are numeric instruc- tions for the 80287 NPX. These numeric instructions for the 80287 are simply encoded into the instruc- tion stream along with 80286 instructions.

All of the CPU memory-addressing modes may be used in programming the NPX, allowing convenient access to record structures, numeric arrays, and other memory-based data structures. All of the memory management and protection features of the CPU are extended to the NPX as well.

Numeric processing in the 80287 centers around the NPX register stack. Programmers can treat these eight 80-bit registers as either a fixed register set, with instructions operating on explicitly-designated registers, or a classical stack, with instructions operating on the top one or two stack elements.

Internally, the 80287 holds all numbers in a uniform 80-bit temporary-real format. Operands that may be represented in memory as 16-, 32-, or 64-bit integers, 32-, 64-, or 80-bit floating-point numbers, or 18-digit packed BCD numbers, are automatically converted into temporary-real format as they are loaded into the NPX registers. Computation results are subsequently converted back into one of these destination data formats when they are stored into memory from the NPX registers.

Table 1-2 lists each of the seven data types supported by the 80287, showing the data format for each type. All operands are stored in memory with the least significant digits starting at the initial (lowest) memory address. Numeric instructions access and store memory operands using only this initial address. For maximum system performance, all operands should start at even memory addresses.

Table 1-3 lists the 80287 instructions by class. No special programming tools are necessary to use the 80287, because all of the NPX instructions and data types are directly supported by the ASM286 Assembler and Intel's appropriate high-level languages.

Software routines for the 80287 may be written in ASM286 Assembler or any of the following higher- level languages:

PL/M-286

PASCAL-286

FORTRAN-286

C-286

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