M80C287

The M80C287 Numeric Processor Extension (NPX) provides arithmetic instructions for a variety of numeric data types. It also executes numerous built-in transcendental functions (e.g. tangent, sine, cosine, and log functions). The M80C287 effectively extends the register and instruction set of the CPU for existing data types and adds several new data types as well. Figure 2 shows the additional registers visible to programs in a system that includes the M80C287. Essentially, the M80C287 can be treated as an additional resource or an extension to the M80C286 CPU. The M80C286 CPU together with an M80C287 NPX can be used as a single unified system.

The M80C287 has two operating modes. After reset, the M80C287 is in the real-address mode. It can be placed into protected mode by executing the FSETPM instruction. It can be switched back to realaddress mode by executing the FRSTPM instruction (note that this feature is useful only with CPU's that can also switch back to real-address mode). These instructions control the format of the administrative instructions FLDENV, FSTENV, FRSTOR, and FSAVE. Regardless of operating mode, all references to memory for numerics data or status information are performed by the M80C286 CPU, and therefore obey the memory-management and protection rules of the M80C286 CPU.

In real-address mode, a system that includes the M80C287 is completely upward compatible with software for the M8086/M8087 and for M80286/ M80287 real-address mode.

In protected mode, a system that includes the M80C287 is completely upward compatible with software for M80286/M80287 protected mode systems.

The only differences of operation that may appear when M8086/M8087 programs are ported to a protected- mode M80C287 system are in the format of operands for the administrative instructions FLDENV, FSTENV, FRSTOR, and FSAVE. These instructions are normally used only by exception handlers and operating systems, not by applications
programs.